|Module 12: Characteristics of a prism. Basic prism optics.|
PAGE References to Optical Formulas Tutorial: (first reference is to edition 1 / second reference is to edition 2).
Basic Prism Optics.
Back when we were discussing refraction and Snell's law we discussed what happens to a ray of light as it passes from one material to another. Then we looked at curved surfaces as if they were a series of flat surfaces angled to each other. Well, what if we have two flat surfaces angled to each other that are not curved?
Look at what happens to the ray of light as it passes through this piece of material. First it travels from air and enters the material, and bends toward the normal. Then it leaves the material travelling back into air, and bends away from the normal. In the first diagram the sides of the material are parallel to each other, so the ray emerges travelling in the same direction as its original travel -- it has been displaced, but not deviated.
In the second diagram the sides of the material are not parallel to
each other, so the ray emerges travelling in a different direction than
its original travel. Now it has been deviated from its original
path. This happens to every ray that we trace through a lens. In the lens
every ray travelling parallel to the original ray is deviated a different
amount, so the vergence of the incident parallel rays changes. In the prism
with straight sides shown here all of the rays that are travelling parallel
to the first ray are deviated the same amount, so they emerge parallel
to each other. This did not happen when the rays were travelling through
a lens. The prism has not changed the vergence of the rays that
travel through it.
Look at the plano prism that you were instructed to acquire for this module. If you are registered in the course and do not have access to one please call your instructor. We are going to determine the characteristics of a prism.
First, look at the edges. One place on the edge, the base of the prism, is thicker than any other. One place on the edge, the apex of the prism, is thinner than any other. The thickest edge is exactly opposite the thinnest edge. Hold the prism at the base and then run your fingers from edge to edge through the center of the prism. The thickness decreases as you go from edge to edge. There is no place on the prism where the thickness is greater than at that one area of the edge [provided your prism has no power].
Hold the prism an inch away from a piece of paper with the thickest part to the left and the thinnest part to the right. Move the prism perpendicular to the edge of the paper. The edge of the paper will appear to be to the right of where it really is, and will stay in that same position consistently as your move the lens. Rotate the prism. The position of the edge will move parallel to the real edge until it lines up with the real edge, then continues to move inside the paper. The plano prism shows no with, against or scissors motion. The image of the edge of the paper is displaced toward the thinnest edge of the prism.
Hold the prism up in front of your eye. Rotate the prism. (You will probably need to close your other eye.) Everything that you look at will be displaced in the direction of the thinnest edge of the prism.
The thin edge of the prism is called the apex of the prism. The
image is displaced toward the apex.
Look at the size of the object that you are looking at through the prism, then without the prism. The size of the object did not change. The vergence of the rays entering your eye from the object did not change.
Look at something close to you through the prism and then move the prism
away. Notice how much the image moved or jumped. Now look at something
several feet away from you with and without the prism. Notice that the
image jumps more when the object that you are looking at is further away
from the prism.
I didn't talk about that last one, did I? That is what happens when you have a light catcher that gives you rainbow colors when it is placed in the sun. It is what is happening in millions of tiny water drops when you look at a rainbow.
We have discussed the fact that the speed of a ray of light in a material determines how much the ray will be deviated, and we have discussed the fact that the higher energy blue and violet waves travel slower in materials than the lower energy red waves do. So, since blue slows more than red it bends more than red, and the result of this difference in refraction is that a narrow beam of light when it travels through a prism will be dispersed into its component colors.
Read pages (71-72 / 93-94) in the Optical Formulas Tutorial.
When you are finished with this click here
the four ways to define the power of a prism.