Now imagine two bulbs sending light through a pinhole (Figure 2). While there are rays of light emanating from each point on the light bulbs, for simplicity, only those rays which get through the pinhole have been drawn. The ray model shows that the light rays from each bulb cross as they go through the pinhole producing reversed images on the screen.
Finally, look at an extended light source, such as a long fluorescent tube or an object. In Figure 3, the extended light source is represented by an arrow. Light rays go out in all directions from every point on the arrow. To illustrate how images are reversed we have simplified this drawing to show only three of the light rays passing through the pinhole. (To show more of the light rays would potentially be confusing, and would not illustrate so clearly what is happening.)
This is what you can see in Figure 3:
The image on the screen is therefore, reversed. This same reasoning explains all reversals; left-to-right, top-to-bottom and everything in between.
Footnotes about Key Conceptual Points
1. Your eye is a light detector
Your eye is a detector of light. All it can sense is the light that enters your eye and the position that it lands on the retina. The key inference here is that if you see something, then light from that object must be entering your eye.
2. Light travels from a source in all directions
Light travels out from a source. It travels out in all directions. We can see a candle flame from many different positions in a room because light travels from that flame out in all those different direction. We know it went from the flame to those positions because we can put our "portable light detector"(our eye) at those positions and detect (see) light from the candle flame.
3. Light must reflect off objects in order for them to be seen
Since all your eye can do is detect light, anything you see has light traveling from it to your eye. You can see objects that are not self-luminous because light from the sun, light bulbs or other sources reflects off these objects and enters your eye.
In a room lit by a candle, you can see an object such as a chair because some of the candle's light that is traveling to your eye, hits the chair, reflects off of the chair and then transports the image of the chair to your eye.
We can see a pinhole image of objects that don't make their own light, such as a pencil placed into one of the holes at the top of a Table Top Light Source, because light from the bulb reflects off the pencil and travels through the pinhole onto a screen or the table.
4. Light travels in straight lines
Textbooks often refer to "light rays". There really is no such thing as a ray, an infinitely thin beam of light. But thinking about light in terms of rays can make understanding it much easier. You can picture light as traveling out from objects in straight lines.
The light ray model of how light travels is useful because it is consistent with many phenomena that we observe. In the case of pinholes, the observation that you can draw a straight line between a light source, a pinhole, and the image of the light source on a screen, is consistent with a straight line, ray model of light.
Some textbooks describe light being able to bend. This can be confusing when trying to reconcile this thought with light traveling in a straight line. Light bends when it travels from one medium to another, such as from air into a prism or lens. This does not happen in the case of pinholes.
Additional Background Information
The notion that light travels in infinitely thin, straight lines and emanates from every point on a light source in all directions is complicated, and an abstraction of the real world. One example (pinholes) is not necessarily convincing, so we are providing an additional situation (shadows) where the model explains the phenomenon. These examples are meant to give you some additional ways to feel more comfortable with the difficult concept of light rays.
The following information is not crucial to understanding or facilitating pinhole investigations.
More on Light Traveling in Straight Lines:
Imagine holding a "point source" of light, such as a "Mini Maglite" with its top removed, in the middle of a room. If you hold up an opaque piece of paper, it will block some of the light and cast a shadow on the wall. If you draw a line from the light to the edge of the paper and then extend this line to the wall, it will hit the edge of the shadow (Figure 4).
Figure 5 shows that if you stand in the shadow looking in the direction of the light you will not see the light. If you move sideways to the point where you just begin to see the light, your eye will be on the line connecting the light, and the edge of the paper.
Figure 6 shows that if you poke a hole in the middle of the paper, you will get a spot of light in the midst of the shadow. If you draw a line from the light source to the hole and extend that line to the wall it will hit the wall right at the spot of light.
These and many other phenomena suggest that light travels outward from a light source in straight lines or rays. A more complete picture of the shadow (Figure 7) would include many rays from the light source The shadow is formed in the area where the rays are blocked.