How does the frequency of a beam of ultraviolet light change when it travels from air into glass?
(A) No change
(B) increases
(C) decreases
(D) remains same

Question

Accepted Answer

Concept understanding — Characteristics of Electromagnetic Waves

Electromagnetic waves share a set of defining properties that follow directly from Maxwell's equations. They are transverse waves: the electric field $\vec{E}$ and magnetic field $\vec{B}$ are always mutually perpendicular, and both are perpendicular to the direction of propagation, with that direction itself given by the cross product $\vec{E}\times\vec{B}$ (which also represents the energy flow of the wave). The two fields vary sinusoidally in phase, reaching their maxima and minima together, and they need no material medium -- EM waves travel through vacuum as readily as through solids, liquids or gases, obeying the principle of superposition like any other wave.

In free space every EM wave travels at the same fixed speed $c=\dfrac{1}{\sqrt{\mu_0\epsilon_0}}\approx3\times10^{8}$ m/s, built entirely from the permeability $\mu_0$ and permittivity $\epsilon_0$ of free space; inside a material medium the speed drops to $v_m=\dfrac{1}{\sqrt{\mu\epsilon}}$ , using that medium's own permeability and permittivity. The amplitudes of the two fields are locked together at every point by the ratio $|\vec{E}_0|/|\vec{B}_0|=c$ , and because the electric field is what dominates optical interactions with matter, it is conventionally singled out as the 'light vector' even though the energy of the wave is, in fact, shared exactly equally between the electric and magnetic fields ( $I_E=I_B$ , since intensity is proportional to the square of the amplitude in each case). Explicitly, a wave travelling along x with E along y and B along z can be written as the pair of in-phase sinusoids $E_y=E_0\sin(kx-\omega t)$ and $B_z=B_0\sin(kx-\omega t)$ , with propagation constant $k=2\pi/\lambda$ and angular frequency $\omega=2\pi\nu$ .

When any wave, including a beam of ultraviolet light, passes from one medium (air) into another (glass), its frequency is set entirely by the source that produced it and does not change at the boundary -- what changes is the wave's speed (it slows down in the denser medium, glass) and correspondingly its wavelength (which shortens, since $v=f\lambda$ with $f$ fixed). This is a completely general wave-propagation result, not specific to ultraviolet light, and follows from the fact that in steady state the number of wave crests arriving at the boundary per second must equal the number leaving it per second, which is exactly what 'frequency' measures. Note that options (A) 'No change' and (D) 'remains same' both express this identical idea, so the question effectively offers the same correct answer twice under different wording. [!ANSWER] (D) remains same

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 29 on this concept.

CBSE 2026Set A1 markMCQ
$Q. Who first explained electromagnetic waves mathematically? (A) Einstein (B) Faraday (C) Maxwell (D) Hertz$
›Reveal solutionSolution

Maxwell predicted EM waves theoretically (1865); Hertz confirmed them experimentally (1887).

James Clerk Maxwell unified electricity and magnetism into four equations and showed mathematically that a changing electric field produces a changing magnetic field and vice versa, propagating as an electromagnetic wave at speed $c = 1/\sqrt{\mu_0\varepsilon_0}$ — equal to the measured speed of light.

Heinrich Hertz later generated and detected these waves experimentally, confirming Maxwell's prediction.

✓Final answer
$(C) Maxwell.$
CBSE 2026Set ANNUAL1 mark
$Q. Mention any one property of electromagnetic waves.$
›Reveal solutionSolution

One key property: electromagnetic waves are transverse - E and B oscillate perpendicular to each other and to the direction the wave travels.

Electromagnetic waves consist of oscillating electric (E) and magnetic (B) fields that are mutually perpendicular, and both are perpendicular to the direction of propagation of the wave - making them transverse waves. Other valid properties (any one suffices to answer): they do not require a material medium and can travel through vacuum; they all travel at the same speed in vacuum, c = 3 x 10^8 m/s; they carry energy and momentum; and they exhibit interference, diffraction, and polarisation like other waves.

✓Final answer
$They are transverse waves - the oscillating electric field E and magnetic field B are perpendicular to each other and to the direction of wave propagation.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. Electromagnetic waves are produced by:$
(a) Charge at rest
(b) Charge in uniform motion
(c) Accelerated charge
(d) None of the above
›Reveal solutionSolution

Electromagnetic waves are radiated only by charges that are accelerating (changing velocity).

