– it can damage the eye and will not give fringes anyway as the Sun subtends an angle of ( / )º. In interference and diffraction, light energy is redistributed. If it reduces in one region, producing a dark fringe, it increases in another region, producing a bright fringe. There is no gain or loss of energy, which is consistent with the principle of conservation of energy.
. . Resolving power of optical instruments In Chapter we had discussed about telescopes. The angular resolution of the telescope is determined by the objective of the telescope.
The stars which are not resolved in the image produced by the objective cannot be resolved by any further magnification produced by the eyepiece. The primary purpose of the eyepiece is to provide magnification of the image produced by the objective. Consider a parallel beam of light falling on a convex lens. If the lens is well corrected for aberrations, then geometrical optics tells us that the beam will get focused to a point.
However, because of diffraction, the beam instead of getting focused to a point gets focused to a spot of finite area. In this case the effects due to diffraction can be taken into account by considering a plane wave incident on a circular aperture followed by a convex lens (Fig. . ).
The analysis of the corresponding diffraction pattern is quite involved; however, in principle, it is similar to the analysis carried out to obtain the single-slit diffraction pattern. Taking into account the effects due to diffraction, the pattern on the focal plane would consist of a central bright region surrounded by concentric dark and bright rings (Fig. . ).
A detailed analysis shows that the radius of the central bright region is approximately given by . . f f r a a ( . ) FIGURE .
A parallel beam of light is incident on a convex lens. Because of diffraction effects, the beam gets focused to a spot of radius . # f/a .