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(i) Long distance radio… (Class 12 Physics 2024, Q26b)

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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(b)

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

Q26(b) · 3 marks · Short answerCompetency-basedElectromagnetic Waves

(i) Long distance radio broadcasts use short-wave bands. Why?
(ii) Optical and radio telescopes are built on the ground, but X-ray astronomy is possible only from satellites orbiting the Earth. Why?

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

Which layer of the upper atmosphere affects radio waves of a few MHz?

Hint 2

Consider which wavelengths can actually get through the atmosphere to the ground.

Hint 3

Short waves bounce off the ionosphere; X-rays are absorbed by the air.

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. (i) Short-wave radio waves (about 3–30 MHz) are reflected back to the Earth by the ionosphere (sky-wave propagation). Repeated reflection between the ionosphere and the ground lets the signal travel very long distances, beyond the curvature of the Earth.

    1½ marks

  2. (ii) The Earth's atmosphere strongly absorbs X-rays, so they cannot reach ground-based telescopes. Visible light and radio waves pass through the atmospheric 'windows'. Hence optical and radio telescopes work on the ground, but X-ray astronomy needs satellites above the atmosphere.

    1½ marks

Answer: (i) Ionosphere reflects short waves (sky waves); (ii) atmosphere absorbs X-rays but transmits visible and radio waves.

Where the marks are

  • Short waves are reflected by the ionosphere, enabling long-distance sky-wave transmission (1½ marks)
  • X-rays are absorbed by the atmosphere while visible and radio waves penetrate it (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.