It’s just a concept in accordance with a question. The detailed explanation will be uploaded on the topic LINE SPECTRUM . After studying the nest topic you will be able to clear the concept of it. Thank you…

Line spectrum is also known as Discontinuous spectrum. It is consisting of difinite lines of definite frequency but not in a continuous manner i.e. doesn’t emit radiation of all types at the same time .
When some of the work done on Hydrogen like heating, electric charge etc. These types heating and electric charge have their own energy. When they are straight against Hydrogen atom, the electron of the hydrogen atom absorb the energy from the exerted energy. Thus, the electron absorbing energy goes to higher energy level.

According to a equation, E = h/lamda . So, the absorbing energy has a definite wavelength or frequency. This means, the portion of the electromagnetic radiation has been absorbed by the electron. Thus, The electron from the atom does not absorb all type of radiation. Again, the electron can never in the higher energy level for long time. So, the electron trends to come back to its original path. Fir this purpose, the electro must emit radiation that it absorbed. And also according to the same relation, it has definite frequency or wavelength. So, it emit radiation in the form of light.

Although, it emit many types of radiation, but the radiation emitted from the visible region can be seen by naked eyes.

Again, if the emitted radiation has definite frequency or wavelength. It has definite wave number. It is denoted by v ( nu ber ) .