UPSC Prelims 2001 Q101: Science & Technology – Physics
Assertion (A): A stick is dipped in water in a slanting position. If observed sideways, the
stick appears short and bent at the surface of water.
Reason (R): The light coming from the stick undergoes scattering from water molecules giving
the stick a short and bent appearance.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is NOT a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (c) A is true but R is false
Explanation
Assertion-Reason: a stick dipped in water looks short and bent
| Statement | Verdict |
|---|---|
| A. A slanting stick in water looks short and bent at the surface | True — a familiar observation |
| R. This is due to scattering of light by water molecules | False — it is due to refraction |
| What happens | Light from the underwater part bends away from the normal as it leaves water for air |
| Result | The submerged part appears raised, so the stick looks bent and shorter |
Light from the part of the stick under water passes from a denser medium (water) to a rarer one (air) and bends at the surface. This refraction makes the underwater part look raised, so the stick appears bent and short. Scattering is what makes the sky blue; it does not bend the image of the stick.
Answer: (c) A is true but R is false.
Why each option is right or wrong
- (a) Incorrect. R is false; the effect comes from refraction, not scattering.
- (b) Incorrect. R is false, so both cannot be true.
- (c) CORRECT. The stick does look bent, but because of refraction.
- (d) Incorrect. A is true; the bent-stick effect is easily seen.
Key point
Refraction effects: bent stick, raised pool bottom, twinkling stars, apparent depth = real depth ÷ refractive index (water about 1.33).
Remember
Bent stick in water = refraction, not scattering.
Common mistakes
- Thinking scattering explains every light effect in water.
- Mixing refraction (bending) with reflection (bouncing back).
- Marking (a) because the reason sounds scientific.
Exam tip
Bending of light at a boundary = refraction; spreading by particles = scattering.
Sources
- NCERT Class X — Science, chapter 'Light: Reflection and Refraction'
- NCERT Class XII — Physics Part II, chapter 'Ray Optics and Optical Instruments'