Exam Lessons Physics

Refraction through triangular prism, Real and apparent depth, Total internal reflection and its application

Refraction Through Triangular Prism

When a ray of light passes through a triangular prism, it is refracted as shown below

12 Prism ideas | prism, refraction of light, refraction

The angle between the incident ray and the emergent ray is known as the angle of deviation. The angle of deviation decreases as the angle of incidence increases

The refractive index, n=sin(A/2+D/2)/sinA/2

Total Internal Reflection - Definitions, Applications and Uses in  Microscopy | Refraction, Wet surface, Mirrors and lenses

When light passes at a small angle of incidence from a denser to a less dense medium e.g. from glass to air, there is a strong refracted ray. There is also a weak ray reflected back into the denser medium.

When the angle of incidence increases, the angle of refraction also increases. At a certain increase of the angle of incidence, the angle of refraction is 900. This angle of incidence in the denser medium for which the angle of refraction in the less dense medium is 900, is referred to as the critical angle ( c). For angle of incidence greater than C, the refracted ray disappears and all the incident light is reflected back into the denser medium. At this point, the ray is said to experience total internal reflection. Example of total internal reflection is the mirage on the road, where the refractive density of warm air is less than that  of cool air and light meets a layer at a critical angle, it suffers total internal reflection.


A thick slab of glass appears to be only two –third of its real thickness when viewed vertically from above. Similarly, water in a pond appears to be only three quarters of its real depth. Rays from a coin at the bottom of a bucket of water are refracted away when they leave water and enter the eyes. They appear as if coming from a virtual image, which is apparent depth while the actual depth of the bottom remains and is referred to as real depth

Refractive index=real depth/apparent depth

General Revision

  1. Distinguish between the resultant and equilibrant forces.
  2. Give two (02) ways to increase the stability of a body.


  1. The direction of a light ray changes as it passes from one medium to another. This phenomenon is called A diffraction B reflection C dispersion D refraction
  2. The velocities of light in air and glass are 3.0 x108 m/s and 1.8 x108 m/s respectively. Calculate the sine of the angle of incidence that will produce an angle of refraction of 300 for a ray of light incident on glass A 1.2  B 1.0  C 0.8  D 0.6
  3. A transparent rectangular block 5.0 cm thick is placed on a black dot. The dot when viewed from above is seen 3.0 cm from the top of the block. Calculate the refractive index of the material of the block A 2/5 B 3/5 C 3/2 D 5/3 E 5/2
  4. When a ray of sunlight passes obliquely through a rectangular glass block

Ait emerges without displacement parallel to the incident ray

B it gets dispersed into seven visible colours without any deviation at all

C it deviates without dispersion

D it gets laterally displaced and the emergent ray is parallel to the incident ray

  1.  The absolute refractive indexes of glass and water are 3/2 and 4/3 respectively. The refractive at the interface when a ray travels from water to glass is

(A) ½ (B) 8/9 (C) 9/8 (D)17/12


  1.  A ray of light passes from air  through a rectangular block of glass with parallel side 4.5 cm apart at an angle of incidence of 520,find A The lateral displacement of the ray B The angle of refraction (Refractive index of glass=1.5)
  2. The critical angle for a transparent is 39. Calculate the refractive index of the substance.

Reading Assignment: New school physics for senior secondary school pages 266-270