Which one of the following types of radiations has the smallest wavelength?
- (a)Microwaves
- (b)Infra-red
- (c)Visible light
- (d)X-rays
Correct — D, X-rays. On the electromagnetic spectrum, arranged from longest to shortest wavelength, the four options run: microwaves (millimetres to centimetres) → infra-red (about 700 nm to 1 mm) → visible light (about 400 to 700 nm) → X-rays (about 0.01 to 10 nm). X-rays therefore have the smallest wavelength of the four. Since the speed is the same for all electromagnetic waves in vacuum (c = fλ), the shortest wavelength also means the highest frequency and the highest photon energy — which is why X-rays are penetrating and ionising.
- (a)Microwaves — Microwaves have the LONGEST wavelength of the four (millimetres to centimetres), the opposite of what is asked.
- (b)Infra-red — Infra-red wavelengths (roughly 700 nm to 1 mm) are longer than visible light and far longer than X-rays, so not the smallest.
- (c)Visible light — Visible light (about 400–700 nm) is shorter than infra-red but still much longer than X-rays, so it is not the smallest wavelength.
The electromagnetic spectrum, in order of increasing frequency (and decreasing wavelength), runs: radio waves → microwaves → infra-red → visible light → ultraviolet → X-rays → gamma rays. All travel at the speed of light in vacuum, so by c = fλ frequency and wavelength are inversely related; by E = hf, shorter wavelength means higher photon energy.
The question only requires ranking the four given options by wavelength. Placing them on the standard spectrum ladder shows X-rays sit far to the short-wavelength, high-energy end, well beyond visible light, infra-red and microwaves.
- Order (long → short wavelength): microwaves > infra-red > visible > X-rays.
- X-ray wavelengths are roughly 0.01–10 nm; visible light 400–700 nm.
- c = fλ: shortest wavelength corresponds to highest frequency.
- E = hf: X-rays carry high energy, making them penetrating and ionising.
- Confusing the wavelength order with the frequency order — they are inverse.
- Assuming visible light has the smallest wavelength because it is the most familiar.
Asked as a ranking recall — place the given radiations on the spectrum and pick the short-wavelength (high-energy) end.
Which one of the following types of waves are used in a night vision apparatus?
- (a) Radio waves
- (b) Microwaves
- (c) Infra-red waves
- (d) None of the above
Answer(c) Infra-red waves
Same concept — identifying and ranking electromagnetic radiations. UPSC 2009 tests the use of infra-red (heat) radiation in night vision; this NDA item tests where such radiations sit by wavelength, with X-rays at the short-wavelength, high-energy end.
Which one of the following radiations has longer wavelength?
- (a) X-rays
- (b) Ultraviolet
- (c) Micro Wave
- (d) Infra-red
Answer(c) Micro Wave
Mirror image of this question. NDA 2025-II asks which has the LONGEST wavelength (microwaves) among a similar set; this 2020 item asks which has the SMALLEST (X-rays). Both reward the same spectrum ordering: microwaves > infra-red > visible > UV > X-rays.
- practice — not a real PYQ
Which of the following electromagnetic radiations has the highest frequency?
- (a)Radio waves
- (b)Microwaves
- (c)Visible light
- (d)X-rays
Answer(d) X-rays — highest frequency of these, since shortest wavelength means highest frequency (c = fλ).
- practice — not a real PYQ
Arrange in order of increasing wavelength: X-rays, visible light, microwaves.
- (a)X-rays < visible light < microwaves
- (b)microwaves < visible light < X-rays
- (c)visible light < X-rays < microwaves
- (d)X-rays < microwaves < visible light
Answer(a) X-rays < visible light < microwaves — X-rays are shortest, microwaves longest.