UPSC Prelims 2013 Q90: Science & Technology – Physics
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical
phenomena are responsible for this?
1. Dispersion
2. Refraction
3. Internal reflection
Select the correct answer using the codes given below.
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (d) 1, 2 and 3
Explanation
How a rainbow is formed
Each raindrop acts like a tiny prism.
| Step | Phenomenon | What happens |
|---|---|---|
| 1 | Refraction | Sunlight bends as it enters the drop |
| 2 | Dispersion | White light splits into seven colours |
| 3 | Internal reflection | Light reflects off the inner back surface of the drop |
| 4 | Refraction again | Light bends as it leaves the drop |
All three phenomena in the question are involved.
Facts about rainbows:
| Feature | Detail |
|---|---|
| Where to look | With the sun behind you |
| Primary rainbow | Red outside, violet inside; one internal reflection |
| Secondary rainbow | Fainter; colours reversed; two internal reflections |
| Colours | VIBGYOR |
Answer: (d) 1, 2 and 3.
Why each option is right or wrong
- (a) Incorrect. Internal reflection inside the drop is also needed.
- (b) Incorrect. Dispersion is what splits the light into colours.
- (c) Incorrect. Refraction at entry and exit is also involved.
- (d) CORRECT. A rainbow needs dispersion, refraction and internal reflection.
Key point
Rainbow: refraction + dispersion + internal reflection in raindrops; seen with the sun behind; red on the outside of the primary bow.
Remember
Rainbow = refraction + dispersion + internal reflection.
Common mistakes
- Leaving out internal reflection.
- Looking for a rainbow towards the sun.
- Forgetting that the secondary rainbow has reversed colours.
Exam tip
A raindrop does three things: bends, splits and reflects light.
Sources
- NCERT Class X — Science, Ch 10 (The Human Eye and the Colourful World)
- NCERT Class XII — Physics Part II (Ray Optics: the rainbow)