Pinhole_Viewer_Explained

http://www.sci-experiments.com/pinhole_camera/pinhole_revised2.html

How does a pinhole let you see the images carried by light? And why are these images upside-down and backward?
 * What's going on**

The simplest way to understand pinhole images is to take a look at what happens to light rays radiating from an object, such as a candle and it's flame, that vou are looking at with your pinhole viewer. Light radiates from all parts of the flame, and bounces off the candle body. A pinhole lets through only a portion of these light rays. Most of these rays are blocked bv the cardboard surrounding the pinhole. Only a few of the rays of light from the flame or candle are heading in just the right direction to shine through the pinhole of your viewer. Light rays from the top of the flame must slope down to pass through the pinhole. These rays hit the **bottom** of the screen. Rays of light from the base of the flame or candle itself must slope up to pass through the pinhole. These upward moving rays will hit the **top** of the screen. Think about this: if the rays from the top of the flame hit the bottom of the screen, and the rays from the bottom of the flame hit the top of the screen, then it seems like you should see an upside-down picture of the flame on your screen. (And you do!) The same logic applies to right and left so that the image is also reversed this way.

The flame has more points than the top and bottom. The same rules apply to light rays emanating from every part of the flame, and reflecting from every point on the body of the candle: only the light rays that are heading in just the right direction will pass through the pinhole. On the other side, these rays of light line up geometrically to make an upside-down and backward image of the flame and candle.

(Light goes in a straight line, so the light from the bottom of the triangle goes to the top and the light from the top of the triangle goes to the bottom.)