Exploring Some Characteristics of Light

Purpose: The purpose is to understand how a light security system works.

Overview: Can you see a beam of light? You can certainly see a lit light bulb, the sun, a candle’s flame, or any source of light. You can also see objects because light is shining on them. But, you are normally not able to see the light beam itself. the path of the beam through the air can, however, be seen by filling the air can, however, be seen by filling the air with tiny particles s the light will reflect off them.

In a dark room, lay a flashlight on a table and shake several facial tissues in the air. The light will reflect (bounce) off the tiny particles of tissue, allowing you to see the path of the light beam. Similarly, have you ever seen the rays of sunlight shine from behind breaks in clouds? Have you ever seen the light beams coming from the headlights you were riding in on a dark, foggy evening?

Because we can’t actually see a beam of light, some home security systems use light to detect if someone walks through the room. A light source shines into an electronic device that detects the light. Everything is fine as long as light is shining on it. But when the light is interrupted when someone walks between the light source and the security device, it senses that the light has gone out, and it sounds an alarm. To make the light less detectable by anyone, a red filter is used to reduce the light reflected by any tiny particles that may be in the air.

Hypothesis: We can detect a person walking in another room using a light source and a mirror.

You need:

  • Lamp;
  • Hand-held mirror;
  • Modeling clay;
  • Small piece of cardboard;
  • Flashlight;
  • Several facial tissues;
  • Dark room;
  • A friend;
  • Table.

Procedure: Set up a demonstration of how a home security system might work. Lay a hand mirror on its side on a piece of cardboard. With modeling clay, build a base around it so the mirror will stand up by itself, as shown.

Set the mirror on a table or bookshelf, and adjust it so that the light from the lamp is reflected into another room. It should be a room that you can make fairly dark. Stand against the wall and look out the door at the mirror. Have a friend adjust the mirror until you can see the lamp in it. Then you know the mirror is lined up. This demonstrates another characteristic about light, light travels in a straight line unless something interferes.

Go into the dark room and close the door until it is only open enough to let a slit of light in to shine against the wall opposite the door.

Watch the light on the wall as you have your friend walk around the room. Can you detect when your friend steps in the path of the light?

Results and Conclusion: Write down the results of your experiment. Come to a conclusion as to whether or not your hypothesis was correct.

Something more: You can tell the direction of a person walking through the room by adding a second mirror a few feet to either side of the first mirror, so that a second light spot shines on the wall. Then, if a person walks by, they will break one beam before the other. The beam that is broken first tells you in which direction the person is walking.