Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): A ray of light going obliquely from water into turpentine bends towards the normal, although turpentine has a lower mass density than water.
Reason (R): The bending of light at a surface depends on optical density, and turpentine is optically denser than water.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are correct and (R) is the correct explanation of (A).
Assertion (A) A ray of light going obliquely from water into turpentine bends towards the normal, although turpentine has a lower mass density than water. — True. Turpentine is lighter than water, yet light travels more slowly in it. So for light going from water into turpentine, \( n_{21}>1 \) and \( r \lt i \).
Reason (R) The bending of light at a surface depends on optical density, and turpentine is optically denser than water. — True. When \( n_{21}>1 \), the second medium is optically denser and the ray bends towards the normal. Turpentine has the higher optical density of the two.
Why Reason (R) explains Assertion (A)?
The direction of bending is decided by \( n_{21} \), which compares the optical densities of the two media. Mass density does not decide it.
R says turpentine is optically denser than water. So \( n_{21}>1 \) and the ray bends towards the normal, as A states.
NCERT (2026-27):
Refraction – “… if n21 > 1, r < i, i.e., the refracted ray bends towards the normal. In such a case medium 2 is said to be optically denser (or denser, in short) than medium 1.”
Refraction – “… For example, turpentine and water. Mass density of turpentine is less than that of water but its optical density is higher.”