Arrange the following electromagnetic radiations in decreasing order of their frequencies.
1. Gamma rays
2. Ultraviolet rays
3. Radio waves
4. X-rays
Select the correct answer from the code given belo

Question

Arrange the following electromagnetic radiations in decreasing order of their frequencies.
1. Gamma rays
2. Ultraviolet rays
3. Radio waves
4. X-rays
Select the correct answer from the code given below :
Code :

UnlockIAS · Accepted Answer

Correct — B, 1, 4, 2, 3 — gamma rays, X-rays, ultraviolet rays, radio waves. Every electromagnetic wave travels at the same speed c in vacuum, so c = νλ ties frequency and wavelength together inversely: the shorter the wave, the higher the frequency. Laid out from the short-wavelength end, the electromagnetic spectrum runs gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves — and that is also the order of decreasing frequency and, because photon energy E = hν, of decreasing energy. Gamma rays occupy the extreme short-wave end, at frequencies above roughly 10 to the 19th hertz. X-rays sit immediately below them. Ultraviolet lies just beyond the violet end of visible light, at wavelengths of tens to a few hundred nanometres. Radio waves are at the opposite extreme altogether, with wavelengths measured in metres and kilometres and the lowest frequencies of the whole spectrum. Picking the four listed items off that ladder in order gives gamma, X-ray, ultraviolet, radio — code 1, 4, 2, 3.

Why the other options are wrong — (a) The first two are right, but this option then places radio waves above ultraviolet. Radio waves are the lowest-frequency and longest-wavelength band of the whole electromagnetic spectrum — far below not just ultraviolet but also visible light, infrared and microwaves. Radio must come last of the four, not third.  (c) This gets the tail of the sequence right — ultraviolet then radio — but inverts the top pair by putting X-rays above gamma rays. In the conventional ordering used in every syllabus, gamma rays sit at the highest-frequency, shortest-wavelength, most energetic end of the spectrum, above X-rays.  (d) This option makes both available mistakes at once. It inverts gamma and X-rays at the top, and it also lifts radio waves above ultraviolet at the bottom. Radio waves are the least energetic radiation on the list and gamma rays the most, so the sequence must open with 1 and close with 3.

Concept: The electromagnetic spectrum is the single continuous family of electromagnetic waves, differing only in wavelength and frequency. All of them travel at the same speed in vacuum, about 3 × 10^8 metres per second, so the relation c = νλ makes frequency inversely proportional to wavelength. Planck's relation E = hν then makes photon energy directly proportional to frequency. The three orderings — increasing frequency, decreasing wavelength and increasing energy — are therefore one and the same ordering, which is why a single remembered sequence answers every question in this area.

State PCS 2025 Q99 — Science & Technology PYQ with Answer

State PCS 2025 Q99 — Science & Technology PYQ with Answer