Purpose
To demonstrate how light rays interact with smooth surfaces to form reflections.
Additional information
Mirrors perform the most common means of specular light reflection. Mirrors are typically constructed of a smooth panel of glass that is in front of a metallic coating. The reflection is enhanced in the metals by suppression of wave propagation beyond their skin depths.
When light rays hit a rough or granular surface, the rays are bounced off in all directions. This is due to the microscopic anomalies of the surface, causing an image (reflection) not to form. This behavior is best known as diffuse reflection, where the energy is retained but the image is not.
When light hits a black object and can't pass through, it is absorbed and changed into heat energy. If light hits a white object and can't pass through, it is reflected back. You can actually feel this phenomena in summer time. If you wear black clothing in the hot sun, you'll feel hotter than if you were wearing light colors or white.
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Required materials
- Roll of aluminum foil
- Scissors
Estimated Experiment Time
Less than 10 minutes
Step-By-Step Procedure
- 1. Cut a 12 inch length of foil from the aluminum foil roll. Be careful to keep the surface of the foil smooth.
- 2. Lay the foil on a flat surface, such as a table or tile floor.
- 3. Look at your reflection on the shinny side of the foil. Even though it's a bit blurred and jagged, you'll still see your reflection clearly.
- 4. Scrunch and wrinkle the foil until the surface is no longer smooth.
- 5. Spread the foil back onto a smooth surface.
- 6. Look at your reflection again. Like magic, your reflection has disappeared!
Note
Make sure to keep the foil smooth when rolling it out and cutting it.
Observation
The disappearing reflection can also be witnessed when a mirror surface is broken and cracked. When the surface is smooth, you can see a clear reflection. As soon as the mirror gets spider-web cracks, your reflection is distorted or goes away completely.
Can you think of other instances when you can see a clear reflection, only to have it disappear when the surface is disturbed? How about ponds and other bodies of water?
Result
Light reflects from a surface in straight lines. When the surface is smooth the light rays reflect back at you. When the surface is scrunched and wrinkled, the reflected light bounces in all directions and your image seems to have disappeared.
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