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LIGHT
The spectrum consists of all wavelengths of electromagnetic energy. Visible
light is only a small portion of this spectrum. The range of visible light
consists of wavelengths between about 0.4 microns (blue) to 0.7 microns
(red).
Normally light travels in a straight line. Here a light
source shines through slits cut in a piece of paper.
If a light wave is incident upon the interface of a more dense
transparent Medium at a 90 degree angle ( normal to the interface)
the wave will slow down, but it will still go straight through the
medium and speed up again when it leaves the other side.
However, if a light wave is incident upon the interface of a
more dense medium at some angle other than 90 degrees, the
wave will slow down and change directions. It will also speed
up again and change directions again when it leaves the new
medium. This change in direction is called Refraction.
To understand how refraction takes place, imagine the wave
crests of a single light wave as lines. In this case, as the wave
enters the Interface between the lower and higher density
media, the Top of the wave slows down first. This “pulls”
the wave towards the slower edge and causes the wave to change
directions.
Remember our beams of light? If we place a piece of
glass with a curved surface into the beams…
The light waves, and therefore the beams, will be refracted.
Only the waves that strike the very center of the glass lens
will strike normal to the surface and travel straight through.
This is a bi-concave lens. It diverges the beams ( spreads
them apart).
This picture shows a Plano-convex lens. Note how the lens
converges the beams to a point (focal point). This lens is
sometimes called a converging lens.
Here a beam of light strikes a mirror at about a 45degree
angle from the normal. Note that the angle of incidence
and the angle of reflection are the same. Angle R=Angle I
Here a laser beam reflects from the surface of a mirror. Note
that the angle of reflection and the angle of incidence are the
same. Both angles are measured from the normal line to the
respective beam.
Here a laser projects a beam of light into the end of a curved
Lucite rod. Because the inner surface of the plastic rod reflects
most of the light, the laser light goes around the curve and shines
out the opposite end of the rod!
On closer examination you can actually see the laser beam reflecting
off of the inner surface of the solid plastic rod! The process at work
here is called internal reflection. This is the basic concept behind
fiber optics technology.
Recall that visible light is a mixture of all the
wavelengths of Light “ROYGBIV”. Each color
is a different wavelength. (*Red is longest & violet is the
shortest visible wavelength.)
Why is the red car red?
Because even though ALL the wavelengths of visible white light
may be incident upon the surface, you see only the red wavelengths
of light reflected from the car. All the other wavelengths
are absorbed by the pigments ( coloring agents) in the paint.
So, what color would the car appear if you looked at it
through a blue plastic filter? ( Hint: remember the car reflects
only red wavelengths while the blue filter transmits only blue
wavelengths and absorbs all others.)
Of course the car appears black because the blue filter
blocks the red wavelengths reflected from the car and
the the car reflects no blue wavelengths!
So, how about if we look at a blue car through
a red filter? What color will the car appear?
If you said black you were right. Even though all the wavelengths
of visible white light strike the car’s surface, only the blue
wavelengths are reflected by the pigments in the paint. Since the
blue wavelengths are blocked by the red filter, no light waves
from the car can reach your eyes, so the car looks black!
This is a gas discharge tube filled with Neon gas. When the
electrons are excited by the electric current, they jump to a higher
energy level. When they fall back down, they give up a distinctive
series of spectrum lines that can be used to identify Neon.
A spectroscope can be used to look at the spectrum emitted by
the excited atoms of gas in the tube.
Here you can see the spectrum of Neon through the
spectroscope.
Here a piece Nichrome wire is coiled and connected to a low
Voltage power source. When electricity passes through the wire
the wire gets hot. The electrons in the wires atoms jump up to
higher energy levels…
…and when they fall back down they give the energy up as heat and
light. This process is called incandescence. This is how light is
produced in light bulbs. Notice that the wire first glows red then as
the electric current is increased it turns yellow…it’s moving up the
spectrum!
This clear light bulb can show how
incandescence works.
Inside the light bulb is a thin coiled wire filament
made of tungsten.
When current is passed through the filament it starts
to glow. As the current is increased…
…it glows brighter and brighter…INCANDESCENCE!!!!
We now have an incandescent light bulb.
Here is how INCANDESCENCE takes place.
1. An electron is excited by absorbing heat from
electrical resistance.
2. The excited electron jumps higher.
3. The excited electron falls back down and gives
off the energy as LIGHT.
I guess this means that incandescent bulbs change electricity
into heat and light!
Here are two plastic light filters. They look like the
plastic sunglasses are made of. Light goes through
both of them.
Now turn one filter ¼ turn you can see that light does
NOT go through the two filters. WHY?
To understand why you must think of light as a wave that
vibrates both horizontally and vertically, like these two
waves made of wire.
The molecules in the filters are aligned parallel to each other.
If they are held so that the molecules in both filters are vertical,
the vertical vibrations of light can get through, but the horizontal
Vibrations can not. The resulting light, that vibrates in one
dimension only, is called “POLARIZED” light.
If one filter is turned ¼ turn, the light that is polarized by
the first filter will not pass through. This is why the two
filters together blocked the light.
Here are the two wire waves. They represent
unpolarized light striking a flat vertical surface.
Notice that the light is vibrating vertically and
horizontally…
…but when it reflects from the surface, the horizontal vibrations
have either been absorbed by the surface or changed to vertical
vibrations. The surface produces polarized light! This polarized
light reflected from a surface is called GLARE.
Click on the picture and you can see how a polarizing
lens blocks glare from a surface.
The little man glows when placed near a black light. This is called
FLUORESCENCE.
Fluorescence happens when the atoms in the little
figure take in UV ( ULTRA VIOLET ) light and
change it to visible light.
If you look carefully, you can see the spectrum
produced by the prism in the foreground.
From above the prism, you can see that the “white light”
beam is refracted by the prism.
The prism refracts the shorter wavelengths of light, the ones towards
The blue end of the spectrum, more than the longer wavelengths.
Thus, a prism sorts light by each colors distinctive wavelength.
The End