Location: Ch 6: Lenses

LENSES IN THIS CHAPTER:  The Color Spectrum Color by Reflection Color by Transmission Sunlight Mixing Colored Light Complementary Colors Mixing Colored Pigments Why the Sky is Blue Why Sunsets are Red Why Water is Bluish-Green

ight rays bend as they enter and exit glass. When light is passed through specially shaped pieces of glass, this refraction can be used to magnify or shrink an image. Such pieces of glass or plastic are known as lenses. There are two types of lenses, convex and concave. The following diagram illustrates what happens when light rays encounter these two types of lenses.

    Cameras use lenses in order to take pictures at very high speeds and to allow the camera to be used in many different situations. In a simple camera, light is bent through a convex lens in order to form a relatively small image on film. This lens is moved in and out so that the camera can focus on objects of different distances. Telescopes also use lenses in order to focus on distant objects such as stars. There are two types of telescopes, astronomical and terrestrial telescopes. Astronomical telescopes always produce virtual images that seem upside down and depend on lenses to focus light. An astronomical telescope uses an objective lens to focus incoming light which then is refracted through an eyepiece lens in order to redirect the image into the eye. What we really see is a virtual, upside down and enlarged image of what the telescope is focused at. Terrestrial telescopes, on the other hand, often use huge concave mirrors in order to focus light. Terrestrial telescopes have the advantage of less stress on the focusing prism. Binoculars are a special compact version of a terrestrial telescope because it uses prisms to focus light. Compound microscopes are similar to astronomical telescopes. Finally, projectors use a combination of lenses in order to project light. The first two lenses are known as condensers because they direct light through a slide or movie film. After the light filters through the semi-transparent light, it is broadcast out into the room by a final convex lens known as a projection lens.

    The eye is the ultimate use of lenses in order to detect light. When light first enters the eye, it refracts through a jelly of the cornea known as the aqueous humor. The light then passes through the iris. The iris acts like a window shade and regulates the amounts of light entering the eye. The iris’s function is to dilate in dim light to let more light in and contract in bright light to avoid overloading the eye’s nerves. Finally, the light encounters the lens. The lens is suspended in place by ciliary muscles which stretch and compress the lens to focus light. If the eye wants to focus on a far object, the ciliary muscles push on the lens, which makes the lens thin. If the eye wants to focus on a near object, the ciliary muscles pull on the lens, thus causing it to thicken. This adjusting of the lens is known as accommodation. The light then passes through another layer of jelly, the vitreous humor, until it reaches the retina. The retina contains many nerve cells necessary for detecting light. There is one spot on the retina that produces the best images. This area is known as the fovea. There are two types of light-sensitive nerve cells, known as cone and rods. The fovea consists mainly of cones, while the surrounding area contains mostly rods. The reason for this is that cone cells work very well in bright light, while rods work well in dim light. When light waves reach these cones and rods, they produce chemical energy. A sodium-potassium pump sends electrical signals to the brain to produce the sensation of light. During the day, the rod cells are overloaded by too much light and thus stop working. Since the eye uses a convex lens to focus light, the resulting image appears upside-down on the retina. The brain simply learns to flip the image the right way. However, there is one spot on the retina where nothing is seen. This is known as a blind spot where there are neither rods nor cones. The brain simply takes the image from the other eye and fills it in.

    However, eyes (and lenses) are not perfect. Normal vision is the ability to see from infinity down to 25cm. The eyes of a farsighted person are too short, foc using light behind the retina and they require a convex lens to refocus the light. A nearsighted person (like me) can see near objects but other objects are focused in front of the retina. Here the eye is too long and requires a concave lens to refocus light as appropriate. When a lens is curved in more in different directions, an astigmatism results and eyes do not form sharp images, thus requiring cylindrical lenses to fix the problem.

    As previously stated, not all lenses are perfect. Distortions in images are called aberrations. By combining several lenses, the effects of aberrations can be minimized. Spherical aberration results from some light focusing in front of or behind the focal point and can be remedied by covering the sides of a lens. Chromatic aberration results when light partially disperses in a lens. In modern times, however, lenses have been replaced by such techniques as contact lenses or RK surgery to correct the eye’s problems.

    In conclusion, lenses refract parallel light rays so they cross or appear to have crossed at the focal point. A converging lens is thicker in the middle and forms magnified virtual images when an object is within the focal length of the lens. Converging lenses produce real images when the object is far from the lens. Diverging lenses, on the other hand, are thin in the middle and always form reduced virtual images. Several instruments that use lenses are telescopes, cameras, compound microscopes and projectors. Lastly, the human eye refracts light and focuses it on the retina with or without the help of corrective lenses.

Location: Ch 6: Lenses