A charge at rest produces only a static electric field; a charge in uniform motion produces steady electric and magnetic fields but radiates no wave. Only when a charge accelerates do its associated electric and magnetic fields change with time in a way that detaches from the charge and propagates outward as a self-sustaining electromagnetic wave (an oscillating/accelerating charge, as in an antenna, is the source of EM waves).

✓Final answer
$Electromagnetic waves are produced by an accelerated charge (option c).$
CBSE 2025Set IMPROVEMENT1 markMCQ
$Q. Electromagnetic waves are produced:$
(a) By static charge
(b) By uniformly moving charge
(c) By accelerated charge
(d) By neutral particles
›Reveal solutionSolution

Electromagnetic waves are radiated only by charges that are accelerating (oscillating charges being a common example).

A charge at rest produces only a static electric field, and a charge moving with uniform velocity produces steady electric and magnetic fields, but neither radiates energy as an electromagnetic wave. Only an accelerated charge — one whose velocity is changing (e.g. an oscillating charge) — produces a time-varying electric field, which in turn produces a time-varying magnetic field, and this mutually sustaining, propagating disturbance constitutes an electromagnetic wave (as predicted by Maxwell's equations).

✓Final answer
$(c) By accelerated charge.$
CBSE 2025Set D1 markMCQ
$Q. Which of the following is not an electromagnetic wave? (A) Alpha rays (B) Gamma rays (C) Infrared rays (D) X-rays$
›Reveal solutionSolution

Gamma rays, infrared and X-rays are all EM waves; alpha rays are charged particles (He nuclei), so they are not EM waves.

Electromagnetic waves are oscillating electric and magnetic fields that travel at the speed of light and carry no charge or rest mass. Gamma rays, X-rays and infrared rays are all parts of the electromagnetic spectrum.

Alpha rays, however, are fast-moving helium nuclei (2 protons + 2 neutrons). They have charge (+2e) and mass, are deflected by electric and magnetic fields, and are therefore particle radiation — not an electromagnetic wave.

✓Final answer
$(A) Alpha rays.$
CBSE 2025Set D1 markMCQ
$Q. The speed of electromagnetic wave in any material medium does not depend (A) Upon its wavelength (B) Upon its frequency (C) Upon its intensity (D) Upon its permittivity$
›Reveal solutionSolution

Wave speed in a medium is v = 1/√(με); it does not depend on how intense (bright) the wave is.

The speed of an electromagnetic wave in a material medium is

v = 1/√(μ ε) = c/n

where ε is the permittivity and μ the permeability of the medium. Because the refractive index n varies with frequency (dispersion), the speed does depend on frequency and hence wavelength, and clearly on permittivity.

Intensity is the energy carried per unit area per second (related to amplitude). It has no effect on the propagation speed — a dim and a bright beam of the same colour travel at the same speed.

✓Final answer
$(C) Upon its intensity.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. Which of the following rays is not an electromagnetic wave?$
(a) Gamma rays
(b) X-rays
(c) Microwaves
(d) Beta rays.
›Reveal solutionSolution

Beta rays are streams of electrons, not electromagnetic radiation, so they are the odd one out.

Gamma rays, X-rays and microwaves are all electromagnetic waves — oscillating electric and magnetic fields that travel through space/vacuum at the speed of light, differing only in frequency/wavelength. Beta rays, however, are high-speed electrons (or positrons) emitted during radioactive beta decay; they are material particles carrying charge and rest mass, travel slower than light, and can be deflected by electric/magnetic fields (unlike EM waves).

✓Final answer
$The correct option is (d) Beta rays — they are particles (electrons), not an electromagnetic wave.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. The magnetic field in a plane electromagnetic wave is given by : B_y = 2 \times 10⁻⁷ sin(0.5 \times 10³x + 3.14 \times 10¹¹ t) Tesla. What is the frequency of the wave ?$
(a) 50 GHz
(b) 50 MHz
(c) 0.5 GHz
(d) 5 MHz
›Reveal solutionSolution

Reading the angular frequency $\omega$ directly off the wave equation and using $f=\omega/2\pi$ gives 50 GHz.

The given wave is $B_y = 2\times10^{-7}\sin(0.5\times10^3 x + 3.14\times10^{11} t)$ T, of the standard form $B_y = B_0\sin(kx+\omega t)$ .

