Question
Arrange the following electromagnetic waves in order of increasing frequency: microwaves, gamma rays, visible light, radio waves, X-rays, infrared, ultraviolet. Give one use of each type.
(NCERT Class 12 — Electromagnetic Waves)
Solution — Step by Step
The order from lowest to highest frequency (longest to shortest wavelength):
Radio waves < Microwaves < Infrared < Visible light < Ultraviolet < X-rays < Gamma rays
All are electromagnetic waves — same nature, same speed ( m/s in vacuum). They differ only in frequency and wavelength, connected by .
| EM Wave | Frequency Range | One Key Use |
|---|---|---|
| Radio waves | to Hz | Broadcasting (TV, FM radio) |
| Microwaves | to Hz | Microwave ovens, radar, satellite communication |
| Infrared (IR) | to Hz | TV remote controls, thermal imaging, night vision |
| Visible light | to Hz | Human vision, optical fibres |
| Ultraviolet (UV) | to Hz | Sterilisation, LASIK surgery, detecting forged currency |
| X-rays | to Hz | Medical imaging (bone fractures), airport security |
| Gamma rays | Hz | Cancer treatment (radiotherapy), sterilising medical equipment |
Real Men In Vests Use X-ray Guns
(Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma — increasing frequency)
Why This Works
All EM waves are produced by accelerating charges and travel at the speed of light. The only difference is frequency, which determines how the wave interacts with matter. Higher frequency means higher energy per photon (), which is why gamma rays are the most penetrating and dangerous, while radio waves are harmless.
Visible light is just a tiny slice of the spectrum — the part our eyes evolved to detect. The Sun’s peak emission is in the visible range, which is why our eyes are tuned to these frequencies.
Alternative Method
Instead of memorising frequency ranges, remember the wavelength order: radio waves have wavelengths from kilometres down to centimetres; gamma rays have wavelengths smaller than atoms ( m). Since and is constant, longer wavelength = lower frequency.
For CBSE 12th boards, a 3-mark question typically asks you to arrange waves and give uses. Write the arrangement clearly, then list uses in a table — this structured format gets full marks with less writing.
Common Mistake
Students often place infrared AFTER visible light (higher frequency). Infrared means “below red” — it has frequency LOWER than red light. Similarly, ultraviolet means “beyond violet” — it has frequency HIGHER than violet light. The names themselves tell you the order if you think about what “infra” (below) and “ultra” (beyond) mean.