In unit 4 we will learn about Waves and Optics. Waves are everywhere in nature – sound waves, visible light waves, earthquakes, water waves, microwaves, Mexican waves… Understanding optics is important in use of lenses – for sight, cameras, entertainment and many other applications. Friday 29th February

Waves & Optics

What do you know? About waves? About optics? Friday 29th February

frequency, wavelength, speed, amplitude, period. Key words: waves, energy transfer, transverse, longitudinal, frequency, speed, wavelength, amplitude. By the end of this lesson you will be able to: State that a wave transfers energy. Use the following terms correctly in context: wave, frequency, wavelength, speed, amplitude, period. State the difference between a longitudinal wave and a transverse wave, and give an example of each. What words are new?

Waves & Energy Transfer Waves transfer energy. The energy transferred by waves can be considerable!

Indian Ocean 2004

Indonesia 2005

Waves transfer energy…

Waves transfer energy…

Properties of Waves What is meant by the axis of a wave? The axis is the line running through the middle of the wave pattern.

What is meant by the crest of the wave? The crest is the top part of the wave

…and the trough? The trough is the bottom part of the wave.

What is the amplitude of the wave? The amplitude is the distance from the axis to the crest

or from axis to trough.

Definition of Wavelength? The wavelength is the distance after which the wave pattern repeats itself – the distance between two identical points on the wave

Wavelength is given the symbol λ pronounced lambda. http://www.teachersdomain.org/resources/lsps07/sci/phys/energy/wavelength/assets/lsps07_int_wavelength/lsps07_int_wavelength_swf.html

Frequency The frequency of the wave is the number of waves each second. It is measured in hertz (Hz) which just means “per second”. http://www.teachersdomain.org/resources/lsps07/sci/phys/energy/frequency/assets/lsps07_int_frequency/lsps07_int_frequency_swf.html http://www.m2m.ecs.soton.ac.uk/Wcb886307c3e67.htm

Frequency Calculate the frequency of each of the following waves: 125 waves passing a point in 10 seconds. 16 waves passing a point in 24 seconds. 30 waves passing a point in 1 minute.

Period The period of a wave is the time taken for one complete wave to pass a point. It is measured in seconds (s).

Frequency & Period The link between the frequency and period of a wave

Frequency & Period Rearranging

Longitudinal Waves In this type of wave, particles vibrate back and forwards along the direction the wave is travelling.

Longitudinal Waves A sound wave is a longitudinal wave. http://www.acoustics.salford.ac.uk/schools/index1.htm

Transverse Waves In this type of wave, particles vibrate at right angles to the direction of motion of the wave.

Transverse Waves Light, and all forms of electromagnetic radiation, are transverse waves. http://www.acoustics.salford.ac.uk/schools/index1.htm

Homework for next lesson Learn: transverse and longitudinal waves properties of waves Axis Crest Trough Amplitude Wavelength Frequency Period

electromagnetic spectrum: gamma rays, X-rays, Key words: electromagnetic spectrum, wavelength, frequency By the end of this lesson you will be able to: State in order of wavelength, the members of the electromagnetic spectrum: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves , TV and radio. State that radio and television signals are transmitted through air at 300000000 m/s and that light is also transmitted at this speed.

About the electromagnetic spectrum? What do you know? About waves? About light? About the electromagnetic spectrum? Friday 29th February

Visible light is the only type of light Myth or Reality? Visible light is the only type of light

Myth! Visible light is a tiny slice of the radiation that makes up the electromagnetic spectrum.

What is radiation?

All radiation is harmful. Myth or Reality? All radiation is harmful.

Light is a form of radiation. All Myth! Not all radiation is harmful. It depends on the dose. Light is a form of radiation. All parts of the electromagnetic spectrum are considered radiation, but only X-rays and gamma rays are ionising radiation.

Myth! Ionising radiation is dangerous because it can penetrate body tissues and cause cell damage. Ultraviolet light from the Sun causes sunburn, which is a common form of “harmful” radiation.

Where does light come from?

What is a light wave? Light is a disturbance of electric and magnetic fields that travels in the form of a wave. Like all waves it can be described as having peaks, troughs, frequency, wavelength, amplitude and it transfers energy.

What is the electromagnetic spectrum?

The electromagnetic spectrum consists of all the different wavelengths of electromagnetic radiation, including light, radio waves, and X-rays. All travel at

Radio Waves The longest wavelength and lowest frequency. Radio waves are longer than 1 mm. Radio wavelengths are found everywhere: in the background radiation of the universe, in interstellar clouds, and in the cool remnants of supernova explosions.

Radio Waves Radio stations use radio wavelengths (10 cm – 1000 m) of electromagnetic radiation to send signals that our radios then translate into sound. These wavelengths are typically 1 m long in the FM band.

Radio Waves Radio stations transmit em radiation, not sound. The radio station encodes a pattern on the em radiation it transmits, and then our radios receive the em radiation, decode the pattern and translate the pattern into sound.

Microwaves 0.1 – 10 cm Basis of almost all space communications.

Infrared 0.1 cm to 0.00007 cm Heat!

Infrared wavelengths are about same size as a single bacteria. Infrared 0.1 cm to 0.00007 cm Infrared wavelengths are about same size as a single bacteria. Heat! http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/irgallery/assets/ess05_int_irgallery/ess05_int_irgallery_swf.html

Visible Light Visible light covers the range of wavelengths from 400 to 700 nm. Our eyes are sensitive only to this small portion of the electromagnetic spectrum.

Visible Light The Sun emits most of its radiation in the visible range, which our eyes perceive as the colours of the rainbow.

Ultraviolet Ultraviolet radiation has wavelengths of 10 to 310 nm (about the size of a virus). Young, hot stars produce a lot of ultraviolet light and bathe interstellar space with this energetic light.

Ultraviolet

X-rays X-rays range in wavelength from 0.01 to 10 nm (about the size of an atom). They are generated, for example, by super heated gas from exploding stars and quasars, where temperatures are near a million to ten million degrees.

Gamma rays Gamma rays have the shortest wavelengths, of less than 0.01 nm (about the size of an atomic nucleus). This is the highest frequency and most energetic region of the em spectrum. Gamma rays can result from nuclear reactions taking place in objects such as pulsars, quasars, and black holes.

Rabbits Radio Mambo Microwaves In Infrared Very Visible Light Unusual Low frequency Long wavelength Rabbits Mambo In Very Unusual eXpensive Gardens Radio Microwaves Infrared Visible Light UV X-rays Gamma Rays High frequency Short wavelength All e-m waves travel at the speed of light!

Transverse Waves In this type of wave, particles vibrate at right angles to the direction of motion of the wave.

Transverse Waves Light, and all forms of electromagnetic radiation, are transverse waves. http://www.acoustics.salford.ac.uk/schools/index1.htm

What have you learned today?

speed of sound in air, using the and speed. Key words: speed of sound, distance, speed, time By the end of this lesson you will be able to: Describe a method of measuring the speed of sound in air, using the relationship between distance, time and speed. describe how sound is produced describe sound as a wave which transfers energy, explaining what is meant by the frequency of a sound

Key words: frequency, wavelength, speed, amplitude, period By the end of this lesson you will be able to: Carry out calculations involving the relationship between distance, time and speed in problems on water waves, sound waves, radio waves and light waves. between speed, wavelength and frequency for waves.

What do we know about sound?

Sound How does sound travel? How fast does sound travel?

Sound Vibrations The aim of this activity is to show how sound energy is produced. Think! How is sound produced by the tuning fork? How does changing the length of the tuning fork affect the sound produced? Look at the frequencies of the tuning forks. As the frequency increases, how does the pitch change?

Energy Transfer The aim of this activity is to show how sound energy is transferred. Think! What is happening to the candle flame? What is moving the flame? Can you explain how sound energy is transferred? Signal generator Loudspeaker

How fast does sound travel? We can use sound switches and an electronic timer to measure the speed of sound. What two things do we need to measure to find the speed of sound? the distance travelled. the time taken.

How fast does sound travel? Why is an electronic timer used when measuring the speed of sound?

Measuring Average Speed Measure the time taken for sound to travel from one microphone to the other. Repeat your measurements to improve reliability.

microphone Electronic timer metre stick metal plate hammer

Oscilloscope Patterns and Frequency Think! How is sound produced by the loudspeaker? At very low frequency, what do you observe? As frequency increases, what happens to the pitch of the note? What is the lowest frequency you can hear? And the highest?

Octaves and Frequency Think! Look at the signals produced for different instruments and different notes. As the pitch increases, what do you notice about the number of waves on the screen? What is happening to the frequency? Look at two notes an octave apart. What happens to the number of waves on the screen. What does this tell you about frequency?

