Activity 8
Effect of Lenses on Light

A light bulb sends out light in all directions. If a few of these rays pass through a pinhole, and the rest are blocked, a dim image of the bulb can be seen on a screen. Opening up the pinhole makes the image brighter but also less sharp. Only if the light is refracted can the image be both bright and sharp. A convex lens bends light to make such an image. Look at the lower ray diagram. It shows two of the countless rays that fan out from each point on the object.

One ray enters the lens parallel to its axis, so this ray is refracted through the focus, the position of the image when the object is extremely distant. The other ray goes through the center of the lens. If the lens is thin, and the object is small, this ray maintains its original direction, since the front and back of the lens are parallel right at the center. Where these two rays meet is the image of the arrowhead. The image is upside down. Also, the image is smaller than the object and is located between 2f and f (f is the focal length) from the lens. The object is beyond 2f. Notice that the object and image could be interchanged, without changing the ray diagram. When the object is close to the lens, the image is far away and larger than the object. When the image is close to the lens, the object is far away and larger than the image. The object and image can be equidistant from the lens, when each is located at 2f. (See the upper ray diagram.) In this symmetrical arrangement, the object is the same size as the image.

Projecting a slide is challenging to analyze. Think about Steps 7 and 8, where a student holds a card with an arrow-shaped cutout between the bulb and the lens. The lens projects the arrow on the screen.

Since the light bulb is an extended source, light rays from the bulb cross at each point in the cutout. In effect, the cutout arrow acts like an extended light source too. This extended source has a sharp edge right at the card, so a sharp arrow can be projected on the screen. See below for notes on changing the size of the projected image.