Pinhole Images --- What's Happening Here?

Pinhole image phenomena can be described using several conceptual models. The two models described here are (1) a model of light as information carried in the form of images, and (2) a more conventional ray model of light. Both models give accurate descriptions of pinhole phenomena. They are meant to help you develop your own conceptual model of pinhole phenomena based on your personal experience with and exploration of pinhole image phenomena.

One or the other of these models may be more useful in helping to understand a particular question or facilitate an investigation of that question. For example, the image model of light may be more useful in understanding why a large pinhole produces a fuzzier image than a small pinhole, while the ray model of light may be more useful in understanding why pinhole images are upside down and left-right reversed. For this reason, we are describing both models, and some major concepts that underlie both of them.

The four basic concepts that are important to consider in understanding pinhole phenomena.

1. Your eye is a light detector

2. Light travels from a source in all directions at once

3. Light must reflect off objects in order for them to be seen

4. Light travels in straight lines

We use footnotes to connect the model descriptions to these underlying concepts.

Light as Images Model

Bob Miller, an artist at the Exploratorium, developed a model of light as information in the form of images. This model starts with the realization that what we see is light.1 The images that we perceive, both of light sources and of objects that don't create their own light, come to us in this light.2,3

In fact, Bob's model describes all light as being made up of images. An image walk with Bob Miller (see the Bob Miller's Image Walk magazine) starts under a tree where sunlight makes round spots of light on the ground which he realized were images of the sun. The reason that we can't see these round images in every shaft of sunlight is not because the images aren't there. It is because there are so many images coming in all directions from each point on the sun that our perception of all the resulting multiple, overlapping images a just a blur of light.

A hole in an opaque sheet of paper (or between leaves on a tree) lets through an image of the sun. More important, the rest of the paper blocks a multitude of images and allows us to see a single image, unconfused by the blur of all the other overlapping images.

The Light Ray Model

One of the aspects of pinhole images that inevitably catches people's attention is that the images are inverted or "flipped," top to bottom and left to right. The ray model4 provides the simplest explanation for this image inversion as well as a powerful way of understanding other phenomena. On the following pages, we will build a step by step description of how the ray model explains inversion of images.

In the workshop, we used light bulbs in the tabletop light sources. However, in this drawing, we show a point source of light instead of a light bulb. On a light bulb, light rays are emanating in all directions from each point on the light bulb so you can think of a light bulb as a collection of point sources of light. This is too complicated to draw so we are illustrating the point source for the sake of simplicity.

Figure 1 shows how a pinhole allows one ray of light from a point source of light (such as a Mini Maglite flashlight with the top removed) through to a wall or screen while surrounding rays are blocked.2 The position of the spot of light is at the end of a straight line drawn from the light through the hole to the screen. The ray that gets through the pinhole determines the position of the image.4

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 light from the top point of the arrow goes through the pinhole and hits the screen on the bottom.
• The light from the middle of the arrow goes through the pinhole and hits the screen in the middle.
• The light from the bottom tail of the arrow goes through the pinhole and hits the screen on the top.

The image on the screen is therefore, reversed. This same reasoning explains all reversals; left-to-right, top-to-bottom and everything in between.

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.

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.

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