Eclipses show that light travels in straight lines
The first example is a common phenomena present with light; shadows. Whenever an opaque object comes into the path of light, the light rays stop. The light does not move around or through the object. The result is a dark area or shadow wherever the object blocks the light. Solar eclipses form because light from the moon blocks the light of the sun casting a shadow on the earth. Watch the video clip to understand how an eclipse and other shadows form and how this illustrates that light travels in straight lines.
Rays of light
We use straight lines with arrows to represent rays of light. The direction of the arrows show the direction of the the light rays. They are important so do not forget them.
Pin hole cameras show light travels in straight lines
A pin hole camera is the second example of how we can see that light travels in straight lines. It is a simple device that consists of a small box that is black on the inside and has a tiny hole, the size of a pin, on one end. On the other end there is a screen for viewing. If you turn the side with the hole towards a distant tree or a candle perhaps and then look at the screen on the opposite end, you will notice an image. The image may be fuzzy depending on how far away or near the object is. But the interesting thing about the image is that it is upside down, or as we say in physics, the image is inverted. This inversion is what is proof that the light travels in straight lines.
As we can see in the diagram, the light rays from the top of the tree that fall through the pin hole hit the bottom of the screen. Similarly, the light rays from the bottom of the tree that pass through the pin hole hit the top of screen. If light did not travel in a straight line, this would not be the case and the image would not be inverted.
Making a pin hole camera
Want to make your own pin hole camera? Watch this short clip and see how easy it is.