What is Color?

Personal Lab 13

Background

We have seen that light normally travels in straight line paths. However, when light encounters a transparent medium with a different index of refraction, such as in passing from air to water, light can bend. We found further that the bending of light always occurred at the interface between the two different transparent media. We have seen that lenses refract or bend light and, if made just the right shape, lenses can form images. We have also seen that mirrors and other shiny smooth surfaces can reflect light and also form images. Are there other situations where the light path can bend?

We noted earlier that the different forms of electromagnetic radiation could be characterized by one number only, namely the wavelength. X-rays have very short wavelengths, and radio waves very long wavelengths, with visible light in the middle of the range of wavelengths of the electromagnetic spectrum. How can light produce different colors?

Purpose

1. To investigate in more detail the path of light.

2. To investigate the colors of light.

3. To understand the function of color filters, and the properties of objects that give them color.

diffraction grating	4 color filters, B, G, Y, R
flashlight	2 index cards
glass slides (2)	liquid soap

Procedure

1. Place one index card on a table top, and shine the flashlight so that a shadow of one edge of the second index card is cast on the first. Record your description of the shadow cast by a corner of the second index card.

2. Find a light source, such as a desk lamp where you can directly view the bulb. Record what type bulb, fluorescent or incandescent (normal light bulb). Hold each color filter up to the light, and record the color you observe.

3. Hold the diffraction grating so that the long part is vertical and between your eyes and the light source. Move the grating slightly up or down until you see colors. Record what you see.

4. Place a color filter between your eyes and the grating as you look at the light source. Record what you now see. Record the color of the filter, the color you see, and any colors that are missing.

5. Carry your diffraction grating with you for a day, and observe different kinds of light bulbs. Record your observations in your log book. City lights at night are particularly interesting, as are fluorescent lights. The daylight sky is also another interesting light source. (Caution: Do not look directly at the Sun, as it could cause eye damage). Record your observations.

Questions

1. What was the appearance of the shadow cast by the corner of the index card? How can you explain the path of light as it passes by the corner of the index card to account for the shadow you observed? Explain your reasoning.

2. What happens to white light when it passes through a color filter?

3. What happens to light when it passes through a diffraction grating?

4. How did the light viewed through the diffraction grating toward street lights differ from what you viewed when looking at skylight? Use sketches to illustrate your answer.

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Last modified 9 Aug 1997
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