Refraction and Lenses Light bends--so you can see better!

Refraction Refraction is the bending of a ray of light at the place where the medium changes. The change in medium results in change in speed. Refraction occurs only when the incident ray hits the boundary at an angle.

Snell’s Law “A ray of light bends in such a way that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.” n i sin  i = n r sin  r

Index of Refraction The index of refraction (abbreviated n) is a measurement of how much light bends when passing into a medium from vacuum. n is calculated as the ratio of the speed of light in vacuum to the speed of light in the substance. n s = c/v s (v s is the speed in the substance) c = 3.00 x 10 8 m/s

Total Internal Reflection Total internal reflection occurs when the angle of refraction is greater than the angle of incidence. When this occurs, there is no refracted ray. At the critical angle, the light travels along the boundary of the substance.

Effects of Refraction Mirage—differences in temperature of air causes light to be refracted, giving the appearance of a puddle. Objects submerged in water appear to be in different position than their actual location.

Dispersion of Light In media other than a vacuum, different colors of light are refracted by differing amounts. Dispersion is the separation of light into a spectrum by refraction. A rainbow is formed by the dispersion of light by water droplets in the atmosphere.

The Convex Lens A convex lens is thicker in the middle than on the ends. Convex lenses are converging lenses. Creates an inverted, smaller real image on the side opposite the object. Creates an erect, magnified virtual image on the same side as the object.

The Concave Lens A concave lens is thinner in the middle and thicker on the ends. Concave lenses are diverging lenses. An erect virtual image is formed on the same side of the lens as the object. The image is reduced is size.

A Lens Comparison A converging or convex lens brings light to the focal point. A diverging or concave lens directs the light away from the focal point.

Chromatic Aberration The edges of a lens can act as a prism. This causes chromatic aberration—the image may be ringed with color. This effect is minimized by combining two lenses.

Eyeglasses Farsighted people see far away objects clearly. Their eyeballs are too short. Farsightedness is corrected by a converging, or convex, lens. Nearsighted people see nearby objects clearly. Their eyeballs are too long. Nearsightedness is corrected by a diverging, or concave, lens.