Activity 7: Refraction of Light

Overview:
Have you ever tried to catch a fish by plunging your hand into the water to grab it with your bare hands? If so, you probably discovered that it's very hard to do - it's as if the fish isn't where it appears to be. That's because light rays bend when they cross the boundary between water and air. This is easy to see if you put a spoon into a bowl of water - the spoon appears to bend where it enters the water. This bending of light rays is called "refraction" and occurs when light rays cross the boundary between two different substances. How much light bending occurs is actually a characteristic property of the material: this is one way a jeweler can tell the difference between a diamond and a piece of glass. The purpose of this lab is to investigate how light bends when it moves from one substance into another.

To understand refraction, consider the following analogy: Let's say that a marching band is marching in a straight line on dry ground, then suddenly encounters a large muddy area. The people who come to the muddy area first will slow down. But, the people who are still on dry ground keep marching fast. If the marching band hits the muddly area at an angle, it will tend to cause a slight change in direction like this:

In the above sketch, "medium 1" is the dry ground, and "medium 2" is the muddy area. The students at the top of the green line hit the mud first and slow down. Beause of this, the top of the line doesn't get as far as the bottom of the line, and the marching band is forced to bend towards the left. The same thing happens to light when it crosses a boundary between substances. For more detailed information, you can visit the GlenBrook Physics Classroom's unit on Light Refraction.

A few important concepts to know:
When light rays move from the air into a transparent substance, they bend towards the normal (a line drawn perpendicular to the boundary where the light it moving through it).

When light rays move from a transparent object into the air, they bend away from the normal.

The angle of incidence is the angle the light rays hit the boundary measured against the normal.

The angle of refraction is the angle the light rays travel through the new substance measured against the normal.

Lab Procedure Notes:
Note that it might be helpful to use lots of sketches when writing your pre-lab's procedure. It will also allow you to use fewer words!

In this lab, you will be tracing light rays from a laser pointer as they travel through air into a block of clear gelatin (called a "BLOX" by the text). Because you can't trace the light when it is inside the BLOX, you will need to make a dot with your pencil where the light enters the BLOX and where it exits. Then, remove the BLOX and use a ruler to connect the dots, and draw your normal. You will aim the light at the BLOX from 3 different angles, measuring the angles of incidence and refraction for each ray pair on a data table like the one shown in your text. You will then need to use a scientific calculator to find the sine of each angle (it's the button that reads "SIN" on many calculators). Then, using the calculator, divide the sine of the incidence angle by the sine of the refraction angle, and enter the value in the last column of your data table. This number is known as the index of refraction.

Finally, you will experiment with other angles to find out what angle the light needs to hit the BLOX so that the refraction actually bends the light ray back into the BLOX and does not escape from it. This angle is called the critical angle. When the light cannot escape the BLOX, it is called total internal reflection.

Safety notes:
Laser beams concentrate light energy into a small point, and should never be aimed at anyone. So that accidents do not happen, only use the lasers to shine them at the mirror on the piece of paper. Do not eat the BLOX - it was not made using clean kitchen utensils, and has been handled by many hands. Yuck!

Discussion Points:
a) Did you get the same index of refraction for all your data? Does this data support the idea that the index of refraction is a characteristic property? Explain.
b) What angles of incidence cause light to bend (refract) the most?
c) Refraction causes sunlight moving through a prism to separate into the colors of the rainbow. Discuss what this suggests about how different colors of light refract.
d) Discuss how this lab relates to other real-life examples and the upcoming sound and light show.


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