What is the approximate wavelength of violet light in air?

Q: What is the answer to CAPF 2021 Paper I Q2?

D. Correct — D, 0·4 µm. Visible light runs from about 400 nm at the violet end to about 700 nm at the red end. One micrometre is 1000 nm, so 400 nm written in micrometres is 0·4 µm. Violet has the shortest wavelength of the seven colours, and 0·4 µm is the only option at that end of the band. (Provisional — pending the official UPSC key.)

(a)0·7 µm
(b)0·6 µm
(c)0·5 µm
(d)0·4 µm

Answer

Why

Correct — D, 0·4 µm. Visible light runs from about 400 nm at the violet end to about 700 nm at the red end. One micrometre is 1000 nm, so 400 nm written in micrometres is 0·4 µm. Violet has the shortest wavelength of the seven colours, and 0·4 µm is the only option at that end of the band.

Why the others are wrong

(a)0·7 µm — 0·7 µm is 700 nm, the far red end of the visible spectrum — the opposite extreme from violet.
(b)0·6 µm — 0·6 µm is 600 nm, in the orange-yellow region. Sodium street lamps emit close to this at 589 nm.
(c)0·5 µm — 0·5 µm is 500 nm, blue-green. It is the middle of the visible band, not its violet edge.

Concept

Wavelength decides colour. The visible band occupies a narrow slice of the electromagnetic spectrum, roughly 400 to 700 nm, with violet shortest and red longest; frequency and photon energy run the other way, so violet photons carry the most energy of the seven. Ultraviolet begins just below 400 nm and infrared just above 700 nm.

Two conversions do all the work. First, 1 µm = 1000 nm, so the whole visible band is 0·4 to 0·7 µm — every option in this question is inside it, which is why the answer turns on position rather than magnitude. Second, VIBGYOR runs from short to long, so the first letter takes the smallest number. Candidates who remember the band but not its direction reliably pick 0·7 µm.

Key facts

The visible spectrum spans roughly 400-700 nm, that is 0·4-0·7 µm.
Violet is the shortest visible wavelength and red the longest; VIBGYOR is the order of increasing wavelength.
1 micrometre = 1000 nanometres = 10⁻⁶ m.
Shorter wavelength means higher frequency and higher photon energy, so violet photons are the most energetic of the seven.
Ultraviolet lies just beyond violet and infrared just beyond red, which is where the two names come from.

The visible band, end to end

Violet→about 400 nm = 0·4 µm — shortest wavelength, highest photon energy

Blue-green→about 500 nm = 0·5 µm

Orange-yellow→about 600 nm = 0·6 µm; the sodium lamp line sits at 589 nm

Red→about 700 nm = 0·7 µm — longest wavelength, lowest photon energy

All four printed options lie inside the visible band, so the question is really about which end violet occupies.

Study next

The full electromagnetic spectrum in order of wavelengthFrequency, wavelength and the relation c = fλWhy the sky is blue — Rayleigh scattering and its dependence on wavelengthUltraviolet and infrared radiation and their everyday usesPhoton energy E = hf and why ultraviolet damages skin while infrared warms it

Common traps

Reading VIBGYOR backwards and assigning violet the longest wavelength.
Mixing up micrometres and nanometres by three orders of magnitude.
Assuming higher energy means longer wavelength.

A recall item that hides a unit conversion; the examiner prints micrometres because the figures candidates memorise are in nanometres.

Related PYQs

NDA_GAT_2018_II_Q902018Open & attempt →

Consider the following statements about visible light, UV light and X-rays : 1. The wavelength of visible light is more than that of X-rays. 2. The energy of X-ray photons is higher than that of UV light photons. 3. The energy of UV light photons is less than that of visible light photons. Which of the statements given above is/are correct?

(a) 1, 2 and 3
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1 only

Answer(b) 1 and 2 only

Tests the inverse link between wavelength and photon energy that decides this CAPF item; statement 3 fails precisely because shorter-wavelength ultraviolet carries more energy than visible light.

CDS_GK_2020_I_Q122020Open & attempt →

Which one of the following types of radiation has the shortest wavelength?

(a) Radio waves
(b) Visible light
(c) Infrared (IR)
(d) Ultraviolet (UV)

Answer(d) Ultraviolet (UV)

The same ruler, one band wider. Place violet at 400 nm and ultraviolet is whatever lies just short of it, which is why UV beats every option there.

Practice

practice — not a real PYQ
What is the approximate wavelength of red light in nanometres?
- (a)400 nm
- (b)500 nm
- (c)600 nm
- (d)700 nm
Answer(d) 700 nm — red occupies the long-wavelength edge of the visible band.
practice — not a real PYQ
Which one of the following has the highest photon energy?
- (a)Red light
- (b)Green light
- (c)Violet light
- (d)Infrared radiation
Answer(c) Violet light — photon energy rises as wavelength falls, and violet is the shortest of these.