Reflection and Refraction

1. Determine the index of refraction of water using Snell’s Law (Law of Refraction) and find the critical angle for total internal reflection.

1. Obtain a specialized refraction tank with a light source.
2. Fill the refraction tank with water and turn on the light source. Direct the beam from the light source into the half of the tank filled with water. It may be necessary to dim the lights in the room.
3. Use the protractor on the refraction tank to measure the beam’s angle of incidence (the angle measured in the half of the tank filled with water) and angle of refraction (the angle measured in the half of the tank filled with air) for the water-air interface in the tank.
4. Use the measured angles and the index of refraction for air (n_air = 1.00) to calculate the index of refraction for water.  
5. Repeat the previous steps for a few incident angles varying from 0° to just under 90°.
6. As the incident angle is increased, one will notice that the refracted light beam can no longer be seen in the half of the container containing air. Slowly rotate the light source about the tank until the point at which the light beam first disappears from the air is reached. This is the critical angle for total internal reflection.
7. If the light source is further rotated, it should be observed that the beam reflects back into the water.
8. Move the light source so that the beam enters the half of the tank filled with air first before traveling into the water. Record a few angles of incidence and refraction under this condition. Note that, previously, the angle of incidence was the angle at which the light traveled through the water. Since the light is now going through the air first, the new angle of incidence is the angle at which it travels through the air, and the new angle of refraction is the angle at which it travels through the water.
9. Observe that total internal reflection does not occur in this configuration. Total internal reflection only occurs when light goes from a medium with a high index of refraction to a medium with a lower index of refraction.

2. Measure the focal length of a lens and create real and virtual images of an object.

1. Obtain a convex lens, a concave lens, a sheet of white paper, a ruler, and a small distinctive object. It also helps to have an optical bench with holders for the lenses and object as well as an apparatus to hold the sheet of paper vertically.
2. Place the convex lens between the object and the piece of paper, all in a line and at the same height.
3. Move the object and paper around until a sharp image of the object appears on the paper. The image on the paper is a real image.
4. Measure the distance from the lens to the object and the distance from the lens to the paper. Use the thin lens equation to determine the focal length of the lens.
5. Place the paper aside and move the object closer to the lens until the distance between the lens and object is less than the focal length of the lens.
6. Look through the lens: a magnified version of the object should be seen. This image is a virtual image.
7. Replace the convex lens with the concave lens. Look through the concave lens: now a demagnified version of the object is visible. This is also a virtual image.