We have seen the electromagnetic spectrum, it is well understood. Take the source of any light That may be the sun or your tableclam’s bulb. This light has a specific color. Three colors are associated with every color of light, wavelength, frequency and energy.

First of all, we understand wavelength and frequency. Understand the light from the picture below:

The upper part of the red line is called the crests and the bottom part is troughs. The horizontal distance between the two consecutive top of a wave is called the wavelength. The maximum length of a top or trough is called the amplitude of the waveform waveform. A head and a trough formed together form a wave cycle. The frequency of a wave is called the number of wave-cycle formed in one second. For example, if we look at the three different waves (two rows of an upward and under it) shown in the picture, then we can say that

1. The wavelength of the first wave is the highest because it has maximum horizontal distance between its two consecutive top or trough.
2. The frequency of the third wave is highest because most waves in a second are becoming.
3. The wavelength the higher the frequency, the lower the frequency. Wavelength and frequency are inverted.

We look at the visible part of the electromagnetic spectrum. The wavelength of visible light is between 400 nm (1 nm = 10-9 m) to 800 nm. Purple color is 400 nm and red at 800 nm.

Suppose the source emits at a specific frequency of light of a specific color like a monochromatic source. This does not mean that the observed wavelength is similar to the wavelength emitted. If the observed wavelength is high then it is called red deviation, if it is less then it is called a blue deviation.

In the above picture the spectrum of the middle is actually emitted by the light source. If we look at the dark absorption lines in the spectrum of the above, we will find that the related lines have become distracted towards the red color. While in the lower spectrum it has diverged towards the blue color. Although this effect is not related to visual light, the increase / decrease in wavelength is always called red / blue deviation. Red / blue deviation is generated by many astrophysical processes. Red deviation in astrophysics is represented by z. Its simple equation: 1 + z = Observed wavelength / real wavelength. The positive value of z represents the blue deviation of the reddish and negative value.

You might be wondering what is the standard reference in this deviation? Which reference spectrum we are comparing to know the red deviation? This is a good and important question. Each element has its own signature spectrum. The most commonly found element in the universe is hydrogen. So if we look at the hydrogen spectrum of a remote galaxy and if the line of that spectrum corresponds to our laboratory spectrum, it means that there is no red deviation. But if these lines are at a certain distance towards red, then there is a reddish flux and that source is going away from us.

Now we look at three types of red deviations and understand the significance of its astrophysics.

We are familiar with Dappler effect in all sound waves. If the sound source is coming towards us, its frequency increases, while it decreases when it goes away. Dappler effect is similar in astrophysics, but it is related to light. Every body is moving relative to Brahma. The stars and galaxies are moving relative to one another. If there is a red deviation in the spectrum of a star / galaxy, it means that it is going away from us. With the formula of Relative Dapler effect, we can calculate the speed of going away from that star or galaxy. When we saw the spectrum of our neighboring galaxy Devyani (Andromeda), we found that it has a blue deviation. This galaxy is coming towards the speed of 140 km / h towards our galaxy way. After the next five billion years, these two galaxies will be dissolved in one another and form a large galaxy.

The red deviation of this is the result of ordinary relativism. According to the gravitational red deviation, when photon travels from low gravitational field to a high-potential area, then its energy decreases. If a star emits light on its surface with a specific wavelength, and we study the spectrum of that star far away from its surface then it will get red deviation. In an effort to escape the gravity of that star, the energy of that photon has decreased, which means increase in wavelengths. Do not understand? Think of it like being tired by a child climbing stairs. Notice that the photon’s energy is decreasing, its frequency will also be lower but the wavelength will increase.

The amount of gravitational red deviation is dependent on the density of that body. Dense bodies such as white dwarfs and neutron stars have more red deviation than normal stars like Sun. Gravitational red deviation of black hole (infinite) is infinite. This kind of reddishness indicates that the photon has a mass which is not a stable mass, but a gravitational mass. This is a classical experimental evidence of Einstein’s general relativity.

Cosmic red deviation is the result of the continuous expansion of space in reality. 1920 Edwin Hubbal had found that the galaxy which is far away from us in the remote space is moving away from us as fast as we can. This is called Hubble’s rule. It is an inspected rule and certifies the continuous expansion of the universe. Space is expanding itself in itself. It is a fact that the spectrum of remote galaxies has given us evidence of the continuous expansion of the universe, in this spectrum a very red divergence has been found.

But we should keep in mind that there is a difference between local familial red deviation and cosmic red deviation. Cosmic red deviation will not occur in two galaxies at relative relative speed. The red deviation in the photons is emerging from the journey in astronomical periods, which are constantly expanding by them. With this continuous expansion, two galaxies moving away from each other can be faster than light speed. But this does not mean that they are violating special relativity.