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"The wavelength of the… (Class 12 Physics 2024, Q26a)

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PhysicsCBSE Class 12 · Board exam 2024, set 55/1/1 · 3 marks · Short answer

CBSE Class 12 Physics 2024: question 26(a)

Set in the CBSE Class 12 board exam of 2024, paper 55/1/1, section C, question 26(a), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26(a) · 3 marks · Short answerElectromagnetic Waves

"The wavelength of the electromagnetic wave is often correlated with the characteristic size of the system that radiates." Give two examples to justify this statement.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 55/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Think about where gamma rays come from and how large that source is.

Hint 2

Compare the length of a radio antenna with the wavelength it transmits.

Hint 3

Pair each part of the EM spectrum with the size of its emitting system.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Example 1 – Gamma rays: they have wavelengths of about – m. This is comparable to nuclear size (≈ 10⁻¹⁵ m), and gamma rays are indeed emitted by atomic nuclei.

    1½ marks

  2. Example 2 – Radio waves: they are produced by accelerating electrons in circuits and antennas. An antenna radiates most efficiently at wavelengths comparable to its own size (metres), so radio wavelengths match antenna dimensions. (Also, X-rays of wavelength ≈ 10⁻¹⁰ m come from inner-shell transitions in heavy atoms, whose size is of the same order.)

    1½ marks

Answer: Gamma rays (~10⁻¹⁵ m) come from nuclei of similar size; radio waves (metres) come from antennas of comparable size (also X-rays from heavy atoms).

Where the marks are

  • First example correctly linking wavelength to source size, e.g. gamma rays – nucleus (1½ marks)
  • Second example, e.g. radio waves – antenna size, or X-rays – heavy atoms (1½ marks)

Revise the chapter

Practise this chapterBoard-style questions from Electromagnetic Waves, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 55/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.