Refraction: A Real Light Bender

Light normally travels in a straight line, but under certain conditions it will bend. The speed at which light is traveling changes when light moves from a medium of one density into a medium of a different density. For example, light slows when it moves from air into water, a denser medium. The slowing causes the light waves to change direction, or bend. The bending of light as it passes from one medium into another is known as refraction. In this activity you will observe this bending phenomenon firsthand.

Materials

4'' × 6'' unruled index card; Black marker; Ruler; Drinking glass; Water

Activity

1. Lay the index card on a table.
2. Use the ruler and marker to draw a vertical line down the center of the index card parallel to the long edges of the card. The line should divide the card into two equal halves.
3. Place the bottom of the drinking glass on top of the line. Looking down into the glass, adjust its position so the line appears to divide the bottom of the glass in half.
4. While you continue to look down into the glass, pour water into the glass. Note what happens to the line in the bottom of the glass as you add water.

Follow-Up Questions

1. What happened to the line in the bottom of the glass as water was added?
2. What caused the change in the appearance of the line?
3. Use what you learned today to explain why a fish you see swimming in the water is not located in exactly the place you think it is.

Answers

1. The line appeared to move to one side as you poured water into the glass.
2. Light slows when it moves from air into water, a denser medium. The slowing causes the light waves to change direction, or bend, causing a change in the appearance of the line.
3. Answers will vary, but students should indicate that the refraction of light from air into denser water creates an illusion.

Extension

Fill a glass half full of water. Place a drinking straw into the water. Look at the straw from both the top and the bottom of the glass. Look at it from the side of the glass. Focus on the point where the straw enters the water. Why does the straw look different depending on the angle of observation?