Which of the following has the longest wavelength?
(a). Radio waves
(b). Sound waves
(c). X-rays
(d). Infra-red rays

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- Hint: Sun emits electromagnetic waves (EM waves). The wavelengths of these waves have a wide range. The arrangement of these waves on the basis of their wavelengths is called electromagnetic spectrum. Take help of the electromagnetic spectrum.

Complete step-by-step solution -
We know energy is transported to us from the sun via a class of waves known as electromagnetic waves. This class includes the familiar visible, ultraviolet and infrared waves. It was the great Scottish physicist James Clerk Maxwell who showed that electric and magnetic fields fluctuating together can form a propagating ‘electromagnetic wave’. All electromagnetic waves move through vacuum at the same speed, and the symbol ‘c’ is used to denote its value. This speed is called the speed of light in vacuum and its value is $c=3\times {{10}^{8}}m{{s}^{-1}}$. In air, electromagnetic waves travel at nearly the same speed as they do in a vacuum, but, in general, they move through a substance such as glass at a speed that is substantially less than c.
The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultra violet light, X-rays and gamma rays.
The different electromagnetic waves can be arranged in increasing order of their wavelengths as;
Gamma rays < X-rays < Ultraviolet light < visible light < infrared < microwaves < radio waves.
The order of the frequencies these waves is the reverse of the above order.
Therefore, the radio waves have the longest wavelength amongst the EM waves, which range from millimetres to kilometres. The wavelength of sound waves range from millimetres to metres.
Hence, the correct option is (a) radio waves.

 Note: Here are some uses of EM waves. The radio captures radio waves emitted by radio stations, bringing your favourite tunes. A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag. Doctors use gamma-ray imaging to see inside your body.