Oscilloscope Patterns and Loudness Think! As the sound becomes louder, what do you expect to happen to the oscilloscope trace? Does loudness affect the frequency of the signal?

What can sound travel through? What do we need for sound to travel? A solid, liquid or a gas. Sound can’t travel through space because it is a vacuum.

What can sound travel through? When the air was pumped out of the container…. it was no longer possible to hear the bell ringing. Virtual Int 1 Sound & Music -> Using Sound -> Vacuum

Longitudinal Waves In this type of wave, particles vibrate back and forwards along the direction the wave is travelling.

Longitudinal Waves A sound wave is a longitudinal wave. http://www.acoustics.salford.ac.uk/schools/index1.htm

Loudness of Sound We measure the loudness of sound in decibels – dB. Spending too long listening to loud sounds can permanently damage your hearing. http://www.tlc-direct.co.uk/Technical/Sounds/Decibles.htm http://www.tlc-direct.co.uk/Technical/Sounds/decible4.swf

Hearing Damage in the Workplace If you are regularly exposed to noise at 80dB or above, your employer must carry out risk assessment. If 85dB or above, they must provide protection. http://www.hse.gov.uk/noise/demonstration.htm

Max recommended length of exposure 85 dB 8 hours 88 dB 4 hours 97 dB 30 minutes 100 dB 15 minutes 103 dB 7.75 minutes 106 dB 3.75 minutes How big is 1dB sound file http://www.phys.unsw.edu.au/jw/dBNoFlash.html Date Accessed 27/03/2008

Nightclub at 106dB 4 minutes unprotected 2 hours protected Gunshot 140dB never unprotected 1.5 minutes protected

Is your music too loud? “Preliminary data on iPods and similar devices have found lower maximum levels - above 100 decibels (the noise volume of a chainsaw; risk of hearing damage after two hours), but not higher than 115 decibels (a football game in a loud stadium; risk of hearing damage after 15 minutes)…To fully understand the potential impact of these devices, it is important to knowthat the sound is travelling a tiny distance from your earbud to your eardrum rather than being diffused in a football stadium or concert arena.” http://www.hearingconservation.org/docs/RealityCheckonMusicNIHL.pdf Extract from article by Gregory Mott, Washington Post. Date Accessed 26/03/2008

Ultrasound What is the normal range of human hearing? 20 Hz – 20000 Hz (or 20 kHz). What is ultrasound? Ultrasound is sound above 20000 Hz.

Imaging with Ultrasound Medical ultrasound used for imaging a foetus is between 1 and 20 MHz (10000000 and 20000000 Hz!) How is ultrasound used for imaging? http://www.layyous.com/ultasound/ultrasound_video.htm

Imaging with Ultrasound http://www.mayoclinic.com/health/ultrasound/MM00084 A handheld transducer is held on the stomach. This changes electrical energy to sound energy which is transmitted in waves into the body.

Imaging with Ultrasound The waves are reflected off the boundaries between materials which have different sound properties (e.g. the amniotic fluid and the foetus). The transducer also acts as a receiver – detecting the echo and turning sound into an electrical signal.

Imaging with Ultrasound By recording the time taken for the echo to be detected, and the intensities of the echoes, and using the speed of sound in tissue, a computer can build up a detailed image of the foetus.

Imaging with Ultrasound The skin is covered in jelly during the ultrasound procedure. Why is this? Good contact is important. Otherwise the ultrasound waves will simply bounce off the outer skin and no image will be obtained. It makes movement of the transducer easier. It ensures a clear picture is obtained.

Imaging with Ultrasound Why is ultrasound used instead of X-rays? Ultrasound causes no harmful effects to the foetus. It is low power so can be used safely to image the foetus.

Other Uses of Ultrasound Doppler ultrasound is a newer technique which can be used to measure blood flow. Ultrasound can also be used to break up kidney stones – faster and safer than surgical removal.

SOund NAvigation and Ranging Using Sonar SOund NAvigation and Ranging Used for underwater ranging – first patented in 1913 – prompted by Titanic disaster.

Using Sonar How does Sonar work? What is it used for? Sonar Use

Radio Communication What are radio (and TV) waves? They are a type of electromagnetic radiation that travel through air at 3 x 108 m/s.

wavelength – remember the Each radio station broadcasts with a different frequency or wavelength – remember the speed is always the same. The frequency of radio waves is much higher than the frequency of sound waves.

