Physics · Electromagnetic Waves · NEET
Not exactly. Infrared is an electromagnetic wave, like light. It is called a 'heat wave' because when molecules of water, CO2 or NH3 absorb IR, their thermal motion increases and they heat up. So IR carries energy that becomes heat when absorbed, but the wave itself is radiation, not heat.
Because most materials contain water molecules, and water absorbs infrared very readily. After absorbing IR, the molecules vibrate faster (more thermal motion) and heat their surroundings. That is why an infrared lamp feels warm on your skin — the IR is absorbed and turned into heat.
NCERT gives infrared from about 700 nm (7 x 10^-7 m) up to about 1 mm (10^-3 m). It lies just after the red end of visible light. So IR wavelength is LONGER than visible light but SHORTER than microwaves. A common exam value is about 10^-4 m (0.1 mm) for the middle of the IR band.
Infrared is produced by hot bodies and by the vibration of atoms and molecules. Any warm object (your body, a heater, the Sun) emits IR. In NEET matching questions the standard answer for IR production is 'vibration of atoms and molecules'.
Both are longer than visible light, but microwaves are longer than infrared. Order by increasing wavelength: visible < infrared < microwave < radio. Microwaves (~10^-2 m, cm range) are made by special vacuum tubes like the klystron or magnetron; infrared (~10^-4 m) is made by hot bodies and molecular vibration.
Visible light from the Sun passes through the atmosphere and is absorbed by Earth's surface. The warm surface then re-radiates energy as infrared. Greenhouse gases (CO2, water vapour) absorb this outgoing IR and trap the heat, keeping Earth warm. This is the greenhouse effect.
NCERT lists thermopiles, the bolometer, and infrared photographic film as IR detectors. These respond to the heating effect of the absorbed radiation. Snakes can also detect infrared to sense warm prey.
Match List I (Electromagnetic waves) with List II (Wavelength) and choose the correct match. List I: (a) AM radio waves (b) Microwaves (c) Infrared waves (d) X-rays. List II: (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) and choose the correct match. List I: (A) Microwave (B) Visible light (C) Gamma rays (D) Infrared. List II: (I) Electron transitions in atoms (II) Radioactive decay (nuclear transitions) (III) Vibration of atoms and molecules (IV) Klystron or magnetron valve
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Infrared is an electromagnetic wave, so in vacuum it travels at c = 3 x 10^8 m/s, exactly like visible light, radio waves and X-rays. Only its wavelength and frequency differ.
Ordinary infrared from warm bodies is not ionising, so it is far safer than UV, X-rays or gamma rays. It mainly produces heating. Very intense IR (like from a strong heat source) can burn, but at normal levels it is used safely in physical therapy lamps.
Visible light has more energy per photon. Energy E = hf, and infrared has a lower frequency than visible light, so each infrared photon carries less energy than a visible-light photon.
Physical therapy (infrared lamps), remote controls, night-vision and thermal imaging, and it plays a role in Earth's greenhouse warming. Detectors include thermopiles, bolometers and IR film.
