Fundamentals of Light

Light• Light is not only comprised of the light we

can see (visible light), but also other types of light waves, called EM waves

Electromagnetic Wave: a wave that is partly electrical and partly magnetic and carries energy. A light wave. EM waves are transverse waves EM waves do NOT require a medium to

transfer energy Ex: gamma rays, UV, infrared, radio waves

The Speed of Light

• The speed of light in a vacuum is 3 x 103 x 1088 m/sm/s. b/c the speed of light is constant in a vacuum,

it is symbolized with the letter “c” c = the speed of light in a vacuum = 3 x 108

m/s NOTHING travels faster than the speed of

light in a vacuum! ALL EM waves travel at this speed

• The speed of light is constant when it travels through one type of medium. Light travels faster in air than in water. Molecules in its way slows light down It is the opposite of sound waves, which

require a medium to move faster

• Light travels the fastest in a vacuum and slowest in a solid The more molecules in its way, the slower it

goes Vacuum > G > L > S

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Why is light slower while in transparent Why is light slower while in transparent materials, like water or glass, than it is materials, like water or glass, than it is in a vacuum?in a vacuum?

The light has to “fight” its way through the molecules of more dense materials. The more molecules in the way, the slower the light will travel.

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Will an EM wave speed up or slow down when going from air to water?

It will slow down.

Which type of medium will allow a sound wave to have the greatest KE?

In a solid (an elastic solid) b/c the molecules are closer together. Sound requires a medium to transfer the energy.

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Which type of medium will allow a light wave to have the greatest KE?

In a vacuum b/c there are no molecules to get in its way. Light waves do not require a medium to transfer energy.

Electromagnetic Spectrum• The range of EM waves is called the

electromagnetic spectrum, or EM spectrum.

Electromagnetic Spectrum: the range of electromagnetic waves extending from radio waves to gamma waves All EM waves are light waves All EM waves travel at the speed of light, 3 x

108 m/s

The EM Spectrum• All of the EM waves are the same in nature,

differ only in the wavelength and frequency

“Ronald McDonald Is Very Ugly X-tra Gross”

• Red light is the lowest frequency of light we can see.

• Violet light is the highest frequency light we can see.

• Infrared waves are lower in frequency than red light. – Heat lamps give off infrared radiation.

• Ultraviolet waves are higher in frequency than violet light. UV rays from the sun cause sunburn

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Which EM wave has the shortest wavelength?

Gamma rays

Which EM wave has the lowest frequency?

Radio waves. B/c they have the longest wavelength, they must have the lowest frequency (remember that frequency is inversely proportional to wavelength).

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Where does sound fit in the EM spectrum?Where does sound fit in the EM spectrum?

It doesn’t! Sound is a mechanical wave (a longitudinal wave), a vibration of material particles. EM waves are not vibrations of material, but are vibrations of pure energy.

Energy Levels of EM Spectrum

• As waves gain more energy, the frequency increases– The wavelength therefore decreases

• This is why UV, X-rays, and Gamma rays are can cause medical problems– They have higher frequencies, and therefore

carry more energy– The increased energy can damage our fragile

cells, sometimes causing cancer

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Which EM wave travels the fastest?None. They all travel at the same speed,

the speed of light!

Which EM wave has the most energy?Gamma rays. They have the highest

frequency because they have the most energy. This is the type of energy usually released during an atomic bomb.

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Are microwaves really dangerous to us?No! They have the second longest

wavelength of the EM waves (behind radio waves), so they do not carry enough energy to do much damage to our cells.

The Color Spectrum

• Visible light is the ONLY part of the EM spectrum we can see.

• Sunlight is a mixture of all the colors of the rainbow.

• Sunlight is an example of white light. – White is not a color, but a combination of all

colors.

• Black is also not a color, but is the absence of light. – An object appears black when all the

frequencies of light are absorbed.

Color by Reflection• Most materials absorb light of some

frequencies and reflect the rest. We see ONLY the reflected light.

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If you shine a red light on a green shirt, what color does it appear?

Black! Only green light can be reflected off of the green shirt; since only red light is being shown on the shirt, all of the light is absorbed.

If you shine a red light on a white shirt, If you shine a red light on a white shirt, what color will the shirt appear?what color will the shirt appear?

Red. White is the combination of all colors, so whatever color you shine on it, it will appear that color.

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Mixing Colored Light

• Light of all the visible frequencies mixed together produces white light.

• White light can also be produced by mixing red, blue, and green light. – Red, blue, and green are the 3 primary colors of

light.

• Color T.V.s produce all the colors we perceive by combining red, blue, and green light in a variety of ways.

Adding Primary Colors

• Red + Green = Yellow• Red + Blue = Magenta (Purple)• Blue+ Green = Cyan (Aqua)• Red + Blue + Green = White

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What color do you get when green and What color do you get when green and red light are mixed?red light are mixed?

Yellow! In a stoplight, this is how they used to make yellow light…by mixing the red light on top and the green light on the bottom to get yellow in the middle.

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What color do you get if you mix red, blue, and green?

White light! When you mix all of the primary colors together, you get white. An absence of all of the colors would be black.