The angular width θ of a distant star can be measured by the Michelson radiofrequency stellar interferometer (as shown in the figure below).
The distance h between the reflectors M1 and M2 (assumed to be much larger than the aperture of the lens), is increased till the interference fringes (at P0, P on the plane as shown) vanish for the first time. This happens for h = 3 m for a star which emits radiowaves of wavelength 2.7 cm. The measured value of θ (in degrees) is closest to
1
0.63
2
0.32
3
0.52
4
0.26