Lens and Light Systems

Figure 141.

Rainbows are made by a combination of reflection and refraction of light, which results in dispersion of the light. In order to make a rainbow, there needs to be water in the atmosphere that will bend light according to the differences in the way light bends by wavelength. It is seen to a greater effect when the background is dark as with dark clouds in stormy weather.

LENS AND LIGHT SYSTEMS

Lenses are found in many different types of optical instruments. They make use of the law of refraction in order to do many things that affect light. Lenses are traditionally thought of as being convex, meaning they have a curvature that is outward in the middle. Convex lenses are referred to as converging lenses. The index of refraction of a lens will be greater than that of air.

As you can see from figure 142, there is a point at which the rays of light cross. This is called the focal point or big letter F of the lens. The distance from the center of the lens to its focal point is called its focal length, shown by the small letter f.

Figure 142.

A more powerful lens has a greater effect on light rays. A powerful converging lens will focus parallel light rays closer to itself and will have a smaller focal length. The light will focus onto a smaller spot in a powerful lens. The power P of a lens is the inverse of its focal length or one divided by its focal length. The unit of power in a lens is called the diopter, given by the big letter D. This is different from the power seen in the rest of physics and is described as such: One diopter equals 1 meter to the minus one power.

Lenses can also be diverging lenses, in which they are concave. Light will bend away from the axis so that light appears to originate from the same point, which is the focal point of the lens. There is also a focal distance, which are defined to be negative because these are found before the lens and before light enters it, rather than after light exits the lens. The power of the lens will be negative in diopters as well. Figure 143 shows a diverging lens:

Figure 143.

One can trace the paths that light rays take through a lens or through any matter. A thin lens is one in which light will refract but will not have a large amount of dispersion or aberrations. An ideal thin lens has two refracting surfaces but the lens is so thin that it can be assumed that light rays will bend only once. A thin lens will have the same focal length on either side of the lens.

There are two distances to note when looking at a lens. There is the object distance and the image distance. The image distance is the distance of the image from the center of a lens. This can determine the height of the object before and after the image is seen through the lens. The height of the image divided by the height of the object in real life is referred to as the magnification, the small letter m, of the lens. This is seen in figure 144: