Question

Mention one use of part of the electromagnetic spectrum to which a wavelength of $21\,cm$ emitted by hydrogen in interstellar space belongs.

Answer 1

In order to solve this question we need to understand the electromagnetic spectrum. Atoms have various levels in which electrons are distributed, suppose one electron excites up to a higher level and then following the principle of conservation of energy, it de excites and thus releases energy equals to difference between energy level, now we know from quantization that this energy is equal to $hv$ so we can find the frequency of emitted wave. So to study frequency we categorized the frequency emitted into different types like low frequency termed as radio waves, high frequency termed as ultraviolet. This is known as the electromagnetic spectrum.

Complete step by step answer:
Let the frequency of emitted wave denoted as $v$ and wavelength denoted as $\lambda$.We know the relation between frequency and wavelength as,
$v\lambda = c$
Here, “c” is the speed of light
So, $\lambda \propto \dfrac{1}{v}$ as $c$ is constant.So wavelength is inversely proportional to frequency.

Given a wavelength $21\,cm$ is very large. And this wavelength belongs to the range of radio waves. As radio waves range from $1\,mm$ to $100\,km$ wavelength.
Use: Radio waves are the waves which are broadly used for communication and satellite transmission. As these waves have comparatively high wavelength so its frequency is low.

Note: It should be remembered that when light travels from one medium to another, frequency remains unchanged, so what changes is the wavelength and speed of light, they change in a manner such that frequency remains unchanged. Also in interstellar space, mostly waves detected are radio waves. Since radio waves have large wavelengths, these waves travel a larger distance.