Consider the following statements about optical fibres:
(A) The refractive index of the core is higher than that of the cladding.
(B) Optical fibres are made of materials that absorb a large part of the light passing through them.
(C) A light signal sent in at a suitable angle at one end comes out at the other end after repeated total internal reflections.
(D) At each reflection inside the fibre, a large part of the light passes into the cladding.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
An optical fibre carries light by total internal reflection at the boundary between its core and its cladding. Light meets this boundary at more than the critical angle, so it keeps getting reflected inside the core.
(A) The refractive index of the core is higher than that of the cladding. — Correct statement. Total internal reflection needs light to go from a denser medium to a rarer one. So the core must have the higher refractive index.
(B) Optical fibres are made of materials that absorb a large part of the light passing through them. — Incorrect statement. Light has to travel long distances inside a fibre. So the material must absorb very little light, and specially purified quartz is used.
(C) A light signal sent in at a suitable angle at one end comes out at the other end after repeated total internal reflections. — Correct statement. The light is totally reflected again and again along the fibre and comes out at the far end.
(D) At each reflection inside the fibre, a large part of the light passes into the cladding. — Incorrect statement. Each reflection is a total internal reflection, so no light passes into the cladding.
NCERT (2026-27):
Total internal reflection in nature and its technological applications – “… Each fibre consists of a core and cladding. The refractive index of the material of the core is higher than that of the cladding.”
Total internal reflection in nature and its technological applications – “The main requirement in fabricating optical fibres is that there should be very little absorption of light as it travels for long distances inside them. …”
Total internal reflection in nature and its technological applications – “When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated total internal reflections along the length of the fibre and finally comes out at the other end …”
Total internal reflection in nature and its technological applications – “… Since light undergoes total internal reflection at each stage, there is no appreciable loss in the intensity of the light signal. …”