Consider the following statements about total internal reflection and its uses:
(A) In a decorative fibre lamp, light enters the free end of each fibre and comes out as a dot at the end fixed over the lamp.
(B) Optical fibres are made of high quality composite glass or quartz fibres.
(C) A mirage is an example of total internal reflection.
(D) Electrical signals can be changed into light by suitable transducers and sent through optical fibres.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) only
In total internal reflection, light inside a denser medium is sent back fully at its boundary with a rarer medium. An optical fibre guides light in this way, so very little light is lost along its length.
(A) In a decorative fibre lamp, light enters the free end of each fibre and comes out as a dot at the end fixed over the lamp. — Incorrect statement. The lamp is fixed at the bottom end of the fibres. Light enters there, travels up each fibre and appears as a dot of light at its free tip.
(B) Optical fibres are made of high quality composite glass or quartz fibres. — Correct statement. The fibres that carry signals are made from high quality composite glass or quartz. The fine plastic fibres of a decorative lamp are also optical fibres.
(C) A mirage is an example of total internal reflection. — Correct statement. The mirage is one of the examples of total internal reflection, along with diamond and totally reflecting prisms.
(D) Electrical signals can be changed into light by suitable transducers and sent through optical fibres. — Correct statement. Transducers change electrical signals into light. The light then travels along the fibre by repeated total internal reflection.
NCERT (2026-27):
Total internal reflection in nature and its technological applications – “… Optical fibres are fabricated with high quality composite glass/quartz fibres.”
Total internal reflection in nature and its technological applications – “… Optical fibres are extensively used for transmitting and receiving electrical signals which are converted to light by suitable transducers.”
Total internal reflection in nature and its technological applications – “You might have seen a commonly available decorative lamp with fine plastic fibres with their free ends forming a fountain like structure. The other end of the fibres is fixed over an electric lamp. When the lamp is switched on, the light travels from the bottom of each fibre and appears at the tip of its free end as a dot of light. The fibres in such decorative lamps are optical fibres.”
Summary – “2. … Multiple internal reflections in diamond (ic ≅ 24.4°), totally reflecting prisms and mirage, are some examples of total internal reflection.”