Imagine a wave where 2 waves pass a point in 1 second. frequency f = 2 Hz. amplitude distance (m) 0.1 0.2 10

speed What is the length of the wave produced in this time? 2 waves in 1 second -> 2 wavelengths. 2 wavelengths = 2 x 0.1 = 0.2 m So two waves are produced in 1 s. That is 0.2 m in 1 s. This links distance and time which gives us….. amplitude speed 10 0.1 0.2 distance (m)

Calculating Wave Speed There are two formulae which can be used to calculate wave speed. Speed is a distance in a given time.

Calculating Wave Speed speed = frequency x wavelength v = fλ speed in m/s wavelength in metres frequency in Hz

How fast is a radio or TV wave? Radio and TV signals are part of the electromagnetic spectrum. This means they travel at the speed of light which is 3 x 108 m/s.

of reflection and normal when a ray of Key words: reflection, angle of incidence, angle of reflection, light rays By the end of this lesson you will be able to: State that light can be reflected. Use the terms angle of incidence, angle of reflection and normal when a ray of light is reflected from a plane mirror. State the principle of reversibility of a ray path.

Reflection of Light reflection? How do they compare? Watch the demonstration with the laser optics kit. What is meant by the normal? What is meant by the angle of incidence? What is meant by the angle of reflection? What do you notice about the angles of incidence and reflection? How do they compare? Virtual Int 2 Physics -> Waves & Optics -> Reflection -> Reflection from a Plane Mirror

The normal, angle of incidence and angle of reflection Complete the sentences: The normal is The angle of incidence is The angle of reflection is

Reflection of Light Law of Reflection The normal is at a right angle (90º) to the surface. Law of Reflection angle of incidence = angle of reflection Θi ΘR Flash animation - reflection

Reversibility of Rays Law of Reflection The normal is at a right angle (90º) to the surface. Law of Reflection angle of incidence = angle of reflection N I R ΘR Θi Flash animation - reflection

What have you learned?

of refraction and normal. direction as light passes from air to glass Key words: refraction, incidence, normal By the end of this lesson you will be able to: State what is meant by the refraction of light. Use the terms angle of incidence, angle of refraction and normal. Draw diagrams to show the change of direction as light passes from air to glass and glass to air.

Refraction http://www.planet-scicast.com/view_clip.cfm?cit_id=2728

Rays of Light In any material (air, glass, water) light will travel in a straight line. We represent light using ray diagrams.

Drawing Light Rays We represent light using ray diagrams. What two things must you remember when drawing light rays?

Refraction of Light speed can cause it to change direction. We saw that light can be reflected from a flat surface. When light passes from one material to another it changes speed. This change in speed can cause it to change direction. We call this refraction.

Refraction of Light – Air to Glass Watch the demonstration with the laser optics kit. What is meant by the normal? What is meant by the angle of incidence? What is meant by the angle of refraction? What do you notice about the angles of incidence and refraction? How do they compare? Virtual Int 2 Physics -> Waves & Optics -> Refraction through blocks and prisms

Activity – Air to Glass incidence and refraction as light By the end of this practical session you will know: how to draw a normal how to identify, and measure, angles of incidence and refraction as light passes from air to glass. what is meant by refraction.

Place the block on the outline provided

Shine the light ray along the line

Accurately mark the path of the refracted ray Then switch off the light source and remove the block

Remove the block and draw in the refracted ray

Carefully draw in the normal

Mark the angles of incidence (i) and refraction (r) Use a protractor to measure the angles of incidence (i) and refraction (r)

Record your results in the space provided Use a protractor to measure the angles of incidence (i) and refraction (r)

Think! incidence? What happens when the ray of light shines into the block along the normal? Is refraction taking place? What is meant by the angle of incidence?

Think! refraction? incidence is greater than 0o? What is meant by the angle of refraction? What happens when the angle of incidence is greater than 0o?

Refraction: Air to Glass What happens when the ray of light enters the block along the normal? There is no change in direction of the light i.e. it goes straight through. The light is refracted – it changes speed. What do we mean by angle of incidence? The angle of incidence is the angle between the incoming ray and the normal.

