In-text Questions — Refraction and Refractive Index — Light - Reflection and Refraction

1. 12 marksNCERT Cl-10 Science, In-text Qs after §9.3.2, Q1

   A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

   Hint. Compare the optical density of air and water before deciding which way the ray bends.

   Step 1 — Compare optical densities. Water is optically denser than air (it has a higher refractive index).

   Step 2 — Apply the bending rule. A ray travelling obliquely from a rarer medium (air) into a denser medium (water) bends towards the normal.

   ✦ Answer: The ray bends towards the normal, since it is entering a denser medium (water) from a rarer one (air).

   Where students slip. Saying it bends away from the normal — that would be the case going from a denser medium into a rarer one, the reverse of this situation.

2. 22 marksNCERT Cl-10 Science, In-text Qs after §9.3.2, Q2

   Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 10⁸ m/s.

   Hint. Rearrange the refractive-index formula to solve for speed rather than for the index itself.

   Step 1 — Recall the refractive index formula. n = (speed of light in vacuum) / (speed of light in the medium) = c/v.

   Step 2 — Rearrange for v. v = c/n.

   Step 3 — Substitute. v = (3 × 10⁸)/1.50 = 2 × 10⁸ m/s, a value less than c, since light always slows down on entering an optically denser medium like glass.

   ✦ Answer: 2 × 10⁸ m/s.

   Where students slip. Multiplying instead of dividing (v = c × n) — the refractive index is defined as c divided by v, so finding v means dividing c by n, not multiplying.

3. 32 marksNCERT Cl-10 Science, In-text Qs after §9.3.2, Q3

   Find out, from Table 9.3, the medium having highest optical density. Also find the medium with lowest optical density.

   Hint. Optical density tracks directly with refractive index — no need to think about mass at all.

   Step 1 — Recall what optical density means. The medium with the larger refractive index is the optically denser one.

   Step 2 — Scan the table for the extremes. Diamond has the highest refractive index in Table 9.3 (2.42), and air has the lowest (1.0003), so these are the optically densest and rarest media listed.

   ✦ Answer: Highest optical density — diamond (n = 2.42). Lowest optical density — air (n = 1.0003).

   Where students slip. Trying to reason about optical density using mass density instead — the table itself warns that these are different; kerosene, for instance, is optically denser than water despite being less mass-dense.

4. 42 marksNCERT Cl-10 Science, In-text Qs after §9.3.2, Q4

   You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 9.3.

   Hint. Speed and refractive index move in opposite directions — the smallest index means the fastest light.

   Step 1 — Look up the three refractive indices. From Table 9.3: kerosene = 1.44, turpentine oil = 1.47, water = 1.33.

   Step 2 — Apply v = c/n. Since speed is inversely related to refractive index, the medium with the smallest n has the largest v.

   Step 3 — Identify the smallest index. Water has the lowest refractive index (1.33) of the three.

   ✦ Answer: Light travels fastest in water.

   Where students slip. Picking kerosene or turpentine because they 'sound less dense than water' intuitively — optical density isn't about how the substance feels; the table's actual refractive index values are what settle it, and water's is the lowest of the three.

5. 52 marksNCERT Cl-10 Science, In-text Qs after §9.3.2, Q5

   The refractive index of diamond is 2.42. What is the meaning of this statement?

   Hint. Refractive index is a ratio of two speeds — say what that ratio actually means for diamond.

   Step 1 — Recall the definition. Refractive index is the ratio of the speed of light in vacuum (or air) to the speed of light in the medium.

   Step 2 — Apply it to diamond. A refractive index of 2.42 means light travels 2.42 times faster in vacuum/air than it does in diamond.

   Step 3 — State the equivalent speed. Since v = c/n, the speed of light in diamond works out to (3 × 10⁸)/2.42 ≈ 1.24 × 10⁸ m/s.

   ✦ Answer: Light travels 2.42 times faster in air/vacuum than in diamond — i.e. its speed in diamond is about 1.24 × 10⁸ m/s.

   Where students slip. Saying diamond 'bends light 2.42 times more' — refractive index is precisely defined as a speed ratio; the bending behaviour follows from this, but the number itself is a ratio of speeds, not a bending amount.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc109.pdf) — four in-text question sets (14 questions total, not 18 as some older manifests claim) plus one end-of-chapter Exercise (17 questions, correctly counted). Total internal reflection (critical angle, optical fibres, mirage) has been removed from the current chapter.. Questions are referenced from the NCERT textbook for identification.