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.
In a demonstration, a laser beam is sent from the top into a long test tube filled with water. A little milk is mixed in so that the beam can be seen. The beam is set so that it is totally reflected every time it strikes the wall, and it travels down the whole tube. The laser is then turned so that the angle of incidence at the wall becomes smaller than the critical angle. Ignore the thin glass wall. What is observed now?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Part of the light comes out through the wall at every reflection
Total internal reflection needs the angle of incidence to be greater than the critical angle. Here the angle of incidence at the wall is now smaller than the critical angle. So refraction into the air becomes possible again.
Option (1) The whole beam still travels down the tube, as the light still goes from water towards air — Incorrect. Light going from water towards air is only one condition. The angle of incidence must also be greater than the critical angle, and now it is smaller.
Option (2) The whole beam travels down the tube, with more reflections than before — Incorrect. The beam does strike the wall more often now. But at each strike the angle of incidence is below the critical angle, so the beam is no longer totally reflected.
Option (3) All the light comes out through the wall at the first point it strikes — Incorrect. Below the critical angle, light meeting the surface is partly reflected and partly refracted. So some light stays in the water after the first strike.
Option (4) Part of the light comes out through the wall at every reflection — Correct. At every strike, part of the light is refracted out into the air and the rest is reflected. So the beam inside the tube grows weaker as it goes down.
NCERT (2026-27):
Total Internal Reflection – “Do you find that it undergoes partial reflection (which is seen as a spot on the table below) and partial refraction [which comes out in the air and is seen as a spot on the roof; …]?”
Total Internal Reflection – “… For values of i larger than ic, Snell’s law of refraction cannot be satisfied, and hence no refraction is possible.”