10. Isolated atoms emit light which is nearly monochromate. But light from a hot gas of atoms is found to be more diffuse in frequency when studied with the help of a spectrometer. Account for this broadening of spectral lines. 11. A galaxy has been observed to be receding away from us at a speed v = c/2. show that the observed wavelength is increased by a factor of 3. 12. An observer A sees a source of light and another observer B to be moving in exactly opposite directions with equal speeds of 0.6 c. Observer A measures the wavelength of light as 4000 . Calculate the natural frequency of the source and the wavelength of the source as measured by observer B. (Ans. 1.5 x 1015 Hz.; 8000 ).