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 :
- (a)1, 4, 3, 2
- (b)1, 4, 2, 3
- (c)4, 1, 2, 3
- (d)4, 1, 3, 2
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.
- (a)1, 4, 3, 2 — 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)4, 1, 2, 3 — 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)4, 1, 3, 2 — 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.
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.
Fix the full sequence once and read questions off it. From highest frequency to lowest: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves. A widely used mnemonic for that order is 'Gamma X-ray Ultraviolet Visible Infrared Microwave Radio'. The single most important discipline in this topic is to check which quantity the question is ordering by: frequency and energy run one way, wavelength runs exactly the other way, so 'decreasing frequency' and 'decreasing wavelength' produce opposite answers from the same four items. Here the question asks for decreasing frequency, so it starts at the gamma end. One honest nuance is worth knowing: the gamma-ray and X-ray wavelength ranges actually overlap, and modern physics distinguishes them by origin rather than by a sharp boundary — gamma rays come from the atomic nucleus, X-rays from electron transitions and from electrons decelerating. Examiners nonetheless use the conventional textbook ladder in which gamma sits above X-rays, and that is what this key rewards.
- All electromagnetic radiation travels at the same speed in vacuum (c ≈ 3 × 10^8 m/s), so c = νλ makes frequency inversely proportional to wavelength
- Photon energy follows Planck's relation E = hν, so the order of decreasing frequency is also the order of decreasing photon energy
- The full spectrum in decreasing order of frequency is: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves
- Visible light occupies roughly 400 to 700 nanometres; ultraviolet lies just beyond the violet end at shorter wavelengths, and infrared just beyond the red end at longer wavelengths
- Radio waves have the longest wavelengths — metres to kilometres — and the lowest frequencies of the entire spectrum
- The gamma-ray and X-ray ranges overlap; they are conventionally distinguished by origin, gamma rays arising from the atomic nucleus and X-rays from electron transitions and from the deceleration of electrons
Reading the four listed items down this ladder gives 1, 4, 2, 3 — option (b). Frequency and energy fall downwards; wavelength rises downwards.
- Ordering by wavelength when the question asks for frequency, or the reverse; the two sequences are exact opposites
- Placing X-rays above gamma rays — in the conventional ordering used by examiners, gamma rays occupy the highest-frequency end
- Forgetting that radio waves are the extreme low-frequency end, and slotting them somewhere in the middle of a short list because no visible, infrared or microwave options are present to anchor them
UPPSC and UPSC both use the electromagnetic spectrum as a ready-made ordering or assertion-reason item — arrange these bands by frequency, wavelength or energy, or decide whether a stated wavelength-energy relationship holds. The underlying test is always the inverse relation between wavelength and frequency.
Assertion (A): In the visible spectrum of light, red light is more energetic than green light. Reason (R): The wavelength of red light is more than that of green light.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(d) A is false but R is true
Exactly the relation this question turns on, tested inside the visible band: longer wavelength means lower frequency and therefore lower photon energy. Getting that inverse relation right is what puts gamma at the top of the sequence and radio at the bottom.
Assertion (A): Radio waves bend in a magnetic field. Reason (R): Radio waves are electromagnetic in nature.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(d) A is false but R is true
Fixes radio waves as one member of the same electromagnetic family — the point that lets you place them on the single spectrum ladder alongside gamma rays, X-rays and ultraviolet rather than treating them as a different kind of thing.
- practice — not a real PYQ
Which one of the following electromagnetic radiations has the longest wavelength?
- (a)Gamma rays
- (b)X-rays
- (c)Ultraviolet rays
- (d)Radio waves
Answer(d) Radio waves — they occupy the long-wavelength, low-frequency end of the electromagnetic spectrum, with wavelengths from metres up to kilometres.
- practice — not a real PYQ
If two electromagnetic radiations travel through vacuum, the one with the higher frequency will necessarily have
- (a)a greater speed
- (b)a longer wavelength
- (c)a higher photon energy
- (d)a lower photon energy
Answer(c) a higher photon energy — all electromagnetic waves share the same speed in vacuum, while E = hν makes photon energy rise with frequency and c = νλ makes wavelength fall with it.