Light – Reflection & Refraction (Extended Question Series – Class 10 CBSE)
Important Formulas
1. Law of Reflection: Angle of incidence (i) = Angle of reflection (r)
2. Refractive Index:
n = sin i / sin r
n = speed of light in vacuum / speed of light in medium
3. Snell’s Law:
n₁ sin i = n₂ sin r
4. Mirror Formula:
1/f = 1/v + 1/u
5. Magnification (mirrors):
m = h₂ / h₁ = v / u
6. Lens Formula (for refraction topics):
1/f = 1/v − 1/u
7. Refractive Index (real depth–apparent depth):
n = Real depth / Apparent depth
Section A — Basic Concept Questions
Q1. What is reflection of light?
Ans. The bouncing back of light into the same medium after striking a smooth surface.
Q2. Define refraction of light.
Ans. The bending of light when it passes obliquely from one medium to another due to change in speed.
Q3. What is lateral inversion?
Ans. The left-right reversal of an image formed in a plane mirror.
Section B — Standard Questions
Q4. Why does refraction occur?
Ans. Because the speed of light changes when it enters a different medium.
Q5. State Snell’s law of refraction.
Ans. The ratio of sine of angle of incidence to sine of angle of refraction is constant.
Q6. Define normal, incident ray, and refracted ray.
Ans. Normal is the perpendicular line to the surface. The ray striking the surface is the incident ray; the ray bending inside the new medium is the refracted ray.
Section C — Numericals
Q7. A light ray enters water (n = 1.33) from air. If angle of incidence is 30°, calculate angle of refraction.
Solution:
Using Snell’s Law: n = sin i / sin r
1.33 = sin 30° / sin r
1.33 = 0.5 / sin r
sin r = 0.5 / 1.33 = 0.376
r ≈ 22°
Q8. An object 15 cm in front of a concave mirror forms an image at 30 cm in front of it. Find focal length.
Solution:
u = −15 cm, v = −30 cm
Mirror formula: 1/f = 1/v + 1/u
1/f = 1/−30 + 1/−15 = −1/30 − 1/15
1/f = −1/30 − 2/30 = −3/30 = −1/10
f = −10 cm (concave mirror)
Q9. A coin appears 2 cm below the water surface. If real depth is 2.6 cm, find refractive index.
Solution:
n = Real depth / Apparent depth = 2.6 / 2 = 1.3
Section D — HOTS / Competency-Based Questions
Q10. Why does a pencil appear bent when it is partially dipped in water?
Ans. Because the light rays from the submerged part bend away from the normal when they move from water to air.
Q11. A convex mirror always gives a virtual, erect, and diminished image. Why is this useful for vehicles?
Ans. Because it provides a wider field of view, allowing drivers to see more area behind them.
Section E — Case Study Based Questions
Case Study 1: A student uses a glass slab to study lateral displacement.
Q12. Why are the incident and emergent rays parallel?
Ans. Because the bending that occurs inside the slab is reversed when the ray exits, causing only a shift but no angular change.
Case Study 2: A shopkeeper uses a convex mirror inside the store.
Q13. What type of image does the convex mirror form?
Ans. Virtual, erect, diminished—helpful for surveillance.
Quick Revision Box
Reflection: i = r
Refraction: n = sin i / sin r
Real Image: Formed by actual intersection of rays.
Virtual Image: Formed by apparent intersection of rays.
Concave Mirror: Can produce real or virtual images.
Convex Mirror: Always virtual, erect, diminished.
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