CAPF AC 2021 Q18: Science & Technology – Physics
- (a) Red
- (b) Violet
- (c) Green
- (d) All suffer by equal amount of deviation
Answer: (b) Violet
Explanation
Which colour bends the most in a prism
How to think: remember the order VIBGYOR.
In a prism, the colour with the shortest wavelength bends the most.
Violet has the shortest wavelength.
Red has the longest and bends the least.
| Colour | Wavelength | Speed in water or glass | Deviation |
|---|---|---|---|
| Violet | Shortest (about 400 nm) | Slowest | Maximum |
| Green | Middle | Middle | Middle |
| Red | Longest (about 700 nm) | Fastest | Minimum |
Why it happens: the refractive index of a medium is higher for violet than for red.
So violet light slows down more and bends more.
This splitting of white light is called dispersion.
The hollow prism: the hollow prism filled with water behaves like a water prism.
The rule stays the same.
Answer: (b) Violet.
Why each option is right or wrong
- (a) Incorrect. Red has the longest wavelength and suffers the least deviation.
- (b) CORRECT. Violet has the shortest wavelength and the highest refractive index, so it bends the most.
- (c) Incorrect. Green lies in the middle of the spectrum and bends by a middle amount.
- (d) Incorrect. If all colours bent equally, white light would not split into seven colours.
Key point
In a prism violet deviates the most and red the least, because refractive index is higher for shorter wavelengths. The splitting is called dispersion.
Remember
Violet bends most, red bends least.
Common mistakes
- Mixing up the two ends and marking red as the colour that bends the most.
- Thinking a water-filled hollow prism behaves differently from a glass prism.
- Forgetting that red deviates least, which is why danger signals are red.
Exam tip
Educated guess: if all colours bent equally there would be no spectrum, so (d) is out. In VIBGYOR, V sits at the bottom of the spectrum from a prism because it bends the most.
Sources
- NCERT Class X Science — The Human Eye and the Colourful World (dispersion of white light by a glass prism)
- NCERT Class XII Physics — Ray Optics and Optical Instruments (dispersion by a prism)