Case: Read the passage and answer the question.
Total internal reflection is the reflection of light back into the denser medium at its interface with a rarer medium. For it to occur, light must travel from the denser medium towards the rarer medium, and the angle of incidence in the denser medium must be greater than the critical angle for the pair of media. The critical angle depends on the nature of the media and on the wavelength of light. It can be shown that \( \mu=1/\sin C \), where C is the critical angle.
Consider the following statements:
(A) When light is partly reflected at a smooth surface, the reflected ray is less intense than the incident ray.
(B) For light going from a denser to a rarer medium, the angle of refraction decreases as the angle of incidence increases.
(C) Light stops travelling along an optical fibre when the fibre is bent.
(D) In silica glass fibres, more than 95% of the light can pass through a fibre 1 km long.
(E) When light goes from water into air, the refracted ray bends away from the normal.
Which of the above statements are correct?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (D) and (E) only
At a surface between two media, light is partly reflected and partly refracted. Total internal reflection is the special case in which no light is refracted.
(A) When light is partly reflected at a smooth surface, the reflected ray is less intense than the incident ray. — True. Some light is always transmitted at an ordinary reflecting surface. So the reflected ray carries less light than the incident ray, however smooth the surface is.
(B) For light going from a denser to a rarer medium, the angle of refraction decreases as the angle of incidence increases. — False. The angle of refraction grows with the angle of incidence. It keeps growing until it becomes 90° at the critical angle.
(C) Light stops travelling along an optical fibre when the fibre is bent. — False. The fibre is made so that light meets the walls at angles larger than the critical angle. Light is totally reflected again and again, so it still travels along a bent fibre.
(D) In silica glass fibres, more than 95% of the light can pass through a fibre 1 km long. — True. The fibre material is purified so that it absorbs very little light. So more than 95% of the light comes out after 1 km of fibre.
(E) When light goes from water into air, the refracted ray bends away from the normal. — True. Water is optically denser than air. A ray going from a denser to a rarer medium bends away from the normal.
NCERT (2026-27):
Total Internal Reflection – “… The reflected ray, therefore, is always less intense than the incident ray, howsoever smooth the reflecting surface may be. …”
Total Internal Reflection – “… As the angle of incidence increases, so does the angle of refraction …”
Total internal reflection in nature and its technological applications – “… Even if the fibre is bent, light can easily travel along its length.”
Total internal reflection in nature and its technological applications – “… In silica glass fibres, it is possible to transmit more than 95% of the light over a fibre length of 1 km. …”
Total Internal Reflection – “When a ray of light enters from a denser medium to a rarer medium, it bends away from the normal …”