Comparing, the angular frequency is

$\omega = 3.14\times10^{11}\ \text{rad/s} \approx \pi\times10^{11}\ \text{rad/s}$

Frequency:

$f = \dfrac{\omega}{2\pi} = \dfrac{\pi\times10^{11}}{2\pi} = 0.5\times10^{11}\ \text{Hz} = 5\times10^{10}\ \text{Hz} = 50\ \text{GHz}$

✓Final answer
$(a) 50 GHz.$
CBSE 2024Set A1 mark
$Q. Write True or False: Electromagnetic waves cannot propagate through any physical medium other than vacuum.$
›Reveal solutionSolution

EM waves are the ONE kind of wave that needs no medium — they travel through vacuum, and also through many material media, just at a different (usually slower) speed.

Unlike mechanical waves (sound, water waves), electromagnetic waves consist of oscillating electric and magnetic fields that regenerate each other, so they require no material medium to propagate — they travel through vacuum at speed $c = 3\times10^8$ m/s. However, this does not mean they cannot travel through other media: light (an EM wave) clearly travels through air, glass, water, etc., though at a reduced speed $v = c/n$ (n = refractive index of the medium). So the statement that EM waves cannot propagate through any medium other than vacuum is incorrect.

✓Final answer
$False.$
CBSE 2024Set A1 mark
$Q. Match Column 'A' with Column 'B' and write the correct pair. Column 'A' item: 'Electromagnetic wave'. Column 'B' options:$
(i) De-Broglie
(ii) Maxwell
(iii) Ohm
(iv) Einstein
(v) Coulomb
(vi) Lenz
(vii) Young.
›Reveal solutionSolution

Electromagnetic waves were theoretically predicted by Maxwell.

James Clerk Maxwell unified electricity, magnetism, and optics into a consistent set of four equations (Maxwell's equations). By adding the displacement-current term, he showed that a changing electric field produces a changing magnetic field and vice versa, self-sustaining a wave that propagates through space at speed $c=1/\sqrt{\mu_0\varepsilon_0}$ — the electromagnetic wave. Maxwell thus theoretically predicted electromagnetic waves before Hertz experimentally verified them.

✓Final answer
$Electromagnetic wave \to (ii) Maxwell .$
CBSE 2024Set ANNUAL1 markMCQ
$Q. The energy in an electromagnetic wave is$
(a) Wholly shared only by electric field vector
(b) Wholly shared only by magnetic field vector
(c) Equally divided between electric and magnetic field
(d) Zero
›Reveal solutionSolution

An electromagnetic wave carries energy in both its electric and magnetic fields, and on time-averaging the two contributions are exactly equal.

The energy density of the electric field part of an EM wave is $u_E = \tfrac12\epsilon_0 E^2$ and of the magnetic field part is $u_B = \tfrac{1}{2}\dfrac{B^2}{\mu_0}$ . Using $E = cB$ and $c = 1/\sqrt{\mu_0\epsilon_0}$ , one finds $u_E = u_B$ at every instant for a plane EM wave in vacuum. So neither field alone carries the wave's energy; it is shared equally between the electric and magnetic field components at all times, not zero and not lopsided toward either field.

✓Final answer
$(c) Equally divided between electric and magnetic field.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Which of the following statements about electromagnetic waves is true?$
(a) Electromagnetic waves travel slower than the speed of light in vacuum
(b) Electromagnetic waves require a medium to propagate
(c) Electromagnetic waves can only be observed in the radio frequency range
(d) Electromagnetic waves can be polarized.
›Reveal solutionSolution
EM waves are transverse waves that travel at the speed of light in vacuum without needing a medium, and span the full spectrum — only the polarisation statement is true.

(a) False — in vacuum, all EM waves travel at exactly $c = 3\times10^8\text{ m/s}$ , not slower.

(b) False — EM waves are self-sustaining oscillations of E and B fields; they need no material medium and can travel through vacuum.

(c) False — EM waves exist across the entire spectrum (radio, microwave, IR, visible, UV, X-ray, gamma), not just radio frequencies.

(d) True — because EM waves are transverse (E and B oscillate perpendicular to the direction of propagation), they can be polarized, unlike longitudinal waves such as sound.

✓Final answer
$Electromagnetic waves can be polarized — option (d).$

Q.How does the frequency of a beam of ultraviolet light change when it travels from air into glass? (A) No change (B) increases (C) decreases (D) remains same

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Q.How does the frequency of a beam of ultraviolet light change when it travels from air into glass?
(A) No change
(B) increases
(C) decreases
(D) remains same

Unlock everything free for 14 days