Lens and Mirror Applet Lens and Mirror Diagrams
Convex Lens in Water H 2 O Glass Glass Air Because glass has a higher index of refraction that water the convex lens at the left will still converge light, but it will converge at a greater distance from the lens that it normally would in air. This is due to the fact that the difference in index of refraction between water and glass is small compared to that of air and glass. A large difference in index of refraction means a greater change in speed of light at the interface and, hence, a more dramatic change of direction.
Air Glass n = 1.5 Air H 2 O n = 1.33 Since water has a higher index of refraction than air, a convex lens made of water will converge light just as a glass lens of the same shape. However, the glass lens will have a smaller focal length than the water lens (provided the lenses are of same shape) because glass has an index of refraction greater than that of water. Since there is a bigger difference in refractive index at the air-glass interface than at the air-water interface, the glass lens will bend light more than the water lens. Convex Lens Made of Water
Air & Water Lenses H 2 O Convex lens made of Air Concave lens made of H 2 O Air On the left is depicted a concave lens filled with water, and light rays entering it from an air-filled environment. Water has a higher index than air, so the rays diverge just like they do with a glass lens. To the right is an air-filled convex lens submerged in water. Instead of converging the light, the rays diverge because air has a lower index than water. What would be the situation with a concave lens made of air submerged in water?
Chromatic Aberration As in a raindrop or a prism, different wave- lengths of light are refracted at different angles (higher frequency ↔ greater bending). The light passing through a lens is slightly dispersed, so objects viewed through lenses will be ringed with color. This is known as chromatic aberration and it will always be present when a single lens is used. Chromatic aberration can be greatly reduced when a convex lens is combined with a concave lens with a different index of refraction. The dispersion caused by the convex lens will be almost canceled by the dispersion caused by the concave lens. Lenses such as this are called achromatic lenses and are used in all precision optical instruments. Chromatic Aberration Achromatic Lens Examples
Human eye Human eye w/rays The human eye is a fluid-filled object that focuses images of objects on the retina. The cornea, with an index of refraction of about 1.38, is where most of the refraction occurs. Some of this light will then passes through the pupil opening into the lens, with an index of refraction of about 1.44. The lens is flexi- ble and the ciliary muscles contract or relax to change its shape and focal length. When the muscles relax, the lens flattens and the focal length becomes longer so that distant objects can be focused on the retina. When the muscles contract, the lens is pushed into a more convex shape and the focal length is shortened so that close objects can be focused on the retina. The retina contains rods and cones to detect the intensity and frequency of the light and send impulses to the brain along the optic nerve.
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- Fall '11
- Light, Total internal reflection, Geometrical optics, rays