Download EM Waves and Color

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 108
m/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
Check Your Understanding
Why is light slower while in transparent
materials, like water or glass, than it is
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.
Check Your Understanding
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.
Check Your Understanding
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
Check Your Understanding
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).
Check Your Understanding
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
Check Your Understanding
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.
Check Your Understanding
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.
Check Your Understanding
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.
Check Your Understanding
If you shine a red light on a white shirt,
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.
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
Check Your Understanding
What color do you get when green and
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.
Check Your Understanding
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.