Dispersion Scattering Colour
Dispersion Scattering Colour
Dispersion Scattering Colour
Dispersion
When light rays are refracted, the change in direction depends on the difference in
speed between the two mediums.
n 2 > n1
Medium 1
v2 < v1
Medium 2
2 < 1
frequency remains constant
However!! The index of refraction (n) depends, to a small degree, on the wavelength of
the light. Normally this effect is so small that it does not result in a noticeable difference
between different wavelengths (i.e. colours) of light, but for glass triangular prisms, the
difference in the index of refraction for different colours of light results in a separation of
the white light into its spectrum of colours. This separation of light into its colours is
called dispersion. (Refer to Pearson pages 675 to 676.)
red
orange
yellow
green
blue
violet
Rainbows are also a result of dispersion. If the sun is shining and there are water
droplets in the air, either due to rain or mist from a waterfall or sprinkler, rainbows can
form. As shown in the diagram below, different wavelengths of light are refracted by
different amounts by the water droplets. In this way, the different colours are separated
resulting in a rainbow. Note that where an observer sees the rainbow depends on the
position of the observer. If the observer moves, the rainbow moves. Thus, you can
never find the pot of gold that is rumoured to exist at the end of a rainbow.
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The dispersion of light demonstrates that white light is actually a combination of all the
colours of visible light combined together. Light is a spectrum of wavelengths and
frequencies. But visible light is only a small part of the light spectrum. As you will learn in
Lesson 24, radio, infra red, ultra violet and x-rays are all light waves they have different
wavelengths and frequencies, but their speeds are all the same: 3.00 x 108 m/s.
Light Spectrum
In order to see the light, some of the light must reflect off of particles like dust and lint in
the air into our eyes. This is the basis of scattering. When light hits particles or
molecules in the atmosphere, it is scattered in all directions by the particle or molecule.
Some of the scattered light goes into our eyes, while the unscattered light continues
without our knowing it was ever there.
unscattered light
scattered light
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The scattering of light in the Earths atmosphere is proportional to the fourth power of
4
the frequency (f ). Therefore, the higher the frequency, the more the light will be
scattered. Blue light and violet light are scattered much more than red or orange, so
the sky looks blue. The entire sky appears blue because the blue light is being
scattered in all directions at the same time. Some blue light is scattered toward our
eyes from every direction in the sky.
At sunset, the suns rays have passed through a maximum length of atmosphere where
much of the blue light is scattered out. Thus the light that reaches the surface of the
earth and scatters off of dust particles in the sky is lacking in blue coloured light. The
remaining light is scattered by the larger particles of dust in the lower atmosphere,
making the sunset appear reddish.
red remains and
is scattered to
where we are
scattered
blue light
Sun
The dependence of scattering on f is valid only if the scattering objects are much
smaller than the wavelength of the light. This is valid for oxygen and nitrogen
molecules. Thus if the atmosphere did not contain oxygen or nitrogen, the sky would
appear quite different. Clouds, however, contain water droplets or crystals that are
much larger they scatter all frequencies of light uniformly. Hence clouds appear
white.
III. Colour
As we saw with dispersion, white light is composed of different colours mixed together.
Black is the absence of light.
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blue + red =
magenta
blue
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IV. Polarisation
When light travels through space it vibrates in all planes. However, when light is
passed through a polarising filter, the filter allows light waves through that are polarised
in one plane. (Refer to Pearson pages 695 and 696.) In other words, the only light that
passes through a polarising filter is the light that was vibrating in the same plane as the
polarising filter.
In the diagram above, unpolarised light passes through a polarising filter that is oriented
to let horizontally polarised light to pass through. Of course if the filter were rotated it
would allow different planes of light to pass through. If the horizontally polarised light
next falls on a second filter that polarises light in the vertical plane, the light energy is
almost completely absorbed. This occurs whenever the axes of the polarising filters are
at right angles to each other. When the axes of the two filters are parallel, the light
polarised by the first filter passes through the second filter without further absorption.
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Polarisation Activity
Carry out the following investigations with the polarisation kit provided.
1.
Hold one of the polarising disks up to a light and observe the effect as you rotate
the disk through 360o. Now hold both disks to the light. Slowly rotate one of the
o
disks through 360 . Note the positions of the axes on the disk rims when
maximum and minimum light is transmitted. Explain, using a description and a
diagram, the observed effects.
2.
3.
Rotate one of the polarised filters over a wristwatch or calculator with a liquid
crystal display. Record your observations. Explain why the LCD changed in
appearance as the polarised filter was rotated.
4.
Look around the classroom for any horizontal surface (such as the floor or a
counter top) where you can see the glare of reflected light. Look at the glare with
a polarised filter and then rotate the filter. Record your observations.
How would you use polarised sunglasses to eliminate or minimise the effects of
glare from a road? Explain.
5.
Observe the mica crystal disc. Place a polarising disk over it. Hold it toward a
light source and rotate the disk. What happens? Now sandwich the mica disk
between two polarising filters. Rotate one of the polarising disks. What happens?
6.
Repeat step 5 substituting the benzoic acid crystal disk for the mica.
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V. Hand-in assignment
1.
2.
What observation indicates that diamond has slightly different refractive indices for
each of the colours of the spectrum?
3.
Considering the relationship between particle size and the scattering of light, is it
better to use red light or blue light when photographing a tiny object through a
microscope when you want to obtain maximum definition? Explain.
4.
Why are you more comfortable on a hot sunny day in light-coloured clothes than
in dark clothes?
5.
6.
A performer wears blue clothes on stage. How could you use spotlights to make
them appear black? Is it possible to make them appear red? Explain.
7.
When white light passes through a flat piece of window glass, it is not dispersed
into its colour spectrum. Why not?
8.
9.
Cats do not see in colour, only black, white and grey. How do cats eyes differ
from human eyes?
10.
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