CAPF AC 2021 Q2: Science & Technology – Physics
What is the approximate wavelength of violet light in air?
- (a) 0·7 µm
- (b) 0·6 µm
- (c) 0·5 µm
- (d) 0·4 µm
Answer: (d) 0·4 µm
Explanation
Wavelength of violet light
How to think: visible light runs from about 0.4 µm to 0.7 µm.
Remember VIBGYOR: violet is at the short end and red is at the long end.
So violet takes the smallest value on offer.
| Colour | Approximate wavelength in air |
|---|---|
| Violet | 0.4 µm (400 nm) |
| Blue | 0.45 to 0.5 µm |
| Green | About 0.5 to 0.55 µm |
| Yellow | About 0.58 µm |
| Orange | About 0.6 µm |
| Red | 0.7 µm (700 nm) |
Violet and red compared:
| Property | Violet | Red |
|---|---|---|
| Wavelength | Shortest | Longest |
| Frequency and energy | Highest | Lowest |
| Bending in a prism | Most | Least |
| Scattering | Most | Least; so used for danger signals |
Units: 1 µm = 1,000 nm.
Answer: (d) 0·4 µm.
Why each option is right or wrong
- (a) Incorrect. 0.7 µm is the wavelength of red light.
- (b) Incorrect. 0.6 µm is in the orange part of the spectrum.
- (c) Incorrect. 0.5 µm is in the blue-green part.
- (d) CORRECT. Violet light has a wavelength of about 0.4 µm, or 400 nm.
Key point
Visible light: 0.4 µm (violet) to 0.7 µm (red). Violet: shortest wavelength, highest frequency, bends and scatters most. Red: the opposite.
Remember
Violet 0.4 µm, red 0.7 µm.
Common mistakes
- Giving violet the longest wavelength.
- Mixing up units: 0.4 µm is 400 nm, not 40 nm.
- Thinking shorter wavelength means lower energy; it means higher.
Exam tip
Educated guess: the four options are the visible range in steps. Violet is at one end, so only 0.4 or 0.7 can be right.
Sources
- NCERT Class X Science — The Human Eye and the Colourful World (dispersion)
- NCERT Class XII Physics — Electromagnetic Waves (electromagnetic spectrum)