Q.Earth's atmosphere is richest in
(A) UV
(B) IR
(C) X-ray
(D) Microwaves

Remote controls use infrared (IR) light-emitting diodes to send commands.

A television remote control contains an infrared LED that emits pulsed infrared radiation (wavelength ~ 850–950 nm). The coded IR pulses are picked up by a photodiode sensor in the TV. Infrared is used because it is invisible, cheap to generate, and does not interfere with radio broadcasts.

Remote controls for TVs, ACs, etc. transmit coded pulses using an infrared LED, so they use infrared electromagnetic waves.

Infrared waves have wavelengths just longer than visible light and are produced by hot bodies and by special LEDs. They are widely used in remote-control units of TVs, ACs and similar devices because they are safe (non-ionising, low energy), directional, and invisible to the eye.

Scattering by small fog droplets is much stronger for shorter wavelengths, so the longer-wavelength infrared rays pass through fog more easily and are used for photography in fog.

Fog consists of tiny water droplets that scatter light strongly, especially shorter wavelengths (this is the same reason visible light struggles to penetrate fog, reducing visibility). Infrared radiation has a longer wavelength than visible light, so it is scattered much less by these small droplets and can pass through fog with far less attenuation. This property is used in infrared photography to capture images through fog/haze, and similarly in infrared astronomy and remote sensing. Radio waves have even longer wavelength but are not used for imaging in this context; ultraviolet and X-rays have shorter wavelengths and would scatter even more, not less.

Infrared radiation is used in remote controls, night-vision equipment and physiotherapy because of its heating effect on tissue and its detectability by heat/IR sensors. Detecting internal flaws or cracks in metal objects requires the much higher-penetrating, shorter-wavelength X-rays, not infrared — so option (d) is the false statement.

Checking each statement about infrared (IR) rays against its real applications:

1. TV remotes — TRUE. Household remote controls use an IR LED to send coded pulses of infrared light to a receiver in the television; IR is safe, cheap and does not interfere with visible-light signals.
2. Fighter aircraft night vision — TRUE. All objects above absolute zero emit infrared radiation (thermal radiation) roughly in proportion to their temperature. Night-vision and thermal-imaging systems (including those used in military/fighter aircraft) detect this emitted IR to "see" objects (such as another aircraft's engine heat) in the dark, when visible light is insufficient.
3. Relieving muscle pulls — TRUE. Infrared lamps are commonly used in physiotherapy: IR radiation penetrates a short distance into the skin and underlying tissue and is absorbed, producing a local heating effect that increases blood circulation and helps relax muscles and relieve pain/muscle pulls.
4. Detecting cracks in metals — FALSE for infrared. To detect internal structural flaws, cracks, or defects inside metal castings/welds, radiation with very high penetrating power and short wavelength is required — this is the job of X-rays (industrial radiography) or gamma rays, not infrared. Infrared has far too long a wavelength and far too little penetrating power to reveal internal defects in metal; it can at best reveal surface temperature variations, not internal cracks. So statement (d) is not a true application of infrared rays.

The Sun’s infrared rays are the primary carriers of heat energy that warm the Earth’s surface. While UV and visible light also arrive, it is the infrared radiation that directly plays the key role in keeping the Earth warm. The correct option is (A).

Scattering by small fog droplets is much weaker for longer-wavelength radiation, so infrared rays penetrate fog better than visible light and are used for photography in fog.

Fog consists of tiny water droplets that strongly scatter visible light (shorter wavelength), which is why visibility drops in fog. Infrared radiation has a longer wavelength than visible light, so it is scattered much less by these small droplets (scattering by particles of this size falls off rapidly as wavelength increases) and penetrates through fog and haze more effectively. This is why infrared-sensitive photographic film/sensors are used to take clear photographs through fog, mist, and haze, and also in aerial/satellite photography for the same reason.

Infrared radiation was discovered by William Herschel, so the statement (attributing it to 'Retar') is false.

William Herschel discovered infrared radiation in 1800 while measuring the temperature of different colours of the visible spectrum split by a prism — he found the temperature kept rising even beyond the red end of the visible spectrum, revealing an invisible form of radiation (infrared) carrying heat. There is no recognised discoverer of infrared radiation named 'Retar' (this appears to be a name garbled in translation/printing); the correct name is Herschel.

Infrared radiation scatters less than visible light in fog (scattering decreases with increasing wavelength), so it is used for fog/haze photography.

Fog consists of fine water droplets that strongly scatter shorter-wavelength visible light (Mie/Rayleigh-type scattering), reducing visibility. Infrared, having a longer wavelength, is scattered much less and can pass through fog more effectively, making infrared photography useful in foggy conditions.

Infrared radiation was discovered by the astronomer William Herschel in 1800, matching option (f).

William Herschel discovered infrared radiation while measuring the temperature of different colours of the visible spectrum (dispersed by a prism); he found the temperature was still rising just beyond the red end of the visible spectrum, revealing an invisible form of radiation — what we now call infrared. This links the term 'Infrared Radiations' to option (f), Herschel.

Infrared radiation is strongly absorbed by the water molecules and other constituents of most materials, producing a heating effect, so it is popularly called "heat radiation."

Among the electromagnetic spectrum bands listed, infrared waves (wavelength roughly 700 nm to 1 mm) are readily absorbed by matter and raise its temperature — this is why they are commonly referred to as heat waves, and are used in devices like heaters, thermal imaging cameras, and greenhouses.

Converting each option to nanometres and comparing against the infrared band (~700 nm – 1 mm) shows only 10⁻⁴ cm qualifies.

The electromagnetic spectrum, in order of increasing frequency (decreasing wavelength) around the visible region is: infrared ($\sim 700\ \text{nm}$ to $1\ \text{mm}$) → visible light ($400$–$700\ \text{nm}$) → ultraviolet ($10$–$400\ \text{nm}$) → soft X-rays → X-rays.

Convert each option ($1\ \text{cm} = 10^{7}\ \text{nm}$):

• (a) $10^{-4}\ \text{cm} = 10^{-4}\times 10^{7}\ \text{nm} = 1000\ \text{nm} = 1\ \mu\text{m}$ — this is in the infrared range.
• (b) $10^{-5}\ \text{cm} = 100\ \text{nm}$ — this is ultraviolet.
• (c) $10^{-6}\ \text{cm} = 10\ \text{nm}$ — this is at the UV/soft-X-ray boundary.
• (d) $10^{-7}\ \text{cm} = 1\ \text{nm}$ — this is X-ray territory.

Infrared radiation's heating effect (readily absorbed by water molecules) is used industrially to dry out moisture from fruit.

Reasoning

Infrared (IR) radiation is strongly absorbed by water and other polar molecules, producing a heating effect. This heating effect is used in industrial dehydration/drying processes, including producing dehydrated fruit, by evaporating the moisture content efficiently.