Download Chapter 27: Light

Physics of Light Notes
27.1 Early Concepts of Light
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There have been many theories about the nature of light ranging from the idea that objects emitted
particles that entered our eyes causing the sensation of light, to the idea that our eyes emitted particles
that fell upon objects lighting them. The current accepted view was put forth by the Greek Empedocles
and is that light travels in waves, and was finally accepted in the 19 the century.
We now know that light consists of massless bundles of concentrated electromagnetic energy called
photons.
It is commonly accepted that light acts both as a wave and as a particle; however in this chapter we will
only discuss its wave properties.
27.2 The Speed of Light
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The finite speed of light was discovered based on measurements and observations of one of Saturn’s
moons. Depending on the location of the earth, the orbit of the moon around Saturn would vary by 22
minutes. This variation of time was caused by the extra time it took for the light to reach the earth when
it was farther away. Using the change in distance and the change in time we can find that the speed of
light is 300,000 km/s (187,500mi/s, or 3.0 x 108 m/s)
o This was proved again my Albert Michelson in 1880 with the use of light, and mirror on a
mountain.
The speed of light that we use of 300,000 km/s represents its speed in a vacuum which is a universal
constant. Because of the constant nature of this speed we can use it to measure distance. We call the
distance light travels in 1 year a light-year.
o It takes light 8 minutes to reach us from the sun, and 4 years to reach us from the next nearest
star.
27.3 Electromagnetic Waves
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Because the light energy that travels in wave form is partly electric and partly magnetic we call it an
electromagnetic wave. There are many different types of electromagnetic waves that make up the
electromagnetic spectrum like x-ray radio wave, and microwave. In Figure 27.4 it shows the relative
frequencies of several different wave types in the electromagnetic spectrum.
o The lowest frequency of visible light is red, and the highest is violet.
o Electromagnetic waves that are at a frequency too low for us to see them are called infrared
(because they are below red light)
o Electromagnetic waves that are at a frequency to high for us to see them are called ultraviolet
(because they are a higher frequency than violet.
27.4 Light and Transparent Materials
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The transfer of light is similar to the transfer of sound, because a vibration emits a wave form which
causes a vibration when it hits, however, there are MANY differences between sound and light.
o Electromagnetic waves are initiated by a vibrating electric charge. When the light wave hits
matter it forces the electrons into vibration.
o How a material responds to an electromagnetic wave depends on the frequency of the wave
AND the natural frequency of the object it hits.
 The natural frequency of the object depends on how tightly its electrons are held to the
atoms and molecules. The looser or more “springy” they are the faster they can vibrate,
thus giving them a higher natural frequency (and vice versa).
When we say that an object is transparent to light it means that light can pass through it (like glass and
water).
o The natural frequency of glass is in the ultraviolet range, which means that when light from the
sun hits the glass the ultraviolet waves cause resonance in the electrons and cause them to vibrate
with a very large amplitude. This causes an exchange of energy that creates heat. Because of
this we say that glass is not transparent to ultraviolet waves.
o Because the natural frequency of glass is not in the visible range, when visible light waves hit it
they simple cause one electron to hit another which hits another until the wave has gone all the
way through the glass and is reemitted from the other side. This process of absorption and
emission of electrons from atom to atom within the glass causes the speed of light to be slightly
decreased in glass.
 The speed of light in glass is 67% what it is in a vacuum, and the speed of light in water
is 75% what it is in a vacuum.
o The infrared waves have even lower frequencies than visible light waves, and they cause not
only the electrons to vibrate but the entire glass structure, which means that they are not able to
pass through, but just generate heat.
27.5 Opaque Materials
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Opaque materials absorb light without reemitting it out the other side, thus we cannot see through them.
In all opaque materials the effect of the rays is to cause vibrations that end up as random internal kinetic
energy, thus increasing their temperature.
o The reason that metals go through the heating process from light so much faster than other
materials is because their electrons are very loosely bound to their atoms and so the surface
electrons that have been exposed to the electromagnetic waves can wander throughout the
material heating all of it, not just the part that has been exposed to light. When the light hits the
metal initially it even sets some electrons free from the material entirely, which is what gives
metal its shiny look.
Our atmosphere is transparent to visible light and some infrared and ultraviolet frequencies, but most
ultraviolet is blocked (the little ultraviolet light that does get through is what is responsible for
sunburns). Clouds block more light than the atmosphere, but still let some visible and ultraviolet light
through.
27.6 Shadows
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A thin beam of light is called a ray. Any actual light beam is just a bundle of rays.
A shadow is formed in the areas that light rays cannot reach. An umbra is a total shadow, and a
penumbra is a partial shadow. A good example of umbra and penumbra is given in your book on page
413 with the different types of lunar eclipse.
27.7 Polarization
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We know that light travels in transverse not longitudinal waves because of a phenomenon called
polarization. Polarization refers to whether the wave is going up and down or side to side. An up and
down wave is vertically polarized and a side to side wave is horizontally polarized. The polarization of
light depends on the direction of vibration of the electron that caused it. Most natural light is emitted by
a source that has electrons vibrating in all directions and so the resulting effect is a nonpolarized light.
We can use polarizing filters to allow only one type of light through. A polarizing filter has slits on it
that go in only one direction, if the slits are aligned vertically they only allow vertically polarized light
through, and horizontally only allow horizontally polarized light through, If you put horizontal and a
vertical polarizing filter on top of each other the result is to let no light through.
o When light hits a horizontal surface, only horizontally polarized light is reflected.
27.8 Polarized Light and 3-D Viewing
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A 3-D projector puts out two images that are taken a short distance apart (so that each eye is seeing a
slightly different image), one shown through a vertical polarization filter and one shown through a
horizontal polarization filter. It looks blurry to your naked eye, but when you put on glasses that have
one lens that is vertically polarized and another that is horizontally polarized; this allows you to see the
image in three dimensions.