Science · Natural phenomena · chapter 9 of 13

Class 10 Science Chapter 9: Light: Reflection and Refraction case study question with answers

One case-based question of the kind the board asks in Section E: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

In a school project, students compared four transparent materials P, Q, R and S. Using data supplied by their teacher, they listed the speed of light in each, taking the speed of light in air as 3 × 10⁸ m/s. They then shone a narrow beam obliquely across the boundary between pairs of these materials and noted whether it bent towards or away from the normal.

Material	Speed of light (× 10⁸ m/s)
P	2.0
Q	2.4
R	1.5
S	1.25

Part 1 · 1 mark

The optically densest of the four materials is

(a) P(b) Q(c) R(d) S

Show answer

Answer: (d) S

Light is slowest in S (1.25 × 10⁸ m/s), so S has the highest refractive index, n = c/v = 2.4, and is optically densest.

Part 2 · 1 mark

The absolute refractive index of Q is

(a) 0.8(b) 1.25(c) 1.5(d) 2.4

Show answer

Answer: (b) 1.25

n = c/v = 3/2.4 = 1.25.

Part 3 · 2 marks

A beam passes obliquely from R into Q. Find the refractive index of Q with respect to R, and state whether the beam bends towards or away from the normal.

Answer and marking scheme

n_QR = (speed in R)/(speed in Q) = 1.5/2.4 = 0.625. Light speeds up on entering Q, which is optically rarer than R, so the beam bends away from the normal.

n_QR = v_R/v_Q = 0.625 (1)
away from the normal, as Q is optically rarer (1)

A beam passes obliquely from P into S. Find the refractive index of S with respect to P, and state whether the beam bends towards or away from the normal.

Answer and marking scheme

n_SP = (speed in P)/(speed in S) = 2.0/1.25 = 1.6. Light slows down on entering S, which is optically denser than P, so the beam bends towards the normal.

n_SP = v_P/v_S = 1.6 (1)
towards the normal, as S is optically denser (1)

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