Refraction: Air to Glass What do we mean by the angle of refraction? The angle of refraction is the angle between the refracted ray and the normal. What happens when the angle of incidence is greater than 0o? When the angle of incidence is greater than 0o the light changes direction due to refraction.

r is less than i refracted ray angle of refraction r angle of incidence i normal incident (or incoming) ray r is less than i

Refraction Light travels in a straight line. When light travels from one material (e.g. air) to another (e.g. glass) it changes speed. This is called refraction. This change in speed sometimes causes it to change direction. Refraction is the change in speed of light as it passes from one medium to another.

Refraction of Light – Glass to Air Watch the demonstration with the laser optics kit. What is meant by the normal? What is meant by the angle of incidence? What is meant by the angle of refraction? What do you notice about the angles of incidence and refraction? How do they compare? Virtual Int 2 Physics -> Waves & Optics -> Refraction through blocks and prisms

Activity – Glass to Air incidence and refraction as light By the end of this practical session you will know: how to draw a normal how to identify, and measure, angles of incidence and refraction as light passes from air to glass. what is meant by refraction.

Place the block on the outline provided

Shine the light ray along the line

Accurately draw in the path of the refracted ray Then switch off the light source and remove the block

Accurately draw in the path of the refracted ray Then switch off the light source and remove the block

Carefully draw in the normal

r i Measure the ANGLE OF INCIDENCE, i Measure the ANGLE OF REFRACTION, r

Record your results in the space provided

0o 15o 35o 42o 75o Refraction – Glass to Air Angle of Incidence (i) Refraction (r) 0o (along the normal) 15o 35o 42o 75o Ray Box

Think! incidence? What happens when the ray of light shines into the block along the normal? Is refraction taking place? What is meant by the angle of incidence?

Think! refraction? incidence is greater than 0o? What do What is meant by the angle of refraction? What happens when the angle of incidence is greater than 0o? What do you notice as you increase the angle of incidence?

Refraction – Glass to Air What happens when the ray of light enters the block along the normal? There is no change in direction of the light i.e. it goes straight through. What do you notice as you increase the angle of incidence? The angle of refraction increases. Some reflection can also be seen.

Refraction – Glass to Air What is the relationship between the angle of incidence and the angle of refraction? The angle of refraction is always larger than the angle of incidence. What happens when the angle of incidence is 42o or above? Total internal reflection occurs and no light escapes.

Blue light refracts more than red light

Why does refraction occur? Virtual Physics Animations -> Refraction of Waves

Why do we need to know about refraction? Refraction explains: how the eye focuses light; sight defects such as long and short sight and how to correct them using contact lenses or glasses.

All lenses refract light All lenses refract light. Lenses are used in cameras, telescopes and binoculars.

Total Internal Reflection and the Amazing Disappearing Coin

transmission system. Key words: total internal reflection, critical angle, optical fibres By the end of this lesson you will be able to: Explain, with the aid of a diagram, what is meant by total internal reflection. by the “critical angle”. Describe the principle of an optical fibre transmission system.

Think! What happens when light passes from one material to another? glass to air? How does the angle of incidence compare with the angle of refraction?

Critical Angle This is called the critical angle. Flash animation We’ve seen that as light passes from glass to air, it changes direction away from the normal. When light passes from glass to air there is an angle beyond which light cannot escape from the glass. This is called the critical angle.

What happens at the critical angle?

What happens at the critical angle? At the critical angle, the angle of refraction is 90°. Beyond the critical angle, Total Internal Reflection (TIR) occurs. When TIR occurs the angle of incidence = angle of reflection.

The normal, angle of incidence and angle of reflection Complete the sentences to give definitions of the normal, and angles of incidence and reflection. The normal is The angle of incidence is The angle of reflection is

Reflection of Light Law of Reflection The normal is at a right angle (90º) to the surface. Law of Reflection angle of incidence = angle of reflection Θi ΘR Flash animation - reflection

Reversibility of Rays Law of Reflection The normal is at a right angle (90º) to the surface. Law of Reflection angle of incidence = angle of reflection N I R ΘR Θi Flash animation - reflection

Total internal reflection is used in fibre optics which are used in: medicine ; cable television ; internet ; telephone access.

Optical Fibres Optical fibres are about the thickness of a human hair. Each fibre is a thin piece of glass, coated with a thin layer (or cladding) of another glass.

