Which of the following statements about electromagnetic waves, sound waves and water waves is/are correct ? 1. They exhibit reflection 2. They carry energy 3. They exert pressure 4. They can travel in vacuum Select the correct answer using the code given below :
- A.1,2 and 3
- B.2 and 4
- C.1 and 3 only
- D.1 only
Correct answer
Explanation
The correct answer is A, 1, 2 and 3. All three kinds of wave reflect, carry energy and exert pressure, but only electromagnetic waves can travel through a vacuum.
Reflection is common to every wave: light bounces off a mirror, sound returns as an echo, and water waves rebound from a harbour wall. Every wave transfers energy without carrying the medium along, which is why sunlight warms the Earth, loud sound can shatter glass and sea waves wear down cliffs. Because waves also carry momentum, they push on the surfaces they strike: light exerts radiation pressure, which drives solar sails, sound waves exert acoustic pressure, and water waves pound breakwaters. Sound and water waves, however, are mechanical waves that need a material medium, so they cannot cross empty space, while light and radio waves reach us from the Sun and distant galaxies.
B is wrong because statement 4 is true only for electromagnetic waves. C is wrong because it leaves out statement 2. D is wrong because statements 2 and 3 are also true.
Exam tip: mechanical waves, sound and water, need a medium; electromagnetic waves travel through a vacuum at 3 × 108 m/s.
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