Waves and wavelength

Some waves need a medium to travel — sea waves need water, sound waves need air. Other waves don’t need any medium to propagate. Electromagnetic waves, which are waves of energy, are like that — they originate somewhere, keep radiating through space unless halted by an object, like earth. Higher the energy, the shorter the wavelength.

Wavelength is the distance between two successive crests of a wave; the number of peaks per second is called ‘frequency’. So, wavelength and frequency are related — the higher the wavelength, the smaller the frequency. Waves are classified according to their wavelength.

Electromagnetic spectrum

Electromagnetic waves come in a variety of wavelengths, depending upon their source. In the descending order of wavelengths (ascending order of frequencies), they are: radio waves, microwaves, infra-red, or IR, (further classified as far, middle and near), visible light (red, orange, yellow, blue, violet), ultra-violet, X-rays and gamma rays.

Visible light is a mixture of the colours mentioned; you split a light ray through a prism, you get a spectrum of these colours.

Radio waves bring music to our pocket radios; microwaves are what we use to talk to each other on the phone; IR, because they come from heat, help us detect if anyone has fever, see people in the dark, and is also used by the Webb telescope to ‘see’; we know visible light, ultra-violet, and X-rays; gamma rays, shortest wavelength and highest energy, are produced when stars explode.

Doppler Effect and red shift

Imagine you and a friend standing a few meters apart. Imagine a wave stretching between the two of you. Assume that the distance between two peaks of the wave — wavelength — is ten centimetres. Now imagine the friend running away from you.

The wave would get elongated, right? In other words, the wavelength would grow bigger. Likewise, if the friend is running towards you, the wavelength will become shorter.

That is why you hear a siren sharper as the ambulance approaches you. This is called ‘Doppler Effect’.

Now, in the example, you are the earth, your friend is a star and the wave, a ray of light. If the star is moving away, then, due to the Doppler Effect, the wave of the light will get elongated and the wavelength will increase, right? The wavelength will ‘shift’ towards the red colour, which is the longest wavelength of visible light.

Simply put, the farther the emitting object, more ‘red’ the spectrum will have. This is called ‘red shift’. The extent of red shift reveals how far the emitter is. If the object pulls away further, it will become invisible — it will move from optical to near IR, and further to middle and far IR. Similarly, if an object is coming towards us, you will have ‘blue shift’.

Webb telescope

Optical telescopes, like the Hubble, pick up light from distant galaxies; Webb is designed to sense IR, which means it can receive radiation from galaxies that have moved very far. For example, it has taken IR radiation from galaxies that are 13.1 billion light years away — it has taken the light (now infra-red) 13.1 billion years to reach the Webb, meaning the Webb is looking at something as it was 13.1 billion years ago. Till now it has taken IR of 5 microns in wavelength; it can go deeper to 28 microns. The universe is believed to have formed 13.8 billion light years ago; the Webb can see the universe as it was just a few hundred million years old.

While the red-shift reveals how far the source is, analysis of the spectrum can tell what the radiation has passed through, because each element has its unique ‘spectral fingerprint’. Scientists can tell whether the source has hydrogen or sulphur or whatever. That’s how they know that an exoplanet that Webb caught has water vapour in its atmosphere.

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