A STORYTELLER : SPECTRA

All of us familiar with the "dispersion of light" experiment. Through this experiment Isaac Newton proved that white light consists of seven colors. The phenomenon which gives rise to the beautiful rainbows is also of great help in modern astronomy.

Spectroscopy is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. Whereas spectrometry is the application of it.

Spectrometry helps us measure certain things such as:

Composition

When spectra from a star is studied, it is observed that it consists of some dark bands which are called absorption lines. These absorption lines arise because light of certain frequencies are absorbed by certain chemical elements. This helps us in determining the chemical composition of a star.

Here, absorption lines can be seen.

Temperature

Stars are black bodies. They absorb all of the EM radiations which strike them and then in order to stay in equilibrium they emit it out in form of heat and light. The wavelength of the emitted light depends upon the amount of energy being emitted.

Different stars have different temperature. The color of the star tells us how hot a star is. When we observe the spectra of a star, the wavelength which is most prevalent tells us about the temperature of the star.

Red stars are less hot than yellow ones and Blue or Violet stars are the hottest. As

E = hν where ν is the frequency of light and h is plank's constant.

Velocity

Spectrum can tell us how fast a galaxy is moving away or towards us. In a spectra it is observed that absorption lines are shifted either towards blue(shorter wavelengths) or red (longer wavelengths). This is known as Doppler effect.

When a body is moving away from us the wavelength appears to be longer ( shifted towards red ). This is called red shift.

And when a body is moving towards us the wavelength appears to be shorter ( shifted towards blue). This is called blue shift.

Velocity of a galaxy can be calculated from the equation given below. This equation has been derived using relativity.

Here z= shift V= velocity of galaxy and c = speed of light