Refraction and Lenses

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

2. 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.

3. 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.” ni sin i = nr sin r

4. 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. ns = c/vs (vs is the speed in the substance) c = 3.00 x 108 m/s

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.