Light is travelling from one medium to other medium as given in the options. In which of the following options total internal reflection will be possible?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Glass to water
Total internal reflection can occur only when light goes from a denser medium to a rarer medium (higher to lower refractive index), at an angle of incidence larger than the critical angle. Compare the refractive indices: air 1, water 1.33, glass 1.5.
Air to water: \( 1 \rightarrow 1.33 \), rarer to denser, not possible
Air to glass: \( 1 \rightarrow 1.5 \), rarer to denser, not possible
Water to glass: \( 1.33 \rightarrow 1.5 \), rarer to denser, not possible
Glass to water: \( 1.5 \rightarrow 1.33 \), denser to rarer, possible
For glass to water the critical angle is given by \( \sin i_c = \frac{1.33}{1.5} = 0.89 \), so \( i_c \approx 62^\circ \); any angle of incidence larger than this gives total internal reflection.
NCERT (2026-27):
Total Internal Reflection – “When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium.”
Total Internal Reflection – “… If the angle of incidence is increased still further (e.g., the ray AO4), refraction is not possible, and the incident ray is totally reflected. This is called total internal reflection.”