i. In thermonuclear fusion experiments, scientists come across extremely hot gases or plasma where the temperature is of the order of millions of degree celsius.

ii. At such high temperatures, molecules of glowing gas are moving away and towards the observer with high speeds.

iii. Due to Doppler effect, the wavelength λ of a particular spectral line is apparently changed.

iv. One edge of the line now corresponds to an apparently increased wavelength λ due to molecules moving directly towards the observer and the other edge to an apparently decreased wavelength λ₂ due to molecules moving directly away from the observer.

v. The line is thus observed to be broadened. The breadth of the line can be measured by using a diffraction grating.

vi. Since ‘λ’ and ‘c’ are known, the velocity ‘v’ can be calculated using the formula, v = \(\sqrt{\cfrac{3RT}M}\), where ‘R’ is the molar gas constant, ‘T’ is absolute temperature and M is the mass of one mole.

Note: Doppler effect in light is symmetric, i.e., it depends only on the relative velocity of the source and the observer. The difference occurs because light does not require a medium for propagation and the speed of light is same for any observer whether the observer and/or the source is moving.