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Chapter 23
Lecture
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Chapter 23: Reflection and
Refraction of Light
•Reflection
•Refraction
•Total Internal Reflection
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§23.2 Reflection of Light
When light is reflected from a smooth surface the rays
incident at a given angle are reflected at the same angle.
This is specular reflection.
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Reflection from a rough surface is called diffuse reflection.
“Smooth” and “rough” are determined based on the
wavelength of the incident rays.
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The angle of incidence equals the angle of reflection. The
incident ray, reflected ray, and normal all lie in the same
plane. The incident ray and reflected ray are on opposite
sides of the normal.
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§23.3 Refraction of Light
When light rays pass from one medium to another they
change direction. This is called refraction.
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Snell’s Law:
n1 sin 1  n2 sin 2
where the subscripts refer
to the two different media.
The angles are measured
from the normal.
When going from high n to low n, the ray will bend away
from the normal.
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The incident ray, transmitted ray, and normal all lie in the
same plane.
The incident and transmitted rays are on opposite sides of
the normal.
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Example (text problem 23.11): Sunlight strikes the surface of
a lake. A diver sees the Sun at an angle of 42.0° with respect
to the vertical. What angle do the Sun’s rays in air make with
the vertical?
incident wave
1
n1 = 1.00; air
surface
n2 = 1.33; water
42°
Transmitted
wave
n1 sin 1  n2 sin  2
1.00sin 1  1.333sin 42
Normal
sin 1  0.8920
1  63.1
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§23.4 Total Internal Reflection
The angle of incidence for when the angle of refraction is
90° is called the critical angle c.
n1 sin 1  n2 sin  2
n1 sin  c  n2 sin 90  n2
n2
sin  c 
n1
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If the angle of incidence is greater than or equal to the
critical angle, then no wave is transmitted into the other
medium. The wave is completely reflected from the
boundary.
Total internal reflection can only occur when the incident
medium has a larger index of refraction than the second
medium.
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Example (text problem 23.22): Calculate the critical angle
for sapphire surrounded by air.
2=90
n2 = 1.0; air
n1 = 1.77; sapphire
surface
1
incident wave
Transmitted wave
Normal
n1 sin 1  n2 sin  2
1.77sin  c  1.00sin 90
sin  c  0.565
 c  34.4
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Summary
•The Laws of Reflection
•The Laws of Refraction
•Condition for Total Internal Reflection
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