UPSC Prelims 2001 Q104: Science & Technology – Physics
When light waves pass from air to glass, the variables affected are
- (a) Wavelength, frequency and velocity
- (b) Velocity and frequency
- (c) Wavelength and frequency
- (d) Wavelength and velocity
Answer: (d) Wavelength and velocity
Explanation
Quantities that change when light passes from air to glass
| Quantity | Changes? |
|---|---|
| Frequency | No — fixed by the source |
| Velocity | Yes — falls from about 3 × 10⁸ m/s to about 2 × 10⁸ m/s |
| Wavelength | Yes — falls in proportion, since v = f λ |
| Refractive index of glass | About 1.5 = speed in air ÷ speed in glass |
The frequency of light depends only on the source and does not change at a boundary. As light enters glass it slows down, and since v = f λ, its wavelength gets shorter by the same factor. So velocity and wavelength change.
Answer: (d) Wavelength and velocity.
Why each option is right or wrong
- (a) Incorrect. Frequency does not change at a boundary.
- (b) Incorrect. Frequency stays the same; wavelength changes.
- (c) Incorrect. Frequency stays the same; velocity changes.
- (d) CORRECT. Wavelength and velocity both change; frequency does not.
Key point
Colour is set by frequency, which is why light keeps its colour when it enters water or glass.
Remember
Air to glass: frequency same; speed and wavelength fall.
Common mistakes
- Thinking frequency changes with the medium.
- Forgetting that v = f λ links speed and wavelength.
- Thinking velocity stays constant in glass.
Exam tip
At a boundary frequency is fixed; speed and wavelength change together.
Sources
- NCERT Class XII — Physics Part II, chapter 'Wave Optics'
- NCERT Class X — Science, chapter 'Light: Reflection and Refraction' (refractive index)