What Path Does Light Follow?

In the mid-nineteenth century James Clerk Maxwell formulated one of the most fundamental theories in science today. Experiments by scientists of that time had established a connection between electricity and magnetism. Electric currents were known to be due to electric charges moving in wires, and these moving electric charges could cause the magnetized needles of compasses to move. Maxwell’s new theory explained the link between electricity and magnetism, and led to his prediction that light is also an electromagnetic phenomenon. Maxwell’s Electromagnetic Theory has been verified by all experiments performed over the past century.

With Maxwell’s unexpected discovery that properties of light such as its speed could be predicted from his theory, Maxwell’s connection between light, electricity and magnetism, became one of the most surprising and important discoveries in the history of science. Maxwell followed up his creative idea by developing an elegant theory that light is a wave which carries both electric and magnetic energy, and propagates through space at a speed of 300,000 km/s. His theory of electromagnetism is one of the most elegant in science because of its great simplicity, and because it connects all electric and magnetic phenomena, and explains what light is.

The regions in the universe between the stars and especially between the galaxies are essentially devoid of material. Because we see stars at night we know that light can propagate through that vacuum in space. In our everyday experiences we also know that light can pass through some objects, and is blocked by others. Through what kinds of materials can light pass? Through what kinds of materials does light not pass? What path does light follow through different materials?

Purpose

1) To observe the path of light through different materials.

2) To investigate what fraction of light is transmitted through different materials.

Materials

Ordinary drinking glass, 8 oz. or larger, or a transparent plastic bag	water
skim or lowfat milk (1 teaspoon)	flashlight
index cards (3 x 5 in.)	polarizing filters (2)
drinking straw, small

Procedure

(Note of caution: In your investigations of light, avoid looking directly at the Sun, as it can cause permanent eye damage.)

1. Shine your flashlight on various objects in a darkened room, and note what you observe about the light path from the flashlight to, through and from the object. While you remain in the darkened room, have someone shine the flashlight down the darkened hallway past the open door to your room. Record notes on all of your observations.

2. Fill the drinking glass or the plastic bag with water, place the drinking straw in the water, with one end protruding above the water. Examine the straw as viewed from all angles, above and through the glass (bag) from the sides. Vary the position of the straw in the water, and repeat your observations. Include sketches of what you observe together with your notes in your Lab Book.

3. Cut a circular disk the size of the end of your flashlight from the index card. Cut a slit about 3 to 4 mm in width, and extending across the center of the circular disk, nearly to the edges. Tape the circular disk with slit to the end of your flashlight, so that the light beam from the flashlight now becomes a slit.

4. Mix about one teaspoon of skim or lowfat milk with the water in the glass (or bag) (straw removed) to make a murky mixture, though not opaque.

5. Shine the flashlight toward the milky water from a variety of angles, from above and from the sides. Record and sketch the light path for a variety of flashlight positions.


6. During the day go outside or find a room with a window. Do not look directly at the Sun. Look toward the sky, away from the Sun, holding the polarizing filters up to your eyes, one at a time, and superimposed. Rotate both filters to various positions as you view the sky through the filters. Record what you observe.

7. Carry one of the polarizers with you during the day, and observe the skylight in various directions away from the Sun at various times during the day, near sunrise(or sunset), near noon, etc. Rotate the polarizing filter through various angles as you look in different directions around the sky each time of the day. Record your observations for different times of the day and different angles from the sun.

Questions

1. Sketch the appearance of the straw in the water when viewed through the glass or bag of water.

2. Based on the experiments in this lab, under what conditions can you see the path of light (i.e. a light beam)? Propose a model which is consistent with your observations that might explain why you can sometimes see the light beam and sometimes not.

3. When you could see the light beam, what path did you observe light to follow? Was there a pattern in the path that light followed? If so, make a general statement about the path of light.

4. Complete the sketch below, by predicting the light paths through the glass or bag of milky water and out the opposite side of the glass. Check your predictions with experiments.

5. Based on your observations write a general rule that describes the path of light from outside a glass (bag) of water, through the water, and out the other side.

6. Did you notice any changes in the skylight when viewed through a single polarizing filter at different times of the day? At different angles from the Sun?

7. Write a statement for a rule describing what happens to light when it passes through two polarizing filters.

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Last modified 9 Aug 1997
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URL: http://acept.la.asu.edu/courses/phs110/ds7/pl10.html