The path of light bends various amounts when it travels from one material to another. The process is called refraction. It can be studied by tracing light rays going from air to water and back.
In this experiment we wish to trace refracted light rays in order to locate a virtual image.
You will need
- a transparent rectangular tank (perhaps food storage container),
- protractor to measure angles,
- a small object to look at such as a small toy, chess piece, or cap to a pen
- a clean piece of paper for recording locations and drawing rays, and
- a way of marking locations. You might want to use several nails that will stand on their flat heads. An alternate would be to use pins and a material under the paper that could hold pins upright.
Fill the rectangular tank with water. To get a rough idea of the effect of the water on light, draw a straight line across a sheet of paper. Put the tank of water on top of the line so the edge of the tank is perpendicular to the water. Here is the hard part! Don't look in through the top surface of the water. Also ignore what is seen near both the side and bottom the edges of the tank caused by those edges. Looking in though one of the sides where the line passes underneath, you may be able to very faintly see the line under the tank. Gradually rotate the tank a few degrees and note the faint line seems to be deflected.
- Place the tank in the middle of a fresh piece of paper, recording its location. Locate (and record on the paper) an object some distance from the tank.
- Look in the opposite side of the tank, again avoiding images near any of the edged. Also ignore what you see of the object both reflected by the top surface and directly over the top of the water. Holding you head steady and using one eye (closing the other) located so you can see the image of the object through the water, align two vertical markers (such as pins or nails standing on their flat heads) as far apart as possible in front the eye side of the tank so that the markers are exactly between the image and your eye. See diagram 2. Draw on the paper a ray from the tank, passing through the marked points towards you eye. This should be part of the route the light took getting from the water to your eye at this location. See diagram 3.
- Temporarily remove the tank. Draw a parallel ray from the object to the side of the tank nearest the object.
- Draw a line segment connecting the tank end of the object ray to the tank end of the ray from the tank to your eye. See diagram 4. This completes the ray diagram for the light when your eye was viewing the object through the water. The light ray started at the object, travelled in the straight line towards the water, then refracted at the edge of the tank, refracted again leaving the tank and traveled to your eye.
- Use a protractor to construct a perpendicular line at the two points where a light ray refracts going into and out of the water. See diagram 5. Measure the angle between the perpendicular and the ray the light approached the water, called the angle of incidence, and the angle between the perpendicular and the ray once in the water, the angle of refraction. How does the angle of incidence compare with the angle of refraction?
- But many light rays start from the object going in other directions. Replace the tank precisely and choose a new location for your eye. Repeat steps 2 through 5. Do this for at least a half dozen eye positions.
- Measure and compare the angle of incidence and the angle of refraction.
- Move the source object to other locations. Are there positions where the source is not visible? Is there a pattern in the refraction?
- Make a table comparing the ratios of the angle of incidence divided by the angle of refraction. Also compare the ratios of the sine of the angle of incidence divided by the sine of the angle of refraction (use your calculator!). The ratio that is most constant is called the INDEX OF REFRACTION; this is a characteristic property of the material (water). Materials with different indices of refraction are used to construct eye glasses, binoculars, telescopes, microscopes, and the like.
Record your observations recorded in your journal. If you need course credit, use the information in your journal to construct a formal report.