# Unit 3: Light and Optical Instruments J. Pulickeel January 2007 Lesson 2: Reflecting on Reflections Reflections and Image Formation.

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Unit 3: Light and Optical Instruments J. Pulickeel January 2007 Lesson 2: Reflecting on Reflections Reflections and Image Formation

Recall... The wavelength (λ) is the distance (m) between successive wave particles that are in phase Frequency (f)is the number of cycles per second (1/s or Hz). Period (T) is the length of time to complete one cycle (s). An example is it takes 31557600 s for the earth to orbit the sun. Its period (1 cycle) is 31557600s

The Electromagnetic Family Light is part of the electromagnetic family. You get an electromagnetic wave when you shake an electron, thus shaking the magnetic field around it. This is similar to shaking a rope (rope waves) or air (sound waves) If you shake an electron faster, you increase its energy and its frequency

Electromagnetic Spectrum Our eyes can sense or wavelength between 400 nm < λ < 700nm or frequencies of 10 14 Hz. The speed of light (c) is 3.00 x 10 8 m/s As the wavelength increases, the frequency decreases (λ α 1 / f ) Some animals can see above and below this spectrum. Some cameras operate in the infrared to take pictures in dark rooms. Wavelength λ (m) Approximate Wavelength Scale Frequency (Hz) Visible Light Spectrum 400 nm 700 nm

Electromagnetic Spectrum Why do things glow? When we heat things up we are shaking the particles. Because the particles are shaking, the electrons are shaking. Electrons are smaller, so they shake faster. If the electrons shake fast enough (at 10 14 Hz), they will glow. This is how the incandescent light bulb works

Question What is the wavelength of electromagnetic radiation that has frequency of 4.62 x 10 14 Hz? Would this be visible light? Click to see answer

Travelling Waves Luminous vs. Non Luminous Luminous objects produce their own energy Non luminous objects do not produce their own energy Luminous Non Luminous

Travelling Waves Reflecting on Reflections Reflections occur when light bounces off a surface. When a room is poorly lit you see less light bounces off the surface

Not all light is reflected – some light is absorbed Dark surface tend to absorb most of the light that strikes them Very little light is reflected from dark clothes. Travelling Waves Reflecting on Reflections

When light strikes a reflecting surface, it bounces off. This ray is called an incident ray Reflecting surface Incident ray Reflected ray Travelling Waves When Light is Reflected

Calculating the Reflected Ray Reflecting surface Incident ray normal Angle of Incident i Angle of Reflection r Reflected ray

Calculating the Reflected Ray Reflecting surface Incident ray normal Angle of Incident i Angle of Reflection r Reflected ray 1.Draw a normal line perpendicular (90°) to the plane 2.Measure the angle of incidence, using a protractor 3.Draw the angle of reflection on the opposite side of the normal. 4.Draw the reflected ray.

Two Laws of Reflection A Law in science is an action or condition that scientists are convinced will ALWAYS happen Laws of Reflection 1.The angle of reflection will ALWAYS equal the angle of incidence

Whats Normal? The normal is always 90° to the reflected surface!!!

Two Laws of Reflection A Law in science is an action or condition that scientists are convinced will ALWAYS happen Laws of Reflection 1.The angle of reflection will ALWAYS equal the angle of incidence 2.The incident ray, the normal and the reflected ray are all in the same plane

Law 2: Rays on A Plane Incident Snake A Plane is an imaginary flat surface (like a piece of paper). The angle of reflection, angle of incidence and the normal will all appear on this surface Im getting tired of these @#\$%! rays on this @#\$%! plane!

Eye can see clearly now… You realize that an object is in front of you because light is spreading out from that object.

Eye can see clearly now… You realize that an object is in front of you because light is spreading out from that object.

Eye can see clearly now… When light from an object is reflected off a mirror, it reflects right back to you. This makes it appear that the object is in front of us Sup yo?

Do you suppose that it would be easier to read from rough pages or glossy pages? Click to see answer... It is much easier to read from rough pages which provide for diffuse reflection. Glossy pages result in specular reflection and cause a glare. The reader typically sees an image of the light bulb which illuminates the page. If you think about, most magazines which use glossy pages are usually the type which people spend more time viewing pictures than they do reading articles.

Glare on rainy nights... The glare on rainy nights is the result of the specular reflection of the beam of light from an oncoming car. Normally a roadway would cause diffuse reflection due to its rough surface. But if the surface is wet, water can fill in the crevices and smooth out the surface. Rays of light from the beam of an oncoming car hit this smooth surface, undergo specular reflection and remain concentrated in a beam. The driver perceives an annoying glare caused by this concentrated beam of reflected light.

Drawing the virtual Image 1.Draw a line from the object and to the mirror 2.Extend the line an equal distance behind the mirror and draw the image I 2 3.Draw a light ray from the I 2 to the eyes. The ray exist on the image side. 4.The rays originated from the I 1. Draw the completed ray

Drawing the virtual Image with 2 mirrors I1I1 I2I2 I 1,2 Multiple images are formed when two mirrors are used. Draw Images I 1, I 2, and I 12, then trace the rays back as illustrated.

The Alien Diet Problem Aliens will eat each person if they can not guess what color hat they are wearing. Each person knows they are wearing either a red or blue hat. Alan will pick first and Jacob will pick last. Alan can see all the hats in front of him, but not his own. Ivan can see Jacobs hat. Jacob can not see anyone's hat. The aliens gave the people 1 night to come up with a plan. It is possible to guarantee that at least 9 people live! AlanBettyCamDaveElmerFredGeorgiaHamzaIvanJacob

Cam counts 3 Red Elmer counts 2 Red Dave counts 3 Red Fred counts 2Red Hamza counts 2Red Geogia counts 2 Red Ivan counts 1 Red The Alien Diet Problem The group decides that Alan will count the number of red hats. Alan is a census taker and has great experience counting things. He can see 9 hats. If the number red hats is odd, Alan will say red. If the number of red hats is even, Alan will say blue. Betty will count the number of red hats. If Alan said red, then Betty know will know Alan saw an odd number of red hats. If Betty counts an even number of red hats, then she must be wearing a red hat. If Betty counts an odd number, then she must have a blue hat. AlanBettyCamDaveElmerFredGeorgiaHamzaIvanJacob Alan counted the 4 red hats and said I am wearing a blue hat. Unfortunately, that was the last thing he ever said.... Betty counts 4 Red

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