Assertion (A): X-rays are produced when slow moving electrons are stopped by a metal target of high atomic num

Question

Q. Assertion (A): X-rays are produced when slow moving electrons are stopped by a metal target of high atomic number. Reason (R): X-rays consist of low-energy photons. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

CBSECBSE Class XII Board 2025MCQ· 1mImportance★★★★★

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Concept understanding — Electromagnetic Spectrum Regions And Uses

The full electromagnetic spectrum is the orderly classification of every possible electromagnetic wave by wavelength or frequency (related by $c=f\lambda$ in vacuum), running continuously from the longest-wavelength, lowest-frequency radio waves to the shortest-wavelength, highest-frequency gamma rays, with visible light forming only a narrow band roughly in the middle. Radio waves (a few Hz to $10^9$ Hz) are produced by accelerated charges oscillating in conducting wires (antennas), show reflection and diffraction, and carry radio and television broadcasts and cellular voice communication in the UHF band. Microwaves ( $10^9$ to $10^{11}$ Hz) are produced by specialised vacuum-tube devices -- the klystron, the magnetron, and the Gunn diode -- undergo reflection and can be polarised, and are used in radar for aircraft navigation and vehicle-speed detection, in microwave ovens, and for long-distance satellite communication. Infrared radiation ( $10^{11}$ to $4\times10^{14}$ Hz), sometimes called heat radiation, is produced by hot bodies and by molecules undergoing rotational and vibrational transitions, and is used in infrared detectors aboard Earth satellites for military surveillance and to observe crop growth, in night-vision/infrared photography, in physiotherapy for muscular strain, to maintain the Earth's warmth through the greenhouse effect, and in remote controls for TVs and other appliances. Visible light ( $4\times10^{14}$ to $8\times10^{14}$ Hz) is produced by incandescent bodies and by excited atoms in gases, obeys the ordinary laws of reflection and refraction, shows interference, diffraction, polarisation and the photoelectric effect, and is what produces the sensation of vision -- it is also used to probe molecular structure and the arrangement of electrons in the outer shells of atoms. Ultraviolet radiation ( $8\times10^{14}$ to $10^{17}$ Hz), from the Sun, arcs and ionized gases, sterilises instruments and destroys bacteria, drives burglar alarms, reveals invisible writing and fingerprints, and aids atomic-structure study, though most is absorbed by atmospheric ozone and can be harmful to the human body. X-rays ( $10^{17}$ to $10^{19}$ Hz), produced by suddenly stopping high-speed electrons at a high-atomic-number target or by tr …

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Both statements are false: X-rays require FAST (not slow) electrons to be decelerated by a high-Z target, and X-ray photons are HIGH-energy (not low-energy). The correct option is (D).

Concept and intuition

X-rays are electromagnetic radiation of very short wavelength (roughly $10^{-11}$ m to $10^{-8}$ m) and correspondingly very HIGH photon energy — from about $100\,\text{eV}$ up to $100\,\text{keV}$ , thousands of times more energetic than visible-light photons ( $\sim2$ – $3\,\text{eV}$ ). They are produced in an X-ray tube by bremsstrahlung ("braking radiation"): electrons are first accelerated through a large potential difference (tens of kilovolts), giving them very HIGH kinetic energy, and are then suddenly decelerated on striking a metal target of high atomic number (e.g. tungsten, $Z=74$ ) — a high- $Z$ target is used because the efficiency of bremsstrahlung production increases with $Z$ .

Evaluating the Assertion

The Assertion states that X-rays are produced when slow moving electrons are stopped by a high- $Z$ target. This gets the mechanism backwards: it is precisely because the electrons are moving very fast (having been accelerated through a large voltage) that stopping them suddenly releases enough energy to produce X-ray photons. A genuinely slow-moving electron carries far too little kinetic energy to produce a photon anywhere in the X-ray range. So the Assertion, as literally worded, is false — the single word "slow" (which should read "fast") is what makes it false; the rest of the statement (high-Z target, sudden stopping) is correct in isolation, but the assertion as a whole is not.

Evaluating the Reason

The Reason states that X-rays consist of low-energy photons. This is also the opposite of the truth: X-ray photons are HIGH-energy compared to visible, infrared, or radio-wave photons — that is exactly why they can penetrate soft tissue and are dangerous in large doses. So the Reason is false as well. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 42 on this concept.

CBSE 2026Set 55/1/11 markMCQ
$1\ \text{nm to } 10^{-3}\ \text{nm}$
›Reveal solutionSolution
The question asks for the wavelength range of electromagnetic waves used in a medical diagnostic tool. X-rays, used in radiography and CT scans, have wavelengths from about $1\ \text{nm}$ down to $10^{-3}\ \text{nm}$ . The correct option is (A).

