Which of the following is not based on total internal reflection?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Real and Apparent depth of a pond
Total internal reflection occurs when light travelling in a denser medium meets a rarer medium at an angle greater than the critical angle.
Option (1) Real and Apparent depth of a pond — Not based on total internal reflection. The real and apparent depth of a pond is a case of refraction at the water surface; the light passes into the air and bends, with no total reflection.
Option (2) Prisms designed to bend light by \( 90^\circ \) or by \( 180^\circ \) — Based on total internal reflection. Right-angled glass prisms turn the light by \( 90^\circ \) or \( 180^\circ \) by total reflection at the glass-air face.
Option (3) Light pipe to facilitate visual examination of internal organs — Based on total internal reflection. The light pipe is a bundle of optical fibres that carry the light by repeated total reflection.
Option (4) Transmission of signals over long distances through optical fibres — Based on total internal reflection. Optical fibres guide the signal by total reflection at the core-cladding boundary.
NCERT (2026-27):
Refraction – “… For viewing near the normal direction, it can be shown that the apparent depth (h1) is real depth (h2) divided by the refractive index of the medium (water).”
Total Internal Reflection in Nature and Its Technological Applications – “(i) Prism: Prisms designed to bend light by 90° or by 180° make use of total internal reflection [Fig. 9.13(a) and (b)].”
Total Internal Reflection in Nature and Its Technological Applications – “… (ii) Optical fibres: Nowadays optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection.”
Total Internal Reflection in Nature and Its Technological Applications – “… For example, these are used as a ‘light pipe’ to facilitate visual examination of internal organs like esophagus, stomach and intestines.”