Lecture 5 refraction of light

Q: What is refraction?

Refraction is the change in direction of light when it passes from one medium to another with a different optical density because its velocity changes.

Q: Why does light refract?

Light changes speed when entering a medium with a different refractive index, causing it to change direction.

Q: Define the angle of incidence and angle of refraction.

Angle of incidence (i) = angle between incident ray and normal.

Angle of refraction (i') = angle between refracted ray and normal.

Q: How does light bend when moving between media of different optical density?

Less dense → more dense = bends towards the normal.

More dense → less dense = bends away from the normal.

Q: State Snell's Law.

nnn = refractive index of first medium

n′n'n′ = refractive index of second medium

iii = angle of incidence

i′i'i′ = angle of refraction

Q: What is refractive index?

Q: What is apparent depth?

Objects under water appear shallower than they actually are because light refracts at the water-air interface.

Q: Why does a straw appear bent in water?

Due to refraction of light at the water surface, making the submerged portion appear displaced.

Q: What happens when light passes through a parallel-sided glass block?

The emergent ray is parallel to the incident ray but shifted sideways.

This sideways shift is called lateral displacement.

Q: What is lateral displacement?

The sideways shift of a ray after passing through a parallel-sided medium.

Q: What is an optical prism?

A transparent optical element with flat polished surfaces that refracts light.

In optometry, prisms can help manage binocular vision problems caused by extraocular muscle dysfunction.

Q: Compare convex and concave refracting surfaces.

Convex surface

Converges light rays

Concave surface

Diverges light rays

Q: What is reduced vergence?

Q: What is the paraxial equation for refraction at a spherical surface?

nnn = first medium refractive index

n′n'n′ = second medium refractive index

rrr = radius of curvature

LLL = object vergence

L′L'L′ = image vergence

FFF = surface power

Q: What is the magnification equation for refraction at a spherical surface?

Q: State the sign convention used in refraction calculations.

Along the principal axis

With direction of light = positive (+)

Against direction of light = negative (−)

Heights

Above axis = positive (+)

Below axis = negative (−)

Angles

Anticlockwise = positive (+)

Clockwise = negative (−)

All distances are measured from the vertex of the surface.