Question

Which of the following electromagnetic waves has the longest wavelength?
A. Heat waves
B. Visible light
C. Radio frequency waves
D. Microwaves

Answer 1

Hint: Here we need to know that the waves which are smaller in frequency have the longest wavelength and vice-versa. Knowing this will solve your problem.

Formula used – Speed of light = frequency ($\nu$) x wavelength ($\lambda$)
$\Rightarrow$Gamma rays - $\nu = 3 \times {10^{23}}Hz,\,\,\,\lambda = {10^{ - 6}}nm$

Complete step-by-step solution -
Gamma rays have the shortest wavelength and the most intensity of any wave in the electromagnetic spectrum. These are formed by the hottest and most energetic events in the universe, such as neutron stars and pulsars, supernova explosions, and black hole regions.
$\Rightarrow$X-ray - $\nu = 3 \times {10^{17}}Hz,\,\,\,\lambda = 1nm$

An X-ray is a simple , painless test that produces images of the structures within your body — especially your bones. X-ray rays pass through the body and are absorbed in varying quantities depending on the composition of the substance they pass through.
$\Rightarrow$Ultraviolet - $\nu = 3 \times {10^{16}}Hz,\,\,\,\lambda = 10nm$

The short-wavelength UVC is the most dangerous form of UV radiation. Nevertheless, the atmosphere is fully filtered and does not touch the surface of the earth. Medium-wavelength UVB is very biologically active but can not penetrate beyond the surface of the skin.
$\Rightarrow$Visible - $\nu = 7.5 \times {10^{14}}Hz,\,\,\,\lambda = 400nm$
The light we can see without any harmfulness is called visible light.
$\Rightarrow$Infrared - $\nu = 3 \times {10^{13}}Hz,\,\,\,\lambda = 10000nm$

Radar technology is considered an active remote sensing system because it actively sends a microwave pulse and senses the energy reflected back. Doppler Radar and Radar Altimeters are examples of active remote sensing instruments that use microwave frequencies.
$\Rightarrow$Microwave - $\nu = 30GHz,\,\,\,\lambda = 1cm$
Radar technology is known to be an active remote sensing device as it continuously sends a microwave pulse and detects the reflected energy back. Doppler Radar and Radar Altimeters are examples of active remote sensing instruments that use microwave frequencies.
$\Rightarrow$Radio-wave - $\nu = 3kHz,\,\,\,\lambda = 100km$

Electromagnetic waves differ from mechanical waves because they do not need a medium to propagate. This means that electromagnetic waves can pass not only through air and solid objects, but also through space vacuum. This has shown that radio waves are a source of light.

So, the right answer is C, radio waves.

Note: To solve this problem we need to know the formula speed of light = frequency ($\nu$) x wavelength ($\lambda$) and speed of light is always constant. You need to either learn the frequency or wavelength of electromagnetic waves since the speed of light is constant that is $3 \times {10^8}m/s$ , so remembering any one, frequency or wavelength you will get the value of the other term easily. Knowing these things will help you ahead and will also solve such problems.