Using Optical Fibres in Communication Systems Optical fibre – sound transmitter and receiver. What are the steps needed to use optical fibres in a communication system? For example in a telephone system?

Transmission The sound signal must be changed into an electrical signal. The signal is a series of electrical pulses. What input device is used to convert sound to an electrical signal? The electrical signal is converted into pulses of light which are transmitted through the optical fibre via total internal reflection.

Receiving The light signal is changed back into an electrical signal using a photodiode. The electrical signal is converted into sound. What output device is used to convert an electrical signal to sound?

What are the advantages of using optical fibres in communications?

The signal that passes along the fibre is not electrical so less likely to suffer from interference. Cheaper to make than copper. Can carry far more information than copper wires. (One optical fibre cable could take all the telephone calls being made in the world at any time!)

There is less loss of signal – so not as many amplifiers required. Disadvantage – can be more difficult to join fibres than copper wires.

Using Fibre Optics in Medicine When light is produced using a normal electric light bulb, what energy transformations take place? Electric energy -> Light energy Electric energy -> Heat energy In an ordinary light bulb, the filament heats to about 2500oC – the bulb is very hot!

You can use fibre optics to provide a “cold” light source inside the body that means the light travels but it doesn’t get hot!

End probe containing coherent bundle, incoherent bundle, lens and surgical instruments Controls Eyepiece Light injected here ENDOSCOPE This is an endoscope image of the inside of the throat. The arrows point to the vocal chords

A COHERENT BUNDLE: A bundle of optical fibres in which the optical fibres are neatly arranged relative to one another. Such a bundle are used for the transmission of images. A NON-COHERENT FIBRE bundle, as you would expect, does not have this neat layout since they need only transmit light for illumination purposes. They are cheaper to produce.

Endoscopes

Light travels along both of these bundles by TOTAL INTERNAL REFLECTION An endoscope consists of two bundles of optical fibres: the LIGHT GUIDE and the IMAGE GUIDE The purpose of the LIGHT GUIDE is to send light (but not heat!) INTO the patient The IMAGE GUIDE brings light back, allowing the doctor to see inside the body Light travels along both of these bundles by TOTAL INTERNAL REFLECTION

reflectors used in telecommunication. Key words: curved reflectors, received signals, transmitted signals By the end of this lesson you will be able to: Explain the action of curved reflectors on certain received signals. certain transmitted signals. Describe an application of curved reflectors used in telecommunication.

Curved Reflectors Watch the demonstration. What do you notice about the light when a curved reflector is used? Where do we make use of curved reflectors?

Dish Aerials A dish aerial can be used either to receive or to transmit a radio (or microwave) signal. Virtual Int 1 Physics -> Telecommunications -> Satellites -> Curved reflectors

receiver receiving dish aerial The receiving dish aerial gathers in parallel rays from a distant object and reflects the rays to one point called the focus. The receiving aerial is placed at the focus to receive the strongest signal.

If we tilt the dish… receiver receiver receiving dish aerial

The transmitting aerial allows a strong transmitter transmitting dish aerial The transmitting aerial allows a strong (concentrated) signal to be sent in a particular direction from the transmitting dish aerial.

transmitter transmitting dish aerial

Dish Aerials What is the advantage of using a dish with a larger area? The dish collects more of the incoming beam and focuses it – resulting in a stronger signal. Satellite TV in Scotland requires a bigger dish than in England!

Dish Aerials A dish aerial can be used either to receive or to transmit a radio (or microwave) signal.

Tasks & Homework for … YPQ2 3.23, 3.28 McCormick & Baillie Int 2 Physics p133 qu 2, p145-146 qu 1, 2 YPQ2 3.25, 3.35 p133 qu 1

What have you learned?

diverging lenses on parallel rays of light. Key words: converging, diverging, lenses, parallel light rays, power, focal length By the end of this lesson you will be able to: Describe the shape of converging and diverging lenses. Describe the effect of converging and diverging lenses on parallel rays of light. Carry out calculations involving the relationship between power and focal length of a lens.

Lenses Lenses refract light. A convex lens is one which is thicker in the middle than at its edge. It bulges out.

Lenses When we shine parallel rays through a convex lens… normal Convex lenses bring parallel light rays to a focus. They are converging lenses.

Lenses The focal length of the lens is the distance between the centre of the lens and the focus. normal Convex lenses bring parallel light rays to a focus. They are converging lenses.

