Chapter 17-1

How Light Behaves at a Boundary

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

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

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

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.

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

Light when it reaches a boundary can be both refracted and reflected:

Refraction: which way will the light bend?

• More dense to less dense:

Bend away from normal

• Less dense to more dense:

Bend toward normal

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

Light

• In a more optically dense medium – Travels slower– has a higher index of refraction– a smaller angle of refraction– bends toward the normal

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

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

Index of refraction of water = 1.33

End 17-1