CBSE Class 12-science Answered

Let us understand the emergence of line spectrum:

Look at the set up given below:

Let us pass electric current through the discharge tube. The gas emits blue light, because the atoms of hydrogen gas gain energy from the current and get excited. When the excited atoms return to the normal state, they emit energy, in the visible region, which appears blue. If this blue light is passed through a prism, the light breaks into 5 colours apearing as lines of four colours. This is known as the line emission spectrum of hydrogen.

When the spectrum of investigated in infra red and ulltraviolet regions, more lines were found, which were given different names. Such as, Lyman, Balmer, Paschen, Brackett and Pfund series.

the wavelengths of the lines followed the expression:

An electron in a Hydrogen atom can be excited to many higher energy levels by absorption of radiation and thus, increasing the energy of electron. When the electron is present in the higher energy levels, to stabilize itself it jumps to lower energy levels and emits radiation. For example, if an electron is present in 5^th energy level, it may jump back either directly, to lowest energy level, or it may undergo succesive downward displacement from 5^th energy level to 4^th energy level and so on. These downward displacements are associated with different amount of energies, and hence lead to emission of radiation of different wavelengths. Thus, giving rise to different lines in hydrogen spectrum.