When parallel light rays enter a CONVEX lens they are CONVERGED (focused) The rays meet at a point called the PRINCIPAL FOCUS (‘F’ in the diagram)

The distance from the centre of the lens to the principal focus is called the FOCAL LENGTH (f)

Lenses A concave lens is one which is thinner in the middle than at its edge. It “caves” in.

Lenses When we shine parallel rays through a concave lens… normal Concave lenses cause parallel light rays to diverge.

Measuring the Focal Length of a Concave Lens The focal length of a lens is the distance from the centre of the lens to where the rays come together. But a concave lens causes the rays to diverge!

Diverging lenses have a negative focal length. Focal length f

When light rays enter a CONCAVE lens they are DIVERGED (spread) This time the principal focus is BEHIND the lens

As before, the distance to the principal focus is the FOCAL LENGTH Because the focal length is in the opposite direction, we give it a NEGATIVE value

Summary (so far) Type of lens What it does Focal length Convex CONVERGES light (brings the rays together) Positive Concave DIVERGES light (spreads the rays) Negative

Thick and Thin Lenses Can you predict which lenses will be Observe the demonstration with the lenses. Can you predict which lenses will be converging? And which will be diverging? Is there a relationship between the thickness of the lens and the focal length?

Measuring the Focal Length of a Converging Lens By the end of this practical session you will be able to measure the focal length of a converging lens.

Measuring the Focal Length of a Converging Lens Step 1: Identify a distant light source. Hold the lens between the light source and screen (piece of paper). Step 2: Move the lens back and forward until a focused image appears on the screen. Step 3: Measure the distance between the centre of the lens and the screen on which the focused image appears – this is the focal length.

Measuring the Focal Length of a Converging Lens Note your results. Why must a distant light source be used? Which lens had the shortest focal length? Which lens had the longest focal length? Which lens is the weakest? Which lens is the strongest? Could you predict this without making measurements?

Compare these 2 convex lenses:

The thicker lens has a shorter focal length

The thicker lens “bends” the light more, so we say it has a greater POWER than the thin lens f f

Power of Lenses A more powerful lens causes more refraction. The power of a lens is measured in dioptres.

To calculate a lens’ power, use this equation: POWER OF A LENS To calculate a lens’ power, use this equation: For short: METRES (m) DIOPTRES (D)

Power of Lenses Converging lenses have a positive focal length and a positive power. Diverging lenses have a negative focal length and a negative power.

Summary Type of lens What it does Focal length Power Convex CONVERGES light (brings the rays together) Positive Concave DIVERGES light (spreads the rays) Negative

A lens has a focal length of 20 cm. Find its power. What do I know? f = 20 cm = 0.2 m What type of lens is this? How do you know?

A lens has a power of -12D. What type of lens is this? Diverging / concave – since the power is negative.

A concave lens has a power of -12D. Find its focal length. What do I know? f = ? P = -12 D

Focal Length and Power Questions For each of the lenses below calculate the focal length or power and state whether the lens is convex (converging) or concave (diverging). Find the focal length of 1-9 Find the power of 10-18 1 Lens power +10D 2 Lens power –10D 3 Lens power +100D 4 Lens power -24D 5 Lens power 3D 6 Lens power 16D 7 Lens power 24D 8 Lens power -32D 9 Lens power -1.33D 10 Focal length 0.5 m 11 Focal length -50cm 12 Focal length 20cm 13 Focal length 0.2cm 14 Focal length 0.4cm 15 Focal length -0.25cm 16 Focal length -0.10cm 17 Focal length 30cm 18 Focal length -15cm Which is the most powerful lens? Which has the longest focal length?

lens forms the image of an object placed at Key words: converging, diverging, lenses, retina, long sight, short sight, ray diagram, image formation, real, virtual, magnified, diminished, inverted By the end of this lesson you will be able to: Draw a ray diagram to show how a converging lens forms the image of an object placed at different distances in front of the lens. Describe the focusing of light on the retina of the eye. State the meaning of long and short sight. Explain the use of lenses to correct long and short sight.

Creating Images with Lenses Two important rules when dealing with lenses. 1. If the light ray enters the centre of the lens then is passes straight through.

Creating Images with Lenses 2. If the light ray enters the lens travelling horizontally, it is refracted through the focal point.

