# Reflection and Refraction

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Reflection and Refraction
Chapter 22

Wave Fronts and Rays Wave fronts are adjacent portions of a wave that are in phase. Rays are used to show the direction of wave travel.

Light / E-M Radiation When light hits a surface, a portion may be reflected, a portion may be transmitted and a portion may be absorbed. Light is actually absorbed and re-emitted Light and E-M radiation travels at c=3.0 X 108 m/s

Reflection The Law of Reflection states that the angle of incidence is equal to the angle of reflection. Angles are measured with respect to the ‘normal’ line. θi = θr Law of Reflection

Reflection When a surface is flat and smooth, regular reflection takes place. θi = θr When the reflecting surface is not smooth, diffuse or irregular reflection occurs.

Reflection Eyes assume rays travel in straight lines. Your eye projects the image into the water.

Example Two mirrors, M1 and M2, are perpendicular to each other. A ray of light is incident on M1 with an angle of 60°. Sketch a diagram to determine the direction of the ray after it leaves M2.

Refraction Refraction is the bending of a wave at a boundary.
Wave speed changes as it passes from one medium to another; if the wave enters at an angle, it will bend. The amount of bending is determined by the relative densities of the two materials.

Refraction When a wave passes into a more dense medium, it will refract towards the normal line. Angles of incidence and refraction are measured from the normal. When a wave passes into a less dense medium, it bends away from the normal line.

Refraction The speed of the wave (and hence the angle of refraction) depends on the density of the medium. The index of refraction, n, gives a measure of the wave speed. n = c/v where c = 3.0 X 108 m/s

Snell’s Law n1sinθ1 = n2sin θ2

Example Light from a laser with wavelength nm travels from air into water. What are the speed and wavelength of laser light in water? nw = 1.33

Example Light is incident on crown glass at an angle of 37º. nc = 1.52
a) will the transmitted ray be bent towards the normal or away? b) find the angle of refraction

Critical Angle When light is refracted from a more dense to a less dense media, it bends away from the normal. At some incident angle, the refracted ray is along the surface. Beyond this incident angle, all of the wave is reflected. This angle is the critical angle.

Total Internal Reflection
Demonstration, Total Internal Reflection When light travels into a less dense medium, it refracts away from the normal line. As the incident angle increases, so does the refracted angle When the refracted angle equals 90º , the incident angle is called the critical angle. Incident angles larger than the critical angle result in total internal reflection (no transmission)

Critical Angle n1sinθ1 = n2sin θ2
At an incident angle called the critical angle, θc, the refracted angle is 90º n1sinθc = n2sin 90 sin θc = 1/n1 for n2 = 1

Example What is the critical angle for light traveling in water and incident on a water-air boundary? If a diver submerged in a pool looked up at the surface of the water at an angle less than the critical angle, what would she see?

Fiber Optics Fiber optic cables are cables designed to transmit light using total internal reflection. See examples on page 719

Dispersion Monochromatic light refers to light of only one frequency.
White light contains all colors (frequencies) of the rainbow. Dispersion is the spreading of light due to a slight difference in speed for each wavelength of e-m radiation.

Dispersion The more the bending, the greater the change in speed.
Index of refraction for red is less than index of refraction for violet.

Rainbow A rainbow is caused by both total internal reflection (inside the droplet of water) and by dispersion by each droplet.

Homework Pg. 726 # 30, , 36, 38, 42, 43, 46, 50, 55, 61, 62, 67