You might have noticed in science fiction movies that the water which surrounds the nuclear reactions is glowing bright blue. Even wondered why that happens? Why is it not in any other colour like yellow, red or anything else? Even though the movies make use of special effects, the glow is because of a scientific reason. It is because the phenomenon is called Cherenkov radiation which we will study in detail in this article.
Introduction to Cherenkov Radiation
A brief flash of light produces when a particle carrying an electric charge travels faster than light in media like air and water. We refer to this flash of light as Cherenkov light. This effect has been named after Pavel Alekseyevich Cherenkov, a Soviet physicist, who was responsible for discovering this phenomenon.
The speed of light in a vacuum is a constant c, however, the speed of light in a medium is lesser than c. This shows that the light can travel faster in the medium, nonetheless slower than the speed of light.
When a high-speed electron will pass through a dielectric medium, the electromagnetic field will be disturbed and electrically polarized. A coherent shock wave is left in the medium in the wake of the particle.
The Cherenkov radiation commonly prevails in the ultraviolet spectrum. Moreover, it creates a constant spectrum, nothing like emission spectra which has spectral peaks.
Reason for Blue Water in a Nuclear Reactor
The charged particles travel faster than light when the Cherenkov radiation passes through water. Consequently, the light has a higher frequency and shorter wavelength than usual. Thus, because of the presence of light with a shorter wavelength, the light seems blue.
Now, the question arises, why is there any light at all? The fast-moving electrons excite electrons of water molecules. These excited electrons absorb energy and release it as photons as they return to equilibrium.
The shock waves which the particles travelling faster than light generate produce a constructive interference which we witness as a glow.
Example of Cherenkov Radiation
If you drop a pebble into a pond, a circular ripple will spread out on the surface of the water. If you continue to drop pebbles at regular intervals then a series of concentric rings all moving outwards with the same speed will form.
But, what will happen if you drop pebbles by moving your hand linearly in one direction or horizontal direction? It will create a series of circular waves that are no longer concentric yet rather crowded in the direction where you are moving your hand.
If you move the hand with the speed with which the rings move then all the rings will touch at a point. What will happen if you move the source of the ripples faster than the speed of rings with which they spread.
Cherenkov radiation is merely a kind of electromagnetic radiation which we observe when a charged particle moves at a faster speed than the speed of light in that medium in which charged particle is moving. Thus, the speed of the particle must be faster than the phase velocity of light instead of overall velocity.
Uses of Cherenkov Radiation
Cherenkov radiation has more uses than just to make the water glow blue in a nuclear lab. Some of them are:
- We use it in particle physics experiments to recognize the nature of the particle being examined.
- In a pool-type reactor, the blue-glow comes in use to gauge the radioactivity of spent fuel rods.
- In astrophysical experiments, we use it for studying cosmic showers.
FAQ on Cherenkov Radiation
Question 1: Is Cherenkov radiation harmful?
Answer 1: Although Cherenkov radiation cannot enter the x-ray and gamma-ray portion of the spectrum, most of it is indeed ultraviolet. As a result, it can be harmful to someone who does not take any precautions to protect themselves.
Question 2: Is Cherenkov radiation always blue?
Answer 2: This is one feature of Cherenkov radiation that it is usually in the ultraviolet spectrum, not bright blue; nevertheless it creates a continuous spectrum, not like emission spectra, which have spectral peaks.
Question 3: State one use of Cherenkov radiation.
Answer 3: We use it for detection of small amounts and low concentrations of labelled biomolecules. Radioactive atoms like phosphorus-32 are readily introduced into biomolecules by enzymatic and synthetic means and consequently, may be easily identified in small quantities.