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Chapter 17-1
How Light Behaves at a Boundary
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The Law of Reflection
The angle of incidence equals the angle of reflection
The angles are measured to the “normal” line which is perpendicular to the reflecting surface
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Regular Reflection• Angle of
incidence equals the angle of reflection
• a smooth surface like a mirror causes regular reflection
• light is reflected back to the observer in parallel beams
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Diffuse Reflection• Angle of incidence
equals angle of reflection
• Reflection from a rough surface results in diffuse reflection
• light is reflected into a wide variety of angles
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Refraction of Light
• Refraction occurs when light changes direction at a boundary between media with different optical densities.
• Remember: wave speeds are different in different media.
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Snell’s Law• How does the angle of refraction depend on the angle
of incidence?• Dutch scientist: Willebrord Snell, 1621• Ratio of sines of angles of incidence and refraction are
a constant• The constant is called the Index of Refraction, n
• or, in the more general case, written as
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Light when it reaches a boundary can be both refracted and reflected:
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Refraction: which way will the light bend?
• More dense to less dense:
Bend away from normal
• Less dense to more dense:
Bend toward normal
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Light
In a less optically dense medium – Travels faster – has a lower index of refraction– a greater angle of refraction– bends away from the normal
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Light
• In a more optically dense medium – Travels slower– has a higher index of refraction– a smaller angle of refraction– bends toward the normal
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Index of Refraction: a measure of the amount that light will bend
n of glass is less than n of water—light bends less in glass than in water
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Index of Refraction and the Speed of Light in that material
Reviewing: Index of Refraction:
• A measure of the opitical density of a material
• How much light bends when it passes into the medium
• How fast light travels in the medium
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Index of refraction of water = 1.33
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End 17-1