Refraction of Light

Refraction: is a property of light in which the speed of light and its direction of travel change as it goes from one medium into another

Angle of incidence (i) Angle of refraction (R) normal Refraction Medium 1 e.g. air Medium 2 e.g. water boundary: surface between 2 media Incident ray Refracted ray Note: Angles are always measured with respect to the normal In this example, the light was refracted “toward the normal” not refracted Refracted “away from the normal” Refracted “toward the normal”

Refraction Note: When light travels from a medium in which its speed is higher (such as air) into a medium in which its speed is lower (such as water), the refracted light ray bends toward the normal.

Refraction Note: When light travels from a medium in which its speed is lower (such as water) into a medium in which its speed is higher (such as air), the refracted light ray bends away from the normal.

Refraction: The Bending of Wavefronts One side of wave front slows down, and the entire train of fronts twists. wave front: the crests, or high points, of the waves

Refraction – An Analogy Refraction can be understood through this analogy. Imagine that the fronts are rows of soldiers and the rays are lines of soldiers. The soldiers move more slowly through mud. If they approach it at an angle, then the soldiers on the left will reach the mud first, and slow down first. This will cause the lines of soldiers (the rays) to bend. See p. 450 Figure 11.4 (an analogy)

Understanding Refraction Fermat’s Principle: When light travels from one point to another, it follows the path that will take the least time. In a single medium  this is a straight line When traveling from one medium to another  this is not a straight line

Refraction in the Real World Here we start with a straight rod used for cleaning pools. By placing this rod into the swimming pool, we see that it appears to bend. This is refraction at work!

Refraction in the Real World Pictured to the right is a polar bear at the San Diego Zoo as seen from the observer’s gallery. The bear’s head is seen through only air (with a layer of glass causing a small amount of lateral displacement), while the bear’s body is seen through water and then air, resulting in refraction of the light rays and an apparent shift in the location of the body. A large stick that the bear is playing with is undergoing a similar distortion.

Refraction in the Real World

Index of Refraction is the ratio of the speed of light in a vacuum to the speed of light in a medium Note: c = 3.00 x 10 8 m/s

Some common indices of refraction: See page 454, Table 11.1 in your text MaterialIndex of refraction, n Vacuum Air (at STP) Water1.33 Normal glass1.52 Crystal glass1.54 – 1.75 Sapphire1.77 Zirconium1.92 Diamond2.42 Speed of light fastest slowest

Wavefronts from a Point Source The region below the gray line has a higher index of refraction and proportionally lower wave velocity than the region above it. Therefore the light ray bends… toward the normal.

Calculate the speed of light in water. n = 1.33 c = 3.00 x 10 8 m/s = 2.26 x 10 8 m/s Sample Problem

Dispersion: the process of separating colours by refraction Note: White light includes all wavelengths of visible light.

glass air Glass Prism normal Incident light ROYGBIVROYGBIV Note: The light refracts twice... 1.when it enters the prism 2.when it leaves the prism Refer to Figure 11.8 on page 453

Dispersion of Light Note: Since each colour of light travels at a slightly different speed in any medium, each colour of light refracts a different amount. The shorter wavelengths of light travel slower & are bent more than the longer wavelengths; blue more than red.

Homework Read page 455 (Sample problem) Do Practice Problems p. 455 # 1 – 4 Page 456 # 1 & 5 Page 482, # 8 & 17