Source of light relatively distant from the lens Virtual Int 2 Physics – Waves and Optics

Source of light relatively distant from the lens We see a real image is formed. The image is smaller than the object (it is diminished), and is inverted. A real image is one which can be formed on a screen.

Source of light more than two focal lengths from the lens We see a real image is formed. The image is smaller than the object (it is diminished), and is inverted. A real image is one which can be formed on a screen.

Source of light between one and two focal lengths from the lens We see a real image is formed. The image is larger than the object (it is magnified), and is inverted. A real image is one which can be formed on a screen.

Source of light between less than one focal length from the lens The rays do not meet.

We trace the rays back to find where the rays would meet.

F F F The image is virtual. It is an illusion of the eye and brain – but you could not place a screen here and see an image

The image is upright and magnified.

Source of light closer to the lens than its focal length The rays do not meet beyond the lens (the rays are diverging). When the rays are projected back they do meet. Since light does not actually pass through this point, it is a virtual image. The human eye will bring the light to focus and the virtual (magnified) image seen. This is the image we see with a magnifying glass.

Ray Diagrams Virtual Int 2 Physics -> Waves & Optics -> Lenses -> Ray Diagrams & More Ray Diagrams

Magnification of a Lens An image formed by a lens is either magnified (bigger) or diminished (smaller).

Summary Objects more than two focal lengths: The image is real, inverted and diminished. Objects between one and two focal lengths: The image is real, inverted and magnified. Object less than one focal length: The image is virtual, upright and magnified.

The Human Eye Light enters the front of the eye at the cornea. It is transparent, and it is here that most of the refraction occurs.

The Human Eye 4 1 2 3

the iris 4 1 2 3 1 controls the amount of light which enters the eye It is called the iris 1 2 3 4

the pupil 4 1 2 3 In bright light will the pupil be large or small? 2 is the hole in the middle of the iris. It allows light to enter the eye. It is called the pupil 1 2 3 4 In bright light will the pupil be large or small? It will be smaller to prevent too much light from getting in to the eye.

1 2 3 4 Along with the cornea, 3 focuses the light. It is called the jelly lens How does the eye lens focus on objects at different distances? Muscles around the lens make it thicker or thinner.

1 2 3 4 4 is where the “light picture” is built up. It is called the retina The retina contains nerve cells which are affected by light and send signals to the brain along the optic nerve.

1 2 3 4 4 is where the “light picture” is built up. It is called the retina To see objects clearly, light must be focused on the retina.

Virtual Int 2 Physics -> Waves and Optics -> Applications of Lenses -> The Eye

Remember! Most of the refraction occurs in the cornea – not the lens!

Long and short sight Common Sight Defects are caused by the failure of the eye’s lens to bring the light rays to a focus on the retina.

Long and Short Sight Observe the demonstration of long and short Where do the rays meet when a person is long sighted? What type of lens can be used to correct this? Where do the rays meet when a person is short

Long Sight A person with long sight can see far away objects clearly. Objects close up appear blurred. Long sight occurs when the light rays are focused “beyond” the retina.

Long Sight Remember that rays do not actually pass beyond the back of the eye! Long sight can occur because the eyeball is shorter than normal from front to back. It can also be caused by the ciliary muscles not relaxing to make the lens fat enough.

Long Sight Long sight can occur because the eyeball is shorter than normal from front to back. It can also be caused by the ciliary muscles not relaxing to make the lens fat enough. Remember the fatter the lens, the more powerful and the more quickly rays are brought to a focus.

Long Sight A person with long sight can see far away objects clearly. The rays are parallel and can be brought to a focus on the retina. Objects close up appear blurred. The rays coming from a close up object are diverging and the eye does not bring them to a focus on the retina.

Long Sight A convex lens converges the light rays before the cornea/lens and allows the eye to focus the light on the retina rather than behind the retina.

Short Sight A person with short sight can see close objects clearly. Objects far away appear blurred. Short sight occurs when the light rays are focused in front the retina.

Short Sight Short sight can occur because the lens in the eye is very curved. It can also be caused by the ciliary muscles not making the lens thin enough.

Short Sight A person with short sight can see close up objects clearly. The rays are diverging and can be focused on the retina. Distant objects appear blurred. The rays coming from a distance object are parallel and the eye brings them to a focus too soon.

Short Sight A concave lens diverges the light rays before the cornea/lens and allows the eye to focus the light on the retina rather than in front of the retina.