It has been known since ancient times that light passing through a small hole in an opaque barrier casts a dim but full-color image of the outside world onto a wall, as shown in FIGURE 34.5a. Such a device is known as a camera obscura, Latin for “dark room.” A pinhole camera is a miniature version of a camera obscura. But there is one problem with viewing the image: It’s upside down!

FIGURE 34.5 A camera obscura.

FIGURE 34.5 Full Alternative Text

A hole through which light passes is called an aperture. FIGURE 34.5b uses the ray model of light passing through a small aperture to explain how the camera obscura works. Each point on an object emits light rays in all directions, but only a very few of these rays pass through the aperture and reach the back wall. As the figure illustrates, the geometry of the rays causes the image to be upside down.

Actually, as you may have realized, each point on the object illuminates a small but extended patch on the wall. This is because the non-zero size of the aperture—needed for the image to be bright enough to see—allows several rays from each point on the object to pass through at slightly different angles. As a result, the image is slightly blurred and out of focus. (Diffraction also becomes an issue if the hole gets too small.) We’ll later discover how a modern camera, with a lens, improves on the camera obscura.

You can see from the similar triangles in Figure 34.5b that the object and image heights are related by

where $d_{\text{o}}$ is the distance to the object and $d_{\text{i}}$ is the depth of the camera obscura. Any realistic camera obscura has $d_{\text{i}}<d_{\text{o}};$ thus the image is smaller than the object.

We can apply the ray model to more complex apertures, such as the L-shaped aperture in FIGURE 34.6. The pattern of light on the screen is found by tracing all the straight-line paths—the ray trajectories—that start from the point source and pass through the aperture. We will see an enlarged L on the screen, with a sharp boundary between the image and the dark shadow.

FIGURE 34.6 Light through an aperture.