Refraction

Refraction Two things happen when a light ray crosses the boundary between the air and the glass: Part of the light reflects from the boundary, obeying the law of reflection. This is how you see the reflections from pools of water or storefront windows, even though water and glass are transparent.

Refraction Part of the light continues into the second medium. It is transmitted rather than reflected, but the transmitted ray changes direction as it crosses the boundary. The transmission of light from one medium to another, but with a change in direction, is called refraction.

Refraction This figure shows the refraction of light rays from a parallel beam of light, such as a laser beam, and rays from a source. An infinite number of rays are incident on the boundary, although for simplicity we focus on a single light ray. Reflection occurs at the boundary, but is usually very weak and is ignored here.

Refraction The angle between the incident ray and the normal is the angle of incidence. The angle on the transmitted side, measured from the normal, is called the angle of refraction.

Refraction The angles are the same for the ray entering Medium 2 in the first figure and the ray exiting Medium 2 in the second figure.

Refraction In 1621, Dutch scientist Willebrord Snell proposed a mathematical statement of the “law of refraction” now called Snell’s Law: The index of refraction determines how much a light ray is bent when crossing the boundary between two different media (a consequence of the change in the speed of light as it crosses a boundary.)

Question 1 A light ray enters a glass prism as shown. Which is a possible path for the ray through the prism? A B C D Answer: D 8

Question 1 A light ray enters a glass prism as shown. Which is a possible path for the ray through the prism? A B C D Answer: D D 9

Total Internal Reflection Crossing a boundary into a material with a lower index of refraction causes the ray to bend away from the normal. As angle 1 increases, the refraction angle  2 approaches 90°. The fraction of light energy that is transmitted decreases while the fraction reflected increases.

Total Internal Reflection A critical angle c is reached when  2 = 90°. The refracted light vanishes at the critical angle; there is only reflected light. There is no critical angle and no total internal reflection if n2 > n1.

Question 2 A fish in an aquarium with flat sides looks out at a hungry cat. To the fish, the distance to the cat appears to be Less than the actual distance. Equal to the actual distance. More than the actual distance. Answer: C 12

Question 2 A fish in an aquarium with flat sides looks out at a hungry cat. To the fish, the distance to the cat appears to be Less than the actual distance. Equal to the actual distance. More than the actual distance. 13

Fiber Optics Fiber optics use total internal reflection for the transmission of light through optical fibers. Light rays pass into the narrow-diameter glass fiber, but then strike the inside wall of the fiber an at angle of incidence approaching 90°. This is larger than the critical angle, so the light undergoes TIR and remains inside the glass.

Fiber Optics When the light rays reach the flat end of the fiber, the angle of incidence is lower and the light can cross the boundary. To protect it from external damage, a glass cladding surrounds the glass core. Light undergoes TIR at the cladding boundary, and remains within the core. Endoscopes made from optical fibers are used for anthroscopic surgery.