Q.(a) Identify electromagnetic waves which :

Showing the 12 most recent of 71 on this concept.

1. CBSE 2026Set 55/1/11 markMCQ
   Q.Electromagnetic waves used in a diagnostic tool in medicine have a wavelength range (A) $1\ \text{nm to } 10^{-3}\ \text{nm}$ (B) $400\ \text{nm to } 1\ \text{nm}$ (C) $1\ \text{mm to } 700\ \text{nm}$ (D) $0.1\ \text{m to } 1\ \text{mm}$

   ›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:

   1. 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.

   2. 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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2. 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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3. 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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4. CBSE 2026Set A1 markMCQ
   Q.The refractive index of water is 1.33. What will be the speed of light in water? (A) 1.33 × 10^8 m/s (B) 4 × 10^8 m/s (C) 2.25 × 10^8 m/s (D) 3 × 10^8 m/s

   ›Reveal solutionSolution

   Speed in a medium = c/n = (3 × 10⁸)/1.33 ≈ 2.25 × 10⁸ m/s.

   The refractive index of a medium is the ratio of the speed of light in vacuum to that in the medium:

   $n=\frac{c}{v}\quad\Rightarrow\quad v=\frac{c}{n}$

   …

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5. CBSE 2026Set ANNUAL1 markMCQ
   Q.The refractive indices of glass and water with respect to air are 3/2 and 4/3, respectively. The refractive index of glass w.r.t. water will be

   (a) 8/9

   (b) 9/8

   (c) 7/6

   (d) 6/7

   ›Reveal solutionSolution

   $n_{g/w} = n_{g/a}/n_{w/a} = (3/2)/(4/3) = 9/8$.

   Refractive index of glass w.r.t. water can be obtained by combining the refractive indices w.r.t. air:

   …

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6. 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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7. CBSE 2026Set ANNUAL1 markMCQ
   Q.Refraction takes place due to

   (a) change in the speed of light

   (b) no change in the speed of light

   (c) change in the colour of light

   (d) polarization

   ›Reveal solutionSolution

   Light bends when it crosses into a medium where its speed is different; that speed change is the fundamental cause of refraction.

   When light travels from one medium into another (e.g. air into glass), its speed changes because the two media have different optical densities (different refractive indices). According to Snell's law, n1sin(theta1) = n2sin(theta2), and refractive index n = c/v (speed of light in vacuum divided by speed in the medium). Because the speed v changes at the boundary, the direction of the light ray bends - this bending is refraction. If the speed did no …

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8. 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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9. CBSE 2026Set ANNUAL1 markMCQ
   Q.The optical density of turpentine is higher than that of water while its mass density is lower. Figure shows a layer of turpentine floating over water in a container. Which of the following four rays incident on turpentine in figure, the path shown is correct?

   (a) 1

   (b) 2

   (c) 3

   (d) 4

   ›Reveal solutionSolution

   Since turpentine is optically the densest of the three (n(turpentine) > n(water) > n(air)), a ray must bend toward the normal at the air–turpentine surface, then bend away from the normal (but not all the way back) at the turpentine–water surface.

   The question states the optical density (refractive index) order is turpentine > water > air (even though turpentine's mass density is lower than water's, which is why it floats — optical density and mass density are unrelated). Refraction at each interface follows Snell's law, $n_1\sin\theta_1 = n_2\sin\theta_2$:

   • Air → Turpentine: going into an optically denser medium, the ray bends towards the normal (angle decreases).
   • Turpentine → Water: water is rarer than turpentine, so going into it the ray bends away from the normal (angle increases) compared to its path inside turpentine. …

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10. CBSE 2026Set ANNUAL1 mark
    Q.In phenomenon of refraction of light, which property of it remains unchanged ?

    ›Reveal solutionSolution

    In refraction, frequency stays constant; speed and wavelength change.

    When light travels from one medium into another, its speed changes (v = c/n) and consequently its wavelength changes (λ = v/f). However, the frequency f is determined by the source of light and does not change when the wave crosses the boundary — the number of wavefronts arriving per second must equal the number leaving per second ( …

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11. 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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12. CBSE 2025Set JS1 markMCQ
    Q.Electro-magnetic wave of minimum frequency is: (A) Ultraviolet rays (B) X-rays (C) Gamma ($\gamma$-) rays (D) Micro waves

    ›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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