Which one of the following statements is correct ? Wavelength of microwaves ranges between

Correct — A, (a) infrared waves and radio waves. Set the electromagnetic spectrum out in order of increasing wavelength and the answer reads straight off it: gamma rays, then X-rays, then ultraviolet, then visible light, then infrared, then MICROWAVES, then radio waves. Microwaves occupy the band from about a millimetre up to about a metre in wavelength, corresponding to frequencies of roughly 300 MHz to 300 GHz. Infrared sits immediately on the short-wavelength side of that band, ending where microwaves begin at about a millimetre; radio waves sit immediately on the long-wavelength side, taking over above about a metre. So microwaves are bounded by infrared below and radio above, which is exactly what this option says. The name itself is the memory aid: they are called MICROwaves because their wavelengths are small compared with the ordinary radio waves they were distinguished from, not because they are small in any absolute sense — a microwave is enormous beside a wave of visible light.

Why the others are wrong

• (b)visible waves and infrared light. — This names the wrong side of infrared. Visible light runs from about 400 to 700 nanometres and infrared from about 700 nanometres to a millimetre, so the band lying between visible and infrared is a narrow strip at the near-infrared boundary — nothing like the microwave band, which begins where infrared ENDS. Placing microwaves here shortens their wavelength by a factor of about a thousand. The error behind this option is thinking of infrared as the far end of the spectrum because it is the last band before we stop being able to see anything, when in fact two whole bands lie beyond it.
• (c)γ-rays and X-rays. — This puts microwaves at the extreme SHORT-wavelength, high-frequency, high-energy end of the spectrum, which is the opposite corner from where they belong. Gamma rays have wavelengths below about 0.01 nanometre and X-rays run from roughly 0.01 to 10 nanometres — both smaller than an atom or a molecule, and both ionising, capable of knocking electrons out of atoms and damaging tissue. Microwaves are around a hundred million times longer in wavelength and are non-ionising; they heat matter but do not ionise it. Anyone who has used a microwave oven and an X-ray machine knows they cannot be neighbours in the spectrum.
• (d)X-rays and visible waves. — The band lying between X-rays and visible light is ULTRAVIOLET, not microwave. Ultraviolet runs from about 10 to 400 nanometres — the radiation the ozone layer absorbs and the reason sunburn happens — and it is still far shorter in wavelength than infrared, let alone microwave. Like the previous option this places microwaves on the high-energy side of visible light, when everything about them, from their use in ovens and radar to their inability to ionise atoms, puts them on the low-energy side. Fixing the sequence gamma, X-ray, ultraviolet, visible, infrared, microwave, radio in memory removes all three of these wrong options at once.

All electromagnetic waves are the same physical thing — oscillating electric and magnetic fields travelling through vacuum at the speed of light, about 3 x 10^8 metres per second — and differ only in wavelength and frequency, related by c = f x lambda. Because c is fixed, a longer wavelength always means a lower frequency and lower photon energy. The conventional bands, in order of increasing wavelength and therefore decreasing energy, are: gamma rays (below about 0.01 nm), X-rays (about 0.01 to 10 nm), ultraviolet (10 to 400 nm), visible light (400 to 700 nm), infrared (700 nm to 1 mm), microwaves (1 mm to 1 m) and radio waves (above 1 m). The boundaries are conventions rather than sharp physical lines, and microwaves are often treated as the short-wavelength end of the radio region rather than as a separate band, which is why the phrase 'between infrared and radio' describes them so well. What sits where has practical consequences. Gamma, X-ray and short ultraviolet radiation are IONISING and therefore biologically damaging; everything from long ultraviolet downwards is non-ionising. Microwaves in particular carry too little energy per photon to ionise, but their frequency happens to drive the rotation of polar molecules such as water, which is how a microwave oven heats food — dielectric heating from within rather than conduction from a hot surface. The same band carries radar, satellite links, mobile telephony and Wi-Fi, because these wavelengths pass through the atmosphere and through clouds with little loss and can be focused by dish antennas of a practical size. The cosmic microwave background, the relic radiation of the early universe, also lies in this band.

Key facts

• Order of the electromagnetic spectrum by increasing wavelength: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.
• Microwave wavelengths run from about 1 millimetre to about 1 metre, or frequencies of about 300 MHz to 300 GHz.
• Infrared bounds the microwave band on the short-wavelength side and radio waves bound it on the long-wavelength side.
• All electromagnetic waves travel at the same speed in vacuum, about 3 x 10^8 metres per second, and obey c = f x lambda.
• Longer wavelength means lower frequency and lower photon energy.
• Gamma rays, X-rays and short ultraviolet are ionising; visible, infrared, microwave and radio radiation are non-ionising.
• A microwave oven heats food by making polar water molecules rotate, a form of dielectric heating that works from within the food.
• Microwaves are used in radar, satellite communication, mobile telephony and Wi-Fi, and the cosmic microwave background lies in this band.

Common traps

• Assuming 'micro' means the wavelengths are among the smallest. Microwaves are among the LONGEST, second only to radio waves.
• Placing microwaves on the high-energy side of visible light. They are on the low-energy side, beyond infrared.
• Forgetting that infrared, not microwave, is the band immediately beyond visible red.
• Mixing up the ultraviolet slot, which lies between X-rays and visible light, with the microwave slot.
• Reading only the first line of a two-line stem and answering a different question from the one printed.

• practice — not a real PYQ

  Which one of the following electromagnetic radiations has the longest wavelength ?

  • (a)Ultraviolet rays
  • (b)Infrared rays
  • (c)Microwaves
  • (d)Radio waves

  Answer(d) Radio waves

• practice — not a real PYQ

  A microwave oven cooks food mainly by

  • (a)ionising the molecules of the food
  • (b)making the polar water molecules in the food rotate rapidly
  • (c)conducting heat inward from the hot metal walls of the oven
  • (d)exposing the food to infrared radiation from a heated filament

  Answer(b) making the polar water molecules in the food rotate rapidly