Refraction

• Refraction is the bending (change in direction) of light when it travels from one medium into another.• Caused by a change in speed

Refraction

How does it bend?

• Wave fronts (dashed lines) slow down when entering glass.• The lower edge slows before the upper edge, so the wave turns to the right.

• Also, the wavelength is shortened.

Lower

edge

Upper

edge

Ray Diagrams

• Light rays reflect and refract.

• If the light slows down, it bends toward the normal line (glass < air).• Angles are measured with the normal line.

• Light rays are reversible.

Law of Refraction

• c = 3 108 m/s

• v is always less than c, so n >1 for all media.• nair = 1.000293

• n is dimensionless.

• n is a measure of the optical density of a material.

Indices of Refraction

Snell’s Law

• Angles must be measured with the normal.

Refraction

• Where does the cat see the fish?

• Where does the fish see the cat?

• Objects appear to be in line with the observed rays.

Practice Problems• A light ray of a wavelength 589nm (produced by a sodium lanp)

traveling through air strikes a smooth, flat slab of crown glass at an angle of 30° to the normal. Find the angle of refraction, θr.

19.2°

Practice Problems• Find the angel refraction for a ray of light that enters a bucket of

water from air at an angle of 25° to the normal.

18.5°

Practice Problems• For an incoming ray of light of vacuum wavelength 589 nm, fill in

the unknown values in the following table.

From Medium To medium θi θr

Flint Glass Crown Glass 25.0°

Air 14.5° 9.80°

Air Diamond 12.5°