To show that light travels in straight lines Procedure1. Place three identical cards with a small hole in each about 10 cm apart.
2. Arrange them so that the holes are in a straight line.
3. Look through the holes to see the light.
4. Move one of the cards slightly to one side.
ResultThe light can only be seem when the cards are in a straight line.
ConclusionLight travels in straight lines

Have you guys not done this?