Consider the two statements given below : Statement-1 : Infrared waves are also called heat waves. Statement-2 : Water molecules readily absorb infrared waves. Select the correct answer using the code given below:
- A.Both the statements are individually true and Statement-2 is the correct explanation of Statement-1.
- B.Both the statements are individually true, but Statement-2 is not the correct explanation of Statement-1.
- C.Statement-1 is true, but Statement-2 is false.
- D.Statement-2 is true, but Statement-1 is false.
Correct answer
Explanation
The correct answer is A. Both statements are true, and the second explains the first: infrared waves are called heat waves because water molecules, present in most materials, absorb them readily and heat up.
Infrared radiation lies just beyond the red end of visible light, with wavelengths from about 700 nm to 1 mm. When water and other molecules absorb it, their thermal motion increases and the material becomes warmer; this is why we feel the warmth of the Sun, a heater or a fire on our skin. Infrared is also used in physiotherapy lamps, night-vision cameras, TV remote controls and weather satellites, and the greenhouse effect depends on gases trapping infrared radiation from the Earth.
B is wrong because the absorption by water molecules is exactly the reason for the name.
C is wrong because Statement-2 is true; water absorbs infrared strongly.
D is wrong because Statement-1 is true; "heat waves" is the usual name for infrared.
Exam tip: infrared = heat waves (absorbed by water molecules); ultraviolet = chemical rays.
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