7. The Doppler effect applies to all types of waves and is named after Christian Johann Doppler, an Austrian scientist who explained this phenomenon in 1842. What is the Doppler effect?
Explain, with the aid of labelled diagrams, how this phenomenon occurs. (18)
Describe a laboratory experiment to demonstrate the Doppler effect. (9)
In the early part of the twentieth century, Hubble and other astronomers made the first measurements on the spectra from distant stars. They observed that these spectra were shifted and they used the Doppler effect to explain these shifts.
What causes the red shift in the spectrum of a distant star? (6)
The yellow line emitted by a helium discharge tube in the laboratory has a wavelength of 587 nm as shown in the diagram. The same yellow line in the helium spectrum of a star has a measured wavelength of 590 nm.
What can you deduce about the motion of the star? Calculate the speed of the moving star.
Give another application of the Doppler effect. (speed of light = 3.00 × 108 m s−1)
(18) (5)
2009 7. When light shines on a compact disc it acts as a diffraction grating causing diffraction and dispersion of the light. Explain the underlined terms.
Derive the diffraction grating formula.
(12) (12)
An interference pattern is formed on a screen when green light from a laser passes normally through a diffraction grating. The grating has 80 lines per mm and the distance from the grating to the screen is 90 cm. The distance between the third order images is 23.8 cm.
Calculate (i) the wavelength of the green light
(ii) the maximum number of images that are formed on the screen. (21)
The laser is replaced with a source of white light and a series of spectra are formed on the screen.
Explain
(iii) how the diffraction grating produces a spectrum (iv) why a spectrum is not formed at the central (zero order) image.
