Diffraction: The Spreading of Light
Light is a form of radiation. Radiation is energy that travels through space in waves that have both electrical and magnetic properties. Energy emitted by stars contains several different forms of radiation, including visible light. Visible light is a part of the electromagnetic spectrum with different colors corresponding to different wavelengths, which is the distance from the point on one wave to the same point on the next.
Diffraction is one of the ways light is affected when it passes through a small opening. In this project, you will determine how diffraction hinders resolution, which is the ability of a telescope to separate the light of adjacent (nearby) stars and the importance of the resolving power of an optical instrument. You will also make and use a spectroscope to view the light spectrum.
Purpose: To observe diffraction of light.
- straight pin
- index card
- yardstick (meterstick)
- desk lamp (with a visible lightbulb)
- Use the pin to make a tiny hole centered near the edge of one short end of the index card.
- Circle the hole so you know where it is.
- Fold the index card in half with the short ends together.
- Partially unfold the card so that the two halves are at right angles to each other.
- Hold the unmarked end of the card at the end of your nose. You should be able to see the circle you marked.
- Close one eye. From 6 feet (1.8 m) or more, look through the pinhole at the illuminated bulb of the desk lamp (see Figure 6.1).
- Observe the appearance inside the hole.
Dark lines appear in the hole.
Diffraction is the spreading of light as it passes the edge of an obstacle and the blurred edges of the obstacle is called diffraction fringe. In this demonstration, some of the light passes directly through the center of the hole, but light hitting the edge of the hole bends (changes direction). Each point on the edge of the hole acts as a source of light. Where light rays from the sources meet, light and dark bands are formed. Think of light as a wave, which is a periodic disturbance in a substance or space that has crests (tops) and troughs (bottoms), as shown in Figure 6.2. When two waves meet, their crests and troughs overlap and the waves combine. If the crests and troughs match, the combined wave has crests and troughs with a greater amplitude (height). This is called constructive interference. The light produced is brighter and light bands are formed. But if the crests and troughs are opposite, the combined wave has less amplitude. This is called destructive interference. The light produced is less bright and can even be totally canceled out if the crests and troughs of the two waves are the same amplitude. Where the light is canceled, dark bands are seen. Despite the name, interference has no effect on the waves. After meeting, they continue to move forward as they did before the encounter.