Properties of EM Waves — Speed, transverse nature
EM Waves
8
JEE Qs
8%
Hard
75
min
Always visualize the perpendicular relationship between E, B, and propagation direction, and use E x B = direction of propagation to solve vector-related problems.
🧮 Key Formulas
✅ Key Points for JEE
- 1Electromagnetic (EM) waves are transverse waves, meaning the oscillating electric field (E), magnetic field (B), and the direction of propagation (k) are mutually perpendicular.
- 2The vectors E, B, and the direction of propagation (k) form a right-handed system, such that E x B gives the direction of propagation.
- 3The speed of EM waves in vacuum (c) is a universal constant, independent of the source or observer's motion, and is approximately 3 x 10^8 m/s.
- 4The speed of EM waves in a material medium (v) is less than 'c' and depends on the electric permittivity (ε) and magnetic permeability (μ) of that medium.
- 5EM waves do not require a material medium for propagation and can travel through vacuum.
⚠️ Common Mistakes
- ✕Confusing the directions of E, B, and propagation, often assuming they are parallel or using an incorrect cross-product relationship.
- ✕Incorrectly applying the speed of light in vacuum (c) for EM waves propagating through a medium instead of the medium-dependent speed (v).
- ✕Forgetting that EM waves are transverse and not longitudinal, which is crucial for understanding phenomena like polarization.
📝 Practice Questions
See allQ39.Arrange the following in the ascending order of wavelength (λ) : (A) Microwaves (λ1) (B) Ultraviolet rays (λ2) (C) Infrared rays (λ3) (D) X-rays (λ4) Choose the most appropriate answer from the options given below : (1) λ4 < λ3 < λ2 < λ1 (2) λ3 < λ4 < λ2 < λ1 (3) λ4 < λ3 < λ1 < λ2 (4) λ4 < λ2 < λ3 < λ1
Q38.The magnetic field of an E.M. wave is given by B = ( √32 ^i + 12 ^j)30 corresponding electric field in S.I. units is : → → (1) 1 (2) 3 1 E = cos [ω (t −zc )] ( 2^i −√32 ^j)30c sin [ω (t −zc )] E = ( 4^i + 4 ^j)30c → → √3 (3) (4) E = + E = sin [ω (t + zc )] sin [ω (t + zc )] ( 12^i 2 ^j)30c ( √32 ^i −12 ^j)30c
Q37.The electric field of an electromagnetic wave in free space is → E = 57 cos [7.5 × 106t −5 × 10−3(3x + 4y)](4^i −3^j)N/C . The associated magnetic field in Tesla is ⇀ 57 (1) B= cos [7.5 × 106t −5 × 10−3(3x + 4y)](^k) 3×108 → (2) 57 B = − cos [7.5 × 106t −5 × 10−3(3x + 4y)](^k) 3×108 → (3) 57 B = − cos [7.5 × 106t −5 × 10−3(3x + 4y)](5^k) 3×108 → (4) 57 B = cos [7.5 × 106t −5 × 10−3(3x + 4y)](5^k) 3×108
Q39.The equation of a transverse wave travelling along a string is y(x, t) = 4.0 sin [20 × 10−3x + 600t]mm , where x is in mm and t is in second. The velocity of the wave is : 2025 (23 Jan Shift 2) JEE Main Previous Year Paper (1) −60 m/s (2) −30 m/s (3) +30 m/s (4) +60 m/s
Q38.Due to presence of an em-wave whose electric component is given by E = 100 sin(ωt −kx)NC−1 , a cylinder of length 200 cm holds certain amount of em-energy inside it. If another cylinder of same length but half diameter than previous one holds same amount of em-energy, the magnitude of the electric field of the corresponding em-wave should be modified as (1) 400 sin(ωt −kx)NC−1 (2) 200 sin(ωt −kx)NC−1 (3) 50 sin(ωt −kx)NC−1 (4) 25 sin(ωt −kx)NC−1
Q30.Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : Electromagnetic waves carry energy but not momentum. Reason ( R): Mass of a photon is zero. In the light of the above statements, choose the most appropriate answer from the options given below : (1) Both (A) and (R) are true and (R) is the correct (2) Both (A) and (R) are true but (R) is not the explanation of (A) correct explanation of (A) (3) (A) is false but (R) is true (4) (A) is true but ( R) is false
NCERT Chapters
- Class 12 Physics Ch 8: Electromagnetic Waves