The critical angle for a beam of
light travelling between water and
air is 49°.
(a) A beam strikes a water or air
boundary and undergoes total
internal reflection. Will the
beam stay in the air or in the
water?
(b) Explain what happens when
a beam of light from the air
strikes the surface of a calm
lake at an angle of 50° from the
normal.
(a) The beam will stay in the water. Total internal reflection occurs when the angle of incidence is greater than the critical angle, in this case 49°. This means that the light beam cannot pass from water to air and will instead reflect back into the water.
(b) When a beam of light from the air strikes the surface of a calm lake at an angle of 50° from the normal, the light beam will refract as it passes from air to water. The angle of incidence is greater than the critical angle of 49°, so total internal reflection does not occur. Instead, the light beam will bend towards the normal as it enters the water due to the higher refractive index of water compared to air. This bending of the light beam is known as refraction.