UPSC Prelims 2002 Q41: Science & Technology – Physics
Consider the following natural phenomena:
1. Terrestrial heating
2. Reflection of light
3. Refraction of light
4. Diffraction of light
Due
to which of these phenomena is mirage formed?
- (a) 1 and 2
- (b) 2, 3 and 4
- (c) 1 and 3
- (d) 4 only
Answer: (c) 1 and 3
Explanation
Natural phenomena responsible for a mirage
| Phenomenon | Role in a mirage |
|---|---|
| Terrestrial heating | Hot ground heats the air layers next to it, so air density and refractive index change with height |
| Refraction of light | Light bends continuously through the layers and curves upward, forming the image |
| Reflection of light | A mirage looks like a reflection, but ordinary reflection is not the cause |
| Diffraction of light | Not involved |
In a mirage the ground heats the air above it, producing layers of different density. Light from the sky refracts (and bends, with total internal reflection at the critical angle) through them, so the sky seems to appear as water on the ground. The accepted pair is 1 and 3.
Answer: (c) 1 and 3.
Why each option is right or wrong
- (a) Incorrect. It adds ordinary reflection, which is not the real cause.
- (b) Incorrect. It adds reflection and diffraction and drops terrestrial heating.
- (c) CORRECT. Terrestrial heating and refraction of light cause a mirage.
- (d) Incorrect. Diffraction plays no role.
Key point
Mirage = refraction in a hot air gradient plus total internal reflection at the grazing angle; seen in deserts and roads.
Remember
Mirage = hot ground + refraction of light.
Common mistakes
- Choosing reflection because the mirage looks like a mirror image.
- Including diffraction by confusion with wave optics.
- Forgetting that heating creates the refractive gradient.
Exam tip
Mirage needs a heated surface and bent light: heating plus refraction.
Sources
- NCERT Class X Science — The Human Eye and the Colourful World (atmospheric refraction)
- NCERT Class XII Physics — Ray Optics and Optical Instruments (total internal reflection and mirage)