Light
A. Electromagnetic Radiation (EMR)
1. The electromagnetic spectrum
a. Radiation is the transfer of energy by electromagnetic waves.
b. They do not need a medium to travel through.
c. They travel at different speeds in different media, but are the fastest waves in nature.
d. They have different frequencies and wavelengths that give them different properties.
e. The electromagnetic spectrum (from longest wavelength to shortest):
1) Radio waves
2) Infrared radiation
3) Visible light
4) Ultraviolet radiation
5) X Rays
6) Gamma rays
f. Photons are tiny packets of radiation of specific amounts of energy that give radiation
a particle-like behavior.
2. Radio waves
a. Longest wavelength, lowest frequency, and the smallest amount of energy.
b. Used for radio, television, cellular phones, cordless telephones, radio astronomy,
microwaves, RADAR.
c. Types of radio:
1) Shortwave (for very long ranges)
2) AM (amplitude modulation)
a) Can bend around large objects such as mountains.
b) AM can reach farther than FM, but is weaker.
c) Fades out in tunnels.
3) FM (frequency modulation)
a) Strong signals anywhere in its range.
b) Has a range of about 50-70 miles.
4) Television broadcast waves (FM for sound & AM for picture)
5) Microwaves
a) Cause water molecules to vibrate very fast and cook food throughout.
b) Used for RADAR (Radio Detection and Ranging).
3. Infrared radiation
a. Heat
b. Thermograms show how much infrared radiation is being given off by a living thing.
c. Used to see things in the dark.
d. Heat lamps use infrared radiation to heat food.
4. Visible light
a. The only part of the electromagnetic spectrum humans can see.
b. Visible spectrum of color (from longest wavelength to shortest):
1) Red
2) Orange
3) Yellow
4) Green
5) Blue
6) Indigo
7) Violet
c. White light is composed of all the colors.
d. Raindroplets act as little prisms to separate white light into a rainbow of colors.
5. Ultraviolet radiation
a. Causes us to make vitamin D in our skin.
b. Can be harmful.
c. We use UV to kill germs in food and sterilize medical equipment.
d. Some minerals glow under UV (used in detergent).
6. X rays
a. Their high energy make them pass through many materials.
b. We commonly use them for X-rays in medicine.
7. Gamma rays
a. The most energetic of all EMR.
b. Used in radiation therapy to kill cancer cells.
B. Light and Color
1. Types of materials:
a. Opaque materials absorb or reflect all light; you cannot see through them.
b. Transparent materials allow light to pass; you can see clearly through them.
c. Translucent materials allow only some light to pass through; you cannot see clearly
through them
2.
a.
b.
c.
Colors
We see color when materials reflect certain wavelengths of light.
The primary colors are red, green, and blue.
White light is produced when the 3 primary colors are shown together.
1) additive color
d. How we see color:
1) Light passes into our eyes and strikes the retina.
2) Cone cells detect the various wavelengths and send messages to the brain.
3. Pigments
a. Pigments are colored materials.
b. The primary pigments are yellow, magenta, and cyan.
c. Any other colors can be made from the primary pigments.
1) subtractive color ... color you see is what’s left after the other colors have been
absorbed (subtracted out)
d. Black is produced when the 3 primary pigments are mixed together.
C. Wave Properties of Light
1. Reflection
a. The bouncing off of a wave from an object it strikes.
b. The law of reflection states that the angle of reflection must equal the angle of
incidence.
2. Refraction
a. The bending of waves (caused by change in speed).
b. Light changes speed as it enters or leaves media of different density.
c. Shorter wavelengths are refracted more.
d. The sky appears blue because blue is scattered more than other colors and our eyes are
most receptive to blue.
3. Diffraction and Interference
a. Diffraction is the bending of waves around a barrier.
b. Longer waves diffract more than shorter waves.
c. Interference of light by a diffraction grating separates it into its component colors.
d. Supports the wave theory of the nature of light.
South Carolina Physical Science standards met in this unit:
C. Interactions of Energy and Matter
1. Waves, including sound and seismic waves, waves on water, and light waves, have
energy and can transfer energy when they interact with matter.
_______a. Identify and show relationships among wave characteristics such as velocity,
period, frequency, amplitude, and wavelength using the formula, v = f? .
_______b. Compare and contrast models of longitudinal (e.g. sound and seismic waves)
and transverse waves (e.g. all electromagnetic and water waves).
