question_answer

Give reasons for the following : (i) Astronomers prefer to use telescopes with large objective diameters to observe astronomical objects.       (ii) Two identical but independent monochromatic sources of light cannot be coherent.  (iii) The value of the Brewster angle for a trans- parent medium is different for lights of different colours.   

Answer:

                (i) The objective of large aperture has a large light gathering capacity and it forms bright images of even distant faint stars. It also increases the resolving power of the telescope.                                  (ii) The phase difference between two independent light sources changes 108 times in one second. Such rapid changes in the positions of maxima and minima cannot be detected by our eyes. The interference pattern is lost and a uniform illumination is seen on the screen,              (iii) According to Brewster law, \[\mu =\tan {{i}_{p}}\] As refractive index \[\mu \] of a transparent medium is different for light of different colours, so Brewster angle \[{{i}_{p}}\] is different for light of different colours.