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JEE Main Electromagnetic Waves — practice questions

Explore 28 free Electromagnetic Waves questions in JEE Main Physics. Open each question or exercise to read its content, reference answers and available explanation.

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Electromagnetic Waves questions and exercises with answers

Showing questions 1–28 of 28.

Two thin lenses of focal lengths $+10$ cm and $+20$ cm are placed in contact. The power of the combination, in diopters, is:Read the answer and worked solution →Two light sources can produce sustained interference fringes on a screen only if they are:Read the answer and worked solution →The speed of an EM wave in vacuum equals:Read the answer and worked solution →Order the steps of EM wave generation by an accelerating chargeRead the answer and worked solution →Which additional current did Maxwell propose to remove the inconsistency?Read the answer and worked solution →Maxwell argued that a magnetic field is generated not only by a current but also by a:Read the answer and worked solution →Who experimentally demonstrated the existence of electromagnetic waves in 1885?Read the answer and worked solution →According to Maxwell's theory, which charges radiate electromagnetic waves?Read the answer and worked solution →A basic source of electromagnetic waves is which arrangement of charge?Read the answer and worked solution →Microwaves are suitable for radar systems in aircraft navigation because of their:Read the answer and worked solution →In an electromagnetic wave, the electric and magnetic fields are:Read the answer and worked solution →Relative to the direction of propagation, the two fields of an electromagnetic wave are:Read the answer and worked solution →The value of c derived by Maxwell equals the speed of light obtained from which measurements?Read the answer and worked solution →The energy carried by a propagating electromagnetic wave comes at the expense of the:Read the answer and worked solution →Cellular phones transmit voice communication using waves in which band?Read the answer and worked solution →Infrared radiation keeps the Earth warm through which effect?Read the answer and worked solution →Most ultraviolet radiation is absorbed in which layer of the atmosphere?Read the answer and worked solution →The electromagnetic spectrum stretches from gamma rays up to which waves?Read the answer and worked solution →In a microwave oven, the frequency is chosen to match the resonant frequency of:Read the answer and worked solution →Ultraviolet radiation can be focussed into narrow beams for which surgical use?Read the answer and worked solution →Whose technological use of electromagnetic waves led to the revolution in communication?Read the answer and worked solution →Which of these were only barely established at the time Maxwell was working?Read the answer and worked solution →Order the spectrumRead the answer and worked solution →Which band does the jobRead the answer and worked solution →Match the name to the contributionRead the answer and worked solution →Spot the false statementRead the answer and worked solution →Pick it fastRead the answer and worked solution →The speed of an EM wave in vacuum equals:Read the answer and worked solution →

Worked practice questions

Two thin lenses of focal lengths $+10$ cm and $+20$ cm are placed in contact. The power of the combination, in diopters, is:

  1. $5$ D
  2. $10$ D
  3. $15$ D
  4. $20$ D
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Correct answer: C. $15$ D

**For thin lenses in contact**, powers add: $P = P_1 + P_2$. $P_1 = \dfrac{1}{f_1\text{ (m)}} = \dfrac{1}{0.10} = 10$ D. $P_2 = \dfrac{1}{0.20} = 5$ D. $P_{\text{total}} = 10 + 5 = 15$ D. Equivalently, $\dfrac{1}{F} = \dfrac{1}{10} + \dfrac{1}{20} = \dfrac{3}{20}$ giving $F = \dfrac{20}{3}$ cm $\approx 6.67$ cm so $P = \dfrac{100}{6.67} = 15$ D.

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Two light sources can produce sustained interference fringes on a screen only if they are:

  1. Coherent (constant phase difference)
  2. Of identical intensity
  3. From the same physical lamp
  4. Polarised in the same direction
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Correct answer: A. Coherent (constant phase difference)

Sustained interference needs sources with a constant phase relationship over time, which is what coherence means. Two ordinary lamps emit light with phases that vary randomly at $10^{-8}$ s scale so their interference pattern washes out almost instantly. The two slits in Young's experiment achieve coherence because both are illuminated by the same primary source, inheriting its phase. Equal intensity (B) improves fringe contrast but is not required. Same lamp (C) is one way to ensure coherence but not the only way (e.g. laser pairs). Polarisation alignment (D) matters for contrast, not for the existence of fringes.

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The speed of an EM wave in vacuum equals:

  1. $1/\sqrt{\mu_0\varepsilon_0}$, approximately $3\times 10^8$ m/s
  2. $\mu_0\varepsilon_0$, the product of two constants here
  3. $\sqrt{\mu_0\varepsilon_0}$, the square root of the product
  4. $\mu_0/\varepsilon_0$, the ratio of the two constants directly
Show answer and solution

Correct answer: A. $1/\sqrt{\mu_0\varepsilon_0}$, approximately $3\times 10^8$ m/s

$c = 1/\sqrt{\mu_0\varepsilon_0} \approx 3\times 10^8$ m/s in vacuum.

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