The key here is to connect the medical diagnostic tool to the specific type of electromagnetic wave it uses. The most common diagnostic tools in medicine that rely on electromagnetic waves are X-ray machines (for bone fractures, chest X-rays, CT scans) and MRI machines (which use radio waves). The question gives wavelength ranges, so we need to match the correct range to the tool.

X-rays are the standard for imaging hard tissues like bones. Their wavelengths are extremely short — much shorter than visible light. Visible light ranges from about $700\ \text{nm}$ (red) to $400\ \text{nm}$ (violet). X-rays lie beyond ultraviolet, in the range of roughly $1\ \text{nm}$ down to $0.001\ \text{nm}$ ( $10^{-3}\ \text{nm}$ ). This matches option (A) exactly.

Let’s check the other options to be sure:

Option (B): $400\ \text{nm to } 1\ \text{nm}$ — This covers the ultraviolet and part of the visible spectrum. Ultraviolet is used in sterilisation, not in routine diagnostic imaging. So this is incorrect.

Option (C): $1\ \text{mm to } 700\ \text{nm}$ — This spans the infrared region. Infrared is used in thermal imaging (thermography), but that is not a primary diagnostic tool like X-rays. The question likely refers to the most common tool, so this is not the best answer. …
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CBSE 2026Set ANNUAL1 mark
$Q. Which electromagnetic wave has the highest energy?$
›Reveal solutionSolution

Energy of an EM wave is proportional to its frequency; gamma rays have the highest frequency.

Photon energy $E=hf$ , so the electromagnetic wave with the highest frequency has the highest energy. In the electromagnetic spectrum, gamma …

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CBSE 2026Set DS1 markMCQ
$Q. Electromagnetic waves of minimum frequency is:$
i) ultraviolet rays
ii) $X$ rays
iii) $\gamma$ -rays
iv) microwaves
›Reveal solutionSolution

Microwaves have the longest wavelength (lowest frequency) of the four, so they are the minimum-frequency wave.

Concept. In the electromagnetic spectrum, frequency increases (and wavelength decreases) in the order:

$\text{radio} < \text{microwave} < \text{infrared} < \text{visible} < \text{ultraviolet} < \text{X-rays} < \gamma\text{-rays}.$ …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. Which of the following has the smallest wavelength?$
(a) Micro-waves
(b) Red light
(c) Ultraviolet radiation
(d) gamma-rays
›Reveal solutionSolution

Across the electromagnetic spectrum, wavelength decreases (and frequency/energy increases) in the order: micro-waves > red light (visible) > ultraviolet > gamma-rays.

The electromagnetic spectrum, arranged from LONGEST to SHORTEST wavelength, runs roughly: radio waves > microwaves > infrared > visible light (red to violet) > ultraviolet > X-rays > gamma rays. Comparing the four given options in that order - micro-waves (longest here), red light (visible, shorter than microwaves), …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. Identify the constituent radiation of electromagnetic spectrum which is used in medicine to destroy cancer cells.$
(a) X-rays
(b) UV rays
(c) IR rays
(d) Gamma rays
›Reveal solutionSolution

Gamma rays, the highest-energy, shortest-wavelength part of the EM spectrum, are used in radiotherapy to destroy cancer cells.

Among the options, X-rays are chiefly used for imaging (and in some radiotherapy), UV rays for sterilisation and detecting fluorescence, and IR rays for heating and thermal imaging. Gamma rays, emitted by radioactive nuclei, carry very high photon energy and deep penetrating power. In medicine this is exploited in gamma-ray therapy (e.g. Cobalt-60 teletherapy, the 'gamm …

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CBSE 2025Set 55/6/11 markMCQ
$Q. Assertion (A): X-rays are produced when slow moving electrons are stopped by a metal target of high atomic number. Reason (R): X-rays consist of low-energy photons. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.$
›Reveal solutionSolution

Both statements are false: X-rays require FAST (not slow) electrons to be decelerated by a high-Z target, and X-ray photons are HIGH-energy (not low-energy). The correct option is (D).

Concept and intuition

X-rays are electromagnetic radiation of very short wavelength (roughly $10^{-11}$ m to $10^{-8}$ m) and correspondingly very HIGH photon energy — from about $100\,\text{eV}$ up to $100\,\text{keV}$ , thousands of times more energetic than visible-light photons ( $\sim2$ – $3\,\text{eV}$ ). They are produced in an X-ray tube by bremsstrahlung ("braking radiation"): electrons are first accelerated through a large potential difference (tens of kilovolts), giving them very HIGH kinetic energy, and are then suddenly decelerated on striking a metal target of high atomic number (e.g. tungsten, $Z=74$ ) — a high- $Z$ target is used because the efficiency of bremsstrahlung production increases with $Z$ .

