Which of the following are the characteristics of electromagnetic waves? 1. They are elastic waves. 2. They can also move in vacuum. 3. They have electric and magnetic components which are mutually perpendicular. 4. They move with a speed equal to 3 lakh meters per second. Select the correct answer using the code given below:
- (a)1, 2, 3 and 4
- (b)1, 2 and 4 only
- (c)2 and 3 only
- (d)3 and 4 only
Correct — C, 2 and 3 only. Electromagnetic waves are NOT elastic (mechanical) waves - they need no material medium - so statement 1 is false. They CAN travel through vacuum (statement 2, true) and they consist of oscillating electric and magnetic fields that are mutually perpendicular, and perpendicular to the direction of travel (statement 3, true). Statement 4 is false: their speed in vacuum is 3 x 10^8 m/s, i.e. 3 lakh KILOMETRES per second, not 3 lakh metres per second. Hence only 2 and 3 are correct.
- (a)1, 2, 3 and 4 — Includes the two false statements - electromagnetic waves are not elastic (1), and their speed is 3 x 10^8 m/s, i.e. 3 lakh km/s, not 3 lakh m/s (4).
- (b)1, 2 and 4 only — Statement 1 (elastic waves) and statement 4 (3 lakh m/s) are both false, and this option drops the correct statement 3 (perpendicular electric and magnetic fields).
- (d)3 and 4 only — Statement 3 is correct, but statement 4 (3 lakh m/s) is wrong, and this option omits the correct statement 2 (they can travel through vacuum).
Electromagnetic waves are non-mechanical (transverse) waves made of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of propagation. They require no medium, travel through vacuum, and move at the speed of light, c = 3 x 10^8 m/s.
Test each statement. 'Elastic waves' describes mechanical waves like sound, not electromagnetic waves - so (1) is false. They do travel through vacuum (2, true) and have mutually perpendicular electric and magnetic components (3, true). The speed is 3 x 10^8 m/s, which is 3 lakh km/s - the unit 'metres per second' in (4) is wrong. Only 2 and 3 survive.
- Electromagnetic waves are non-mechanical, transverse waves - they need no medium and travel through vacuum.
- They consist of mutually perpendicular oscillating electric and magnetic fields, both perpendicular to the direction of propagation.
- Their speed in vacuum is c = 3 x 10^8 m/s = 3 lakh km/s.
- Examples: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Only statements 2 and 3 are correct - hence (c).
- '3 lakh per second' - the unit matters: c is 3 lakh km/s (3 x 10^8 m/s), not 3 lakh m/s.
- Calling electromagnetic waves elastic or mechanical - they need no medium.
Electromagnetic-wave questions test which properties are true (no medium, transverse, perpendicular E and B, speed c) - watch the units on the speed.
Assertion (A): Radio waves bend in a magnetic field. Reason (R): Radio waves are electromagnetic in nature.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(d) A is false but R is true - radio waves are electromagnetic, but being uncharged they are not bent by a magnetic field.
UPSC 2008 on the electromagnetic nature of waves - reinforces that EM waves (radio waves) carry no charge and are unaffected by a magnetic field.
- practice — not a real PYQ
The speed of electromagnetic waves in vacuum is
- (a)3 x 10^8 m/s
- (b)3 x 10^5 m/s
- (c)3 x 10^6 m/s
- (d)3 x 10^10 m/s
Answer(a) 3 x 10^8 m/s - the speed of light.
- practice — not a real PYQ
Electromagnetic waves can travel through
- (a)solids only
- (b)liquids only
- (c)vacuum as well as material media
- (d)gases only
Answer(c) vacuum as well as material media - they need no medium.