_______c. Distinguish among electromagnetic spectrum, seismic waves, water waves,
and sound waves based on their properties and behaviors.
_______d. Understand factors affecting wave energy (wavelength, amplitude, and
frequency) and its effects on everyday life (e.g. health issues, medical diagnostics, and
treatments).
2. Electromagnetic waves result when a charged object is accelerated or decelerated.
Electromagnetic waves include radio waves (the longest wavelength), microwaves,
infrared radiation (radiant heat), visible light, ultraviolet radiation, x-rays, and gamma
rays. The energy of electromagnetic waves is carried in packets whose magnitude is
inversely proportional to the wavelength.
_______a. Compare and contrast the parts of the electromagnetic spectrum in terms of
velocity, wavelength, frequency, and energy using the formula, v = frequency x
wavelength.
3. Each kind of atom or molecule can gain or lose energy only in particular discrete
amounts and thus can absorb and emit light only at wavelengths corresponding to these
amounts. These wavelengths can be used to identify the substance.
_______a. Understand how releasing of energy by electrons produces light (e.g.
fireworks, neon lights, florescent lights, and halogen lights).
I understand that light waves cause electrically charged particles in matter to vibrate so that these
particles can absorb and reemit light, even in transparent materials. But doesn't that explanation
contradict quantum theory, which states that only specific photons corresponding to allowed electronic
transitions can be absorbed? -- GS, Akron, OH
When a light wave passes through matter, the charged particles in that matter do respond--the
light wave contains an electric field that pushes on electrically charged particles. But how a
particular charged particle responds to the light wave depends on the frequency of the light
wave and on the quantum states available to the charged particle. While the charged particle
will begin to vibrate back and forth at the light wave's frequency and will begin to take energy
from the light wave, the charged particle can only retain this energy permanently if doing so
will promote it to another permanent quantum state. Since light energy comes in discrete
quanta known as photons and the energy of a photon depends on the light's frequency, it's
quite possible that the charged particle will be unable to absorb the light permanently. In that
case, the charged particle will soon reemit the light.
In effect, the charged particle "plays" with the photon of light, trying to see if it can absorb
that photon. As it plays, the charged particle begins to shift into a new quantum state--a
"virtual" state. This virtual state may or may not be permanently allowed. If it is, it's called a
real state and the charged particle may remain in it indefinitely. In that case, the charged
particle can truly absorb the photon and may never reemit it at all. But if the virtual state turns
out not to be a permanently allowed quantum state, the charged particle can't remain in it long
and must quickly return to its original state. In doing so, this charged particle reemits the
photon it was playing with. The closer the photon is to one that it can absorb permanently,
meaning the closer the virtual quantum state is to one of the real quantum states, the longer
the charged particle can play with the photon before recognizing that it must give the photon
up.
A colored material is one in which the charged particles can permanently absorb certain
photons of visible light. Because this material only absorbs certain photons of light, it
separates the components of white light and gives that material a colored appearance.
A transparent material is one in which the charged particles can't permanently absorb any
photons of visible light. While these charged particles all try to absorb the visible light
photons, they find that there are no permanent quantum states available to them when they do.
Instead, they play with the photons briefly and then let them continue on their way. This
playing process slows the light down. In general blue light slows down more than red light in
a transparent material because blue light photons contain more energy than red light photons.
The charged particles in the transparent material do have real permanent states available to
them, but to reach those states, the charged particles would have to absorb high-energy
photons of ultraviolet light. While blue photons don't have as much energy as ultraviolet
photons, they have more energy than red photons do. As a result, the charged particles in a
transparent material can play with a blue photon longer than they can play with a red photon-the virtual state produced by a blue photon is closer to the real states than is the virtual state
produced by a red photon. Because of this effect, the speed at which blue light passes through
a transparent material is significantly less than the speed at which red light passes through
that material.
Finally, about quantum states: you can think of the real states of one of these charged
particles the way you think about the possible pitches of a guitar string. While you can jiggle
the guitar string back and forth at any frequency you like with your fingers, it will only
vibrate naturally at certain specific frequencies. You can hear these frequencies by plucking
the string. If you whistle at the string and choose one of these specific frequencies for your
pitch, you can set the string vibrating. In effect, the string is absorbing the sound wave from
your whistle. But if you whistle at some other frequency, the string will only play briefly with
your sound wave and then send it on its way. The string playing with your sound waves is just
like a charged particle in a transparent material playing with a light wave. The physics of
these two situations is remarkably similar.