# The Electromagnetic Spectrum and Light

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The Electromagnetic Spectrum and Light

Electromagnetic Waves
Electromagnetic waves- transverse waves consisting of changing electric and magnetic fields Carry ENERGY from place to place Different from mechanical waves…how?

How are EM Waves produced?
An electric field exerts forces on charged particles Electric fields are produced by charged particles changing magnetic fields A magnetic field produces magnetic forces Magnetic fields are produced by magnets changing electric fields vibrating charges

How do EM waves travel? As the two fields regenerate each other, their E travels in the form of a wave. EM waves DON’T need a medium!!! Can travel through a vacuum (=empty space) OR through matter EM radiation – the transfer of E by EM waves traveling through matter or across space.

What is faster…speed of sound or light?
Light travels much faster than sound. For example: Thunder and lightning start at the same time, but we will see the lightning first. 2) When a starting pistol is fired we see the smoke first and then hear the bang.

Speed of Light First accurate estimations were in 1926 when Albert Michelson completed his experiment in California.

Speed of Light c = 3.00 x 108 m/s Or 300,000,000 m/s
Light (and all EM waves) travel the same speed in a vacuum… c = 3.00 x 108 m/s Or 300,000,000 m/s (compared to ~ 340 m/s) That’s 8 times around the earth in 1 second!!!!

l and frequency EM waves vary in l and frequency c = l * frequency
Since the speed of light (c) is always x 108 m/s, you can always calculate l from frequency and vice versa.

Wave or Particle??? EM radiation sometimes behaves like a wave, and sometimes like a stream of particles. There is evidence for both theories... That’s called wave-particle duality In some experiments, the wave model works best. In other experiments, the particle model works best. Thus, we use both

Wave… Double Slit Experiment Interference is a property of waves!
Pass light through two slits and an interference pattern is observed Interference is a property of waves!

Particle… Photons- particles of light
Photoelectric Effect Light shown on a metal can cause electrons to be emitted from the metal Photons- particles of light The greater the frequency of an EM wave, the more E each of its photons has

Intensity Intensity- the rate at which a wave’s energy flows through a given unit of area…basically, it is brightness of light. As you leave the source of light, does intensity increase or decrease?

Farther from the source, the photons spread out over a larger area, and intensity decreases.

The Electromagnetic Spectrum

The Waves of the Spectrum
What happens when you put a prism in front of a window?

The EM Spectrum EM spectrum- the full range of frequencies of EM radiation How many different types of EM waves can you think of? EM Spectrum includes: Radio waves Infrared rays Visible light UV rays X-rays Gamma Rays

Television Shorter than radio, also used to carry messages (pictures & sound) to our TV sets. We can sense the TV waves around us with our televisions.

Microwave Emitted by: Detected by Gas clouds collapsing into stars
Microwave Ovens Radar Stations Cell Phones Detected by Microwave Telescopes Food (heated) Cell phones Radar (systems)

Infrared (Heat or Thermal) Are you a source of infrared? YES you are!
Emitted by Sun and stars (Near) TV Remote Controls Food Warming Lights (Thermal) *Everything at room temperature or above,=HEAT Detected by Infrared Cameras TVs, VCRs, Your skin

Visible Each color is a different size wave
Visible Each color is a different size wave. Red the longest & violet the shortest Emitted by The sun and other astronomical objects Laser pointers Light bulbs Detected by Cameras Human eyes Plants (red light) Telescopes RoyG.Biv

Ultraviolet Sunburn / black light
He can get skin cancer! Emitted by Tanning booths (A) The sun (A) Black light bulbs (B) UV lamps Detected by Space based UV detectors UV Cameras Flying insects (flies)

X-ray Emitted by Detected by Astronomical objects X-ray machines
CAT scan machines Radioactive minerals Airport luggage scanners Detected by Space based X-ray detectors X-ray film

Gamma Ray (Short electromagnetic waves but more energetic)
Emitted by Radioactive materials Exploding nuclear weapons Solar flares Detected by Geiger counters Gamma detectors and astronomical satellites Medical imaging detectors

COSMIC Rays (The highest energy waves and the deadliest)
Cosmic rays come from deep space and can pass through the Earth.

Behavior of Light

Light and Materials When you are looking at things, anything, what you are really seeing is light. We can’t see through walls because light doesn’t pass through walls.

We see things because they reflect light into our eyes:
Homework

Light and Materials, ctd.
How light behaves when it strikes an object depends on many factors, including the material the object is made of Materials can be translucent, transparent, or opaque.

Transparent objects- Translucent- Opaque- No scattering
Color transmitted is color you see and all other colors are absorbed Translucent- Light is scattered and transmitted some Opaque- Light is either totally reflected or absorbed Color of opaque objects is color it reflects

Interactions of Light When light strikes a new medium, the light can be reflected, absorbed, or transmitted. When light is transmitted, it can be refracted, polarized, or scattered.

Reflection Mirror Reflection from a mirror: Normal Angle of incidence
Incident ray Reflected ray Angle of incidence Angle of reflection Mirror

Angle of incidence = Angle of reflection
The Law of Reflection Angle of incidence = Angle of reflection The same !!!

Clear vs. Diffuse Reflection
Smooth, shiny surfaces have a specular reflection: Rough, dull surfaces have a diffuse reflection Diffuse reflection is when light is scattered in different directions

Using mirrors Two examples: 2) A car headlight 1) A periscope

Refraction The bending of light waves as they pass from one medium to another Results in mirages, which are false or distorted images

Inferior Mirages: formed when the air near the ground is very warm compared to the air just above it.

Polarized Polarized light is light that all vibrate in the same plane (or direction!)

Scattering Light is redirected as it passes through a medium.
This is responsible for our red sunsets!

Color

We can demonstrate this by splitting white light with a prism
White light- not a single color; it is made up of a mixture of the seven colors of the rainbow We can demonstrate this by splitting white light with a prism This is how rainbows are formed: sunlight is “split up” by raindrops .

Red Orange Yellow Green Blue Indigo Violet

Adding Colors White light can be split up to make separate colors. These colours can be added together again The primary colors of light are red, blue and green: Adding blue and red makes magenta (purple) Adding blue and green makes cyan (light blue) Adding red and green makes yellow Adding all three makes white again

Only red light is reflected
Seeing Color The color an object appears depends on the colors of light it reflects For example, a red book only reflects red light: Homework White light Only red light is reflected

A white hat would reflect all seven colors:
A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue): Purple light A white hat would reflect all seven colors: White light

Testing Color Blind Tests

Optical Illusions

Which circle in the middle is bigger?

Do you see gray areas in between the squares
Do you see gray areas in between the squares? Now where did they come from?

If you continue to focus on the sign in the centre of the image you will notice that the circle of violet circles will soon disappear completely, and you will see only the green spot (which is actually violet)

LIGHT & ITS USES: Lenses
Convex lenses Thicker in the center than edges. Lens that converges (brings together) light rays. Forms real images and virtual images depending on position of the object The images formed are upside down

LIGHT & ITS USES: Lenses
Focal Point Object Convex Lenses Ray Tracing Two rays usually define an image Ray #1: Light ray comes from top of object; travels parallel to optic axis; bends thru focal point. © 2000 D. L. Power Lens

LIGHT & ITS USES: Lenses
Ray #1 Convex Lenses Ray Tracing Two rays define an image Ray 2: Light ray comes from top of object & travels through center of lens. © 2000 D. L. Power Ray #2

LIGHT & ITS USES: Lenses
© 2000 D. L. Power Concave lenses – Lens that is thicker at the edges and thinner in the center. Diverges light rays All images are upright and reduced

Vision Retina Rods Cones The eye is a convex lens
Lens refracts light to converge on the retina and then nerves transmit the image Rods Nerve cells in the retina. Very sensitive to light & dark Cones Nerve cells help to see light/color

PARTS OF EYE CONTINUED…
Rods – responsible for black and white vision and detection of motion. Cones – Seeing in color and visual acuity. We have three types of cones: cones that see red, cones that see blue, and cones that see green.

How You See Near Sighted – Eyeball is too long and image focuses in front of the retina Far Sighted – Eyeball is too short so image is focused behind the retina.

LIGHT & USES: Optical Instruments
LASERS Holography – Use of Lasers to create 3-D images Fiber Optics – Light energy transferred through long, flexible fibers of glass/plastic Uses – Communications, medicine, t.v. transmission, data processing.

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