EM Spectrum — Wavelengths and uses
EM Waves
8
JEE Qs
8%
Hard
60
min
Create mnemonics for the order of the EM spectrum and compile a concise table of wavelength ranges and primary uses for quick revision.
🧮 Key Formulas
✅ Key Points for JEE
- 1Memorize the exact order of the EM spectrum (Gamma rays, X-rays, Ultraviolet, Visible light, Infrared, Microwaves, Radio waves) in terms of increasing wavelength (which implies decreasing frequency and energy).
- 2Associate the approximate wavelength ranges and specific applications/uses for each major segment of the EM spectrum (e.g., visible light ~400-700 nm, X-rays for medical imaging, Microwaves for radar).
- 3Understand that all electromagnetic waves travel at the speed of light 'c' in a vacuum and are transverse waves, requiring no medium for propagation.
- 4Higher frequency/shorter wavelength EM waves carry more photon energy (E=hf) and are generally more penetrating and potentially harmful (e.g., ionizing radiation like Gamma and X-rays).
- 5Be aware of the different methods of production for various EM waves (e.g., accelerated charges for radio waves, electronic transitions for UV/Visible, nuclear decay for gamma rays).
⚠️ Common Mistakes
- ✕Mixing up the order of different parts of the spectrum, especially adjacent regions like UV, Visible, and IR.
- ✕Confusing the applications or hazards associated with different types of EM radiation (e.g., attributing X-ray uses to UV or vice-versa).
- ✕Incorrectly relating frequency, wavelength, and energy, particularly forgetting that higher frequency means higher energy and shorter wavelength.
📝 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