Physics · Electromagnetic Waves · NEET
Microwaves are shorter. NCERT calls them 'short-wavelength radio waves'. In the full spectrum the order by wavelength (longest to shortest) is: radio waves > microwaves > infrared > visible > ultraviolet > X-rays > gamma rays. So AM radio ~10^2 m, microwaves ~10^-2 m (a few cm), infrared ~10^-4 m. Microwaves sit between radio and infrared.
Remember about 10^-2 m (roughly 1 cm). Their frequency is in the GHz (10^9 Hz) range. In a matching question, microwaves are the 'centimetre' entry: AM radio ~10^2 m, microwave ~10^-2 m, infrared ~10^-4 m, X-rays ~10^-10 m. Do NOT swap microwave (10^-2 m) with infrared (10^-4 m).
The oven's microwave frequency is chosen to match the resonant frequency of water molecules. At resonance, energy is transferred efficiently into the kinetic energy of the water molecules, so their motion increases and the temperature rises. Food without water absorbs far less, so it heats poorly. This is why NEET matches 'microwave -> warming food (heats water molecules)'.
All three. NCERT says microwaves are produced by special vacuum tubes called klystrons, magnetrons and Gunn diodes. In a match-the-column PYQ, 'microwave' pairs with 'klystron or magnetron valve'. Do not confuse this with visible light (electron transitions in atoms) or infrared (vibration of atoms and molecules).
Microwaves have longer wavelength (~10^-2 m, GHz) and are made by electronic devices (klystron/magnetron). Infrared has shorter wavelength (~10^-4 m) and is produced by vibration of atoms and molecules (heat radiation). Microwaves -> radar and ovens; infrared -> heaters, remote controls and thermal imaging. Both are non-ionising.
Match List I (Electromagnetic waves) with List II (Wavelength): (a) AM radio waves, (b) Microwaves, (c) Infrared waves, (d) X-rays; (i) 10^-10 m, (ii) 10^2 m, (iii) 10^-2 m, (iv) 10^-4 m.
Match List I (Electromagnetic wave) with List II (Production): (A) Microwave, (B) Visible light, (C) Gamma rays, (D) Infrared; (I) Electron transitions in atoms, (II) Radioactive decay, (III) Vibration of atoms and molecules, (IV) Klystron or magnetron valve.
Match List-I with List-II: P. Microwave, Q. UV rays, R. Gamma rays, S. Radio wave; I. For purifying the water, II. For warming the food, III. For AM and FM communication, IV. For treating the cancer cells.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Microwaves have frequencies in the gigahertz (GHz, 10^9 Hz) range. Their wavelength is about 10^-2 m (a few centimetres), which is why NCERT calls them short-wavelength radio waves.
Because of their short wavelength, microwaves travel in narrow beams and reflect well off objects. This makes them suitable for radar systems in aircraft navigation and for speed guns that time balls, tennis serves and cars.
The microwave frequency is chosen to match the resonant frequency of water molecules. Energy is transferred efficiently into the kinetic energy of the water molecules, raising the temperature of any food containing water.
No. Microwaves have much lower frequency and energy than X-rays or gamma rays, so they are non-ionising. They can heat tissue but do not ionise atoms the way X-rays and gamma rays do.
No. Like all electromagnetic waves, microwaves travel through vacuum at the speed of light c = 1/sqrt(mu0*epsilon0) = 3 x 10^8 m/s. They do not need any material medium.