Evaluating the Assertion

The Assertion states that X-rays are produced when slow moving electrons are stopped by a high- $Z$ target. This gets the mechanism backwards: it is precisely because the electrons are moving very fast (having been accelerated through a large voltage) that stopping them suddenly releases enough energy to produce X-ray photons. A genuinely slow-moving electron carries far too little kinetic energy to produce a photon anywhere in the X-ray range. So the Assertion, as literally worded, is false — the single word "slow" (which should read "fast") is what makes it false; the rest of the statement (high-Z target, sudden stopping) is correct in isolation, but the assertion as a whole is not.

Evaluating the Reason

The Reason states that X-rays consist of low-energy photons. This is also the opposite of the truth: X-ray photons are HIGH-energy compared to visible, infrared, or radio-wave photons — that is exactly why they can penetrate soft tissue and are dangerous in large doses. So the Reason is false as well. …
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CBSE 2025Set JS1 markMCQ
$\gamma$
›Reveal solutionSolution

In the EM spectrum the order of increasing frequency is microwave < ultraviolet < X-ray < $\gamma$ -ray, so microwaves have the minimum frequency — option (D).

Concept. For all electromagnetic waves $c=\nu\lambda$ , so a larger wavelength means a smaller frequency. Arranged by increasing frequency:

$\text{radio} < \text{micro} < \text{infrared} < \text{visible} < \text{UV} < \text{X-ray} < \gamma\text{-ray}.$

…

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CBSE 2025Set A1 mark
$Q. Write True or False: Frequency is maximum for long radio waves.$
›Reveal solutionSolution

The statement is False: frequency is minimum, not maximum, for long radio waves.

In the electromagnetic spectrum, wavelength and frequency are inversely related (c = νλ). Radio waves have the longest wavelengths and hence the LOWEST frequencies (as low as a few Hz to MHz), while at the other end, gamma rays have the shortest wavelengths and the HIGH …

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CBSE 2025Set A1 mark
$Q. Write answer in one sentence: What is the degree of order of frequency of visible light?$
›Reveal solutionSolution

The frequency of visible light is of the order of 10¹⁴ Hz.

Visible light has wavelengths roughly between 400 nm (violet) and 700 nm (red). Using $c = \nu\lambda$ with $c = 3\times10^8$ m/s:

For red light ( $\lambda \approx 700$ nm): $\nu = \dfrac{3\times10^8}{700\times10^{-9}} \approx 4.3\times10^{14}$ Hz

For violet light ( $\lambda \approx 400$ nm): $\nu = \dfrac{3\times10^8}{400\times10^{-9}} \approx 7.5\times10^{14}$ Hz

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. The frequencies of gamma-rays, ultraviolet rays, and X-rays are ν1, ν2 and ν3 respectively. Then$
(i) ν1 = ν2 = ν3
(ii) ν1 > ν3 > ν2
(iii) ν1 > ν2 > ν3
(iv) ν3 > ν2 > ν1
›Reveal solutionSolution

In the electromagnetic spectrum, frequency increases in the order: ultraviolet < X-rays < gamma rays.

The electromagnetic spectrum is ordered by wavelength/frequency as follows (increasing frequency): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Gamma rays (emitted in nuclear transitions) have the shortest wavelength and hence the highest frequency; X-rays (produced by high-energy electron transitions/deceleration) come next; ultraviolet rays have a comparatively lower fr …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Which has the longest wavelength ?$
(a) X-rays
(b) Micro waves
(c) Infrared rays
(d) Radio waves
›Reveal solutionSolution

In the electromagnetic spectrum, wavelength increases in the order X-rays < infrared < microwaves < radio waves, so radio waves are longest.

The electromagnetic spectrum, arranged by increasing wavelength (decreasing frequency/energy), runs roughly:

Gamma rays < X-rays < Ultraviolet < Visible light < Infrared < Microwaves < Radio waves. …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. The frequency of gamma-rays, x-rays and ultra-violet rays are a, b and c respectively. Then -$
(a) a < b < c
(b) a > b > c
(c) a = b = c
(d) a > c > b
›Reveal solutionSolution

In the electromagnetic spectrum, gamma rays have the highest frequency, then X-rays, then ultraviolet.

Ordering the electromagnetic spectrum by increasing frequency (equivalently, decreasing wavelength): radio waves < microwaves < infrared < visible < ultraviolet < X-rays < gamma rays.

…

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Concept and intuition

Evaluating the Assertion

Evaluating the Reason

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