Reflection and Refraction of Light

Question 1

The law of reflection states that:

Accepted Answer

the angle of incidence equals the angle of reflection — The law of reflection states that the angle of incidence equals the angle of reflection, both measured from the normal.

Question 2

The image formed in a plane mirror is:

Accepted Answer

virtual and the same size as the object — A plane mirror forms a virtual, upright image that is the same size as the object and the same distance behind the mirror.

Question 3

When a ray of light passes from air into glass, it:

Accepted Answer

slows down and bends towards the normal — Glass is denser (optically) than air, so light slows down on entering it and refracts towards the normal.

Question 4

A ray enters glass with an angle of incidence of 50 degrees and refracts at 30 degrees. What is the refractive index? (sin 50 = 0.766, sin 30 = 0.500)

Accepted Answer

1.53 — n = sin i / sin r = 0.766 / 0.500 = 1.53.

Question 5

When light passes from glass into air, it bends:

Accepted Answer

away from the normal — Going from a denser to a less dense medium the light speeds up and bends away from the normal.

Question 6

The word 'AMBULANCE' is written back-to-front on the front of a vehicle so that in a mirror it reads correctly. This is because a plane mirror image is:

Accepted Answer

laterally inverted — A plane mirror image is laterally inverted (left and right are swapped), so back-to-front writing appears the right way round in a driver's mirror.

Question 7

The refractive index of a medium can also be found from:

Accepted Answer

speed of light in vacuum / speed of light in the medium — Refractive index n = speed of light in a vacuum / speed of light in the medium. Because light slows in the medium, n is always greater than 1.

Question 8

Total internal reflection can occur only when light travels:

Accepted Answer

from a denser to a less dense medium at an angle greater than the critical angle — Total internal reflection needs light going from a denser to a less dense medium with an angle of incidence greater than the critical angle.

Question 9

In problems on reflection and refraction, angles are always measured from:

Accepted Answer

the normal — By convention, the angle of incidence, reflection and refraction are all measured between the ray and the normal (the line at 90 degrees to the surface).

Question 10

When light passes from air into water, which quantity stays the SAME?

Accepted Answer

Its frequency — On refraction the speed and wavelength both change, and usually the direction changes too, but the frequency of the light stays the same.

Reflection at a plane mirror

Refraction