NDA (II) 2016 GAT Q111: Which one of the following statements is not correct

Q: What is the answer to NDA (II) 2016 GAT Q111?

C. Correct — C, the claim that gamma rays have a longer wavelength than X-rays, which is the false statement. Gamma rays sit at the far end of the electromagnetic spectrum, beyond X-rays, with the shortest wavelengths and the highest frequencies and photon energies of all. Running the spectrum from long to short gives radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and finally gamma rays, so the wavelength of gamma radiation is shorter than that of X-rays, not longer. The two are also distinguished by where they come from — X-rays are produced when fast electrons are stopped or when inner-shell electrons drop, while gamma rays come out of the nucleus itself.

NDA (II) 2016 General Ability Test · Question 111 of 150

Question text verified against the official NDA question paper.

Which one of the following statements is not correct ?

(a)The longest wavelength of light visible to human eye is about 700 nm
(b)The shortest wavelength of light visible to human eye is about 400 nm
(c)The wavelength of gamma rays is longer than that of X-rays
(d)The ability of a telescope to form separable images of close objects is called its resolving power

Answer

Why

Correct — C, the claim that gamma rays have a longer wavelength than X-rays, which is the false statement. Gamma rays sit at the far end of the electromagnetic spectrum, beyond X-rays, with the shortest wavelengths and the highest frequencies and photon energies of all. Running the spectrum from long to short gives radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and finally gamma rays, so the wavelength of gamma radiation is shorter than that of X-rays, not longer. The two are also distinguished by where they come from — X-rays are produced when fast electrons are stopped or when inner-shell electrons drop, while gamma rays come out of the nucleus itself.

Why the others are wrong

(a)The longest wavelength of light visible to human eye is about 700 nm — This is correct, so it cannot answer a 'not correct' question. The red end of the visible spectrum lies at roughly 700 nanometres; beyond it the radiation is infrared and the eye no longer responds.
(b)The shortest wavelength of light visible to human eye is about 400 nm — Also correct. The violet end of the visible band is at about 400 nanometres, and shorter wavelengths fall into the ultraviolet, which the eye cannot see.
(d)The ability of a telescope to form separable images of close objects is called its resolving power — Correct as stated. Resolving power is the ability of an optical instrument to show two closely spaced objects as two rather than as one blur, and for a telescope it improves as the aperture is made larger.

Concept

The electromagnetic spectrum is one continuous family of waves that all travel at 3 × 10⁸ metres per second in vacuum and differ only in wavelength and frequency. Since the speed is fixed, wavelength and frequency are inversely related, and photon energy rises as wavelength falls. Visible light occupies a very narrow slice of the whole range, roughly 400 to 700 nanometres, which is why so much of astronomy depends on instruments that see outside it.

The safe way to handle any spectrum question is to keep the order memorised in one direction and never reverse it under exam pressure. Going from long wavelength to short: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Frequency and energy increase in exactly the opposite direction. Two useful anchors for the visible band are that red is the longest visible wavelength and violet the shortest, and that this is why red light is scattered least and is used for danger signals, while blue is scattered most and makes the sky blue.

Key facts

In order of decreasing wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
Gamma rays have the shortest wavelength, the highest frequency and the greatest photon energy in the spectrum.
The visible band runs from about 400 nm (violet) to about 700 nm (red).
X-rays originate in the electron shells of an atom while gamma rays originate in the nucleus, which is the physical rather than the numerical distinction between them.

The electromagnetic spectrum, longest wavelength first

Radio waves, then microwaves, then infrared→Longest wavelengths, lowest frequencies and lowest photon energies; used for broadcasting, radar, cooking and heat sensing

Visible light, about 700 nm down to about 400 nm→The narrow band the eye responds to, red at the long end and violet at the short end — options (a) and (b) of this item

Ultraviolet, then X-rays→Shorter than visible, energetic enough to ionise; X-rays come from the electron shells of atoms

Gamma rays→Shortest wavelength of all, shorter than X-rays — which is why option (c) is the false statement (the key's answer)

Wavelength falls from top to bottom; frequency and photon energy rise in the same direction.

Study next

The full electromagnetic spectrum, its ordering and one use for each bandHow X-rays and gamma rays are produced, and how they differ in originResolving power of the eye, the microscope and the telescope, and how aperture affects itScattering of light — why the sky is blue and the setting sun redThe photoelectric effect and the photon picture of light

Common traps

Reversing the wavelength order at the short end and putting gamma rays before X-rays.
Confusing frequency order with wavelength order — they run in opposite directions.
Rushing a 'not correct' item and marking the first statement that looks familiar rather than the one that is false.

The GAT asks for the spectrum's ordering, for the band with the largest or smallest wavelength, or for a use of a named band, and almost always as a single-statement item.

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

The same ordering tested across three bands at once, and with the energy relation added — solve it beside this item to lock in that shorter wavelength means higher energy.

Practice

practice — not a real PYQ
Which one of the following electromagnetic radiations has the highest frequency?
- (a)Microwaves
- (b)Ultraviolet rays
- (c)X-rays
- (d)Gamma rays
Answer(d) Gamma rays — they have the shortest wavelength of all, and frequency rises as wavelength falls.
practice — not a real PYQ
The resolving power of a telescope increases when
- (a)the aperture of the objective is increased
- (b)the focal length of the eyepiece is increased
- (c)the wavelength of the light used is increased
- (d)the length of the tube is reduced
Answer(a) the aperture of the objective is increased — a wider objective narrows the diffraction pattern and lets two close objects be seen separately.