If it is not easily seen, sprinkle baby powder or flour in the air above it. Discuss what is seen. Have students draw a conclusion about what they have seen. Give each group a set of 3 index cards and chalk erasers. Have them set the cards in the erasers.
Students pull string through the holes to make sure they are line up.
Instruct students that they are going to prove that light travels in straight lines. One member of the group holds the sheet of construction paper at the back of the cards. Another student shines a flashlight at the opposite end. Remaining group members tell what they observe about the light. Group members then trade places and repeat until everyone has had a turn to use the flashlight.