Location: Ch 3: Light
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Location: Ch 3: Light
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LIGHT
IN THIS CHAPTER:
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nergy can travel on electromagnetic waves. There are several types of
waves on the electromagnetic spectrum. These include electricity; radio;
microwaves; infrared; visible light; ultraviolet light; x-rays and gamma
rays. As the frequency of a wave increases, more energy is required to
sustain the wave. However, light is not just a wave. It also can travel
as a particle (photon) in a discrete packet of energy known as quanta.
Until the 20th century, it was debated as to whether light is
a wave or a bunch of photons. It had been proven that light was photons
because UV rays bounce electrons off a sheet of zinc. The debate raged
on and on until in 1905, when Einstein suggested that it was both. Now
it is known that electromagnetic energy is both photons and waves. Electrical
and radio waves have a relatively low frequency and do not carry too much
energy. A radio antenna receives energy from a radio wave to power a speaker.
We use microwaves every day to add KE to foods in a microwave. When infrared
light hits us, we feel heat. Human can only detect the energy in infrared
and visible light. When we see an image of an object, light that reflected
off that object has entered our eyes and triggered some nerve cells. Overexposure
to any waves with higher energy levels than visible light are dangerous
to living tissue. If we stay in the path of ultraviolet light, x-rays or
gamma rays, the energy from those waves changes our cell configuration
and cancers develop. However, when taken in very low quantities, these
rays are not harmful. When we take X-rays, high-energy radiation waves
go through our bodies onto photographic film. Since all electromagnetic
waves travel at 300,000 km/s, the only difference
between electromagnetic waves is the amount of energy the wave can hold.
Energy cannot pass through any material opaque to the type of wave it is traveling on. Light energy cannot pass through a brick wall, but it can go through a window. Neither ultraviolet nor infrared light can pass through glass. Electrons in glass have a natural frequency in the ultraviolet range. Therefore, when ultraviolet light hits the glass, resonance occurs and the electrons build a greater and greater amplitude. Thus, the energy in the ultraviolet light is converted into KE and ultimately into heat. Fortunately, our atmosphere will not allow much ultraviolet light through. Any light with frequencies lower than that of ultraviolet light however, should pass through without losing much energy. This results in visible light’s ability to pass through glass relatively easily. This means that in glass there is very little reflection of electromagnetic energy at the frequencies of visible light. Such a substance is transparent. An object that doesn’t allow a wave to pass through is opaque. Opaqueness is defined as any material that reflects or absorbs a lot of electromagnetic energy at certain frequencies. Any object that is neither transparent nor opaque is translucent. Unfortunately, glass is not transparent to infrared light. Infrared light vibrates not only the electrons, but the entire structure of the glass. Consequently the glass gains energy and warms up.
Greenhouses are a simple demonstration of opacity and transparency. When sunlight shines on a greenhouse, the glass is transparent to infrared and visible light. When the infrared light hits the ground, the ground absorbs the infrared and heats up. The hot ground then heats up the air inside the greenhouse. The glass, however, does not conduct much of the heat out. Thus, a greenhouse can get hot and stay hot even during the winter.
If an opaque object sits in front of a light source, some light rays
will be blocked and prevented from shining on whatever is behind the object.
The area where no light shines is known as a shadow. Some shadows have
a sharp outline. These are known as an umbra. However, a shadow usually
is not completely dark. Such a shadow is known as a penumbra. Penumbras
result from light bouncing around and partially
filling in where there is no light. Penumbras can also be caused by light
from a broad source only being partially blocked. A penumbra occurs on
the earth during a solar eclipse. While some parts of the world go dark,
other parts are in the moon’s shadow and only get a partial eclipse. However,
when the earth casts its shadow on the moon in a lunar eclipse, an umbra
occurs. The earth is so massive that the only light that can reach the
moon is light that curves around our atmosphere.
When an electromagnetic wave vibrates in only one direction, the wave is said to be polarized. A single electron emits a polarized electromagnetic wave. Most light sources emit light that is not polarized. This is because the electrons that produce these waves vibrate in a random direction. Light will pass through a pair of polarizing filters only when their polarization axes are aligned. It is much like filtering a vibrating rope through a picket fence. However, light does not need a polarization filter to become polarized. If light beam hits any horizontal surface and reflects off, the reflected light will have a polarization axes that is parallel to the surface it reflected off of. Thus, sunglasses with vertical polarization axes are most effective against glare. Polarization can be put to use in other ways. Three-dimensional movies use polarized filters to project two images at different angles onto the same screen. When the viewer puts on specially polarized glasses, the image on the screen appears to be "jumping out" at the viewer.
In conclusion, light is energy that travels in electromagnetic waves within certain frequencies. Light is produced by vibrating electrons in atoms. It can pass through materials whose atoms absorb the energy and reemit it a light wave. Light cannot pass through a substance when it is opaque. Shadows are created by absences of light behind an object. Polarization filters a wave so that it vibrates in only one direction. Since light waves are transverse, they can be polarized.