Q1. A microwave oven produces waves of frequency 2450 MHz. Find their wavelength.
λ = c/f = 3×10⁸ / (2450×10⁶)
λ = 300 / 2450
λ ≈ 0.122 m ≈ 12.2 cm
Electromagnetic Waves — Class 12
Radio, light, X-rays — all the same kind of wave at different frequencies. Learn how EM waves are made and compute their wavelength instantly.
Displacement currentA changing electric field acts like a current (Maxwell's idea), producing a magnetic field just as a real current does.
Nature of EM wavesOscillating electric and magnetic fields, perpendicular to each other and to the direction of travel (transverse). They need no medium.
Speed of EM wavesAll EM waves travel at c = 3×10⁸ m/s in vacuum, where c = 1/√(μ₀ε₀) = fλ, and E₀/B₀ = c.
EM spectrumIn order of increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
Wavelength calculator Interactive
Enter the frequency (in MHz) to find the wavelength of an electromagnetic wave (c = 3×10⁸ m/s).
Wavelength, λ = c/f—m
Higher frequency ⇒ shorter wavelength ⇒ more energetic wave (radio → light → X-rays → gamma).
Numericals practice Interactive
Wavelength and frequency problems with steps.
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Solved numericals
Q2. Arrange in order of increasing frequency: X-rays, radio waves, visible light, microwaves.
Frequency increases as: radio < microwave < visible light < X-rays
So: radio waves, microwaves, visible light, X-rays.
Formula sheet
| Quantity | Formula | SI unit |
|---|---|---|
| Speed of an EM wave | c = f λ | m/s |
| Speed from constants | c = 1 / √(μ₀ε₀) | m/s |
| Field ratio | E₀ / B₀ = c | — |
| Speed of light | c = 3 × 10⁸ | m/s |
Common mistakes & exam wins
- All electromagnetic waves travel at the same speed c = 3×10⁸ m/s in vacuum.
- EM waves are transverse — E and B are perpendicular to each other and to the direction of propagation.
- Order of the spectrum (low → high frequency): Radio, Micro, Infra-red, Visible, Ultra-violet, X-ray, Gamma.
- A changing electric field produces a magnetic field (displacement current) and vice-versa — this keeps the wave going.
Frequently asked questions
What are electromagnetic waves?
Waves of oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel, that propagate through vacuum at 3×10⁸ m/s.
What is displacement current?
A quantity introduced by Maxwell, produced by a changing electric field, that acts like a current in producing a magnetic field.
What is the order of the electromagnetic spectrum?
By increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Do electromagnetic waves need a medium?
No. Unlike sound, EM waves can travel through vacuum, which is how sunlight reaches the Earth.