Saved Bookmarks
| 1. |
What is resolving power of Optical Instruments ? Derive the condition under which images are resolved. |
|
Answer» Solution :Resolving power : The resolving power of a LENS is its ability to resolve two points that are close to each other. Resolving power of optical instruments (i) Consider a parallel beam of light falling on a convex lens. Due to DIFFRACTION EFFECT, thebeam focussed to a spot of finite area. (ii) . Taking into account the effects due to diffraction, the pattern on the focal plane A parallel beam of light is incident on a convex lens. Because of diffraction effects, the beam gets focused to a spot of radius - 0.61 Af/a. would CONSIST of a central bright region (circular) surrounded by a concentric dark and bright rings. (iii) The radius of the central bright region is given by r, = where f is focal length of the lens 2a = diameter of the lens. Derive the condition under which images are resolved : The size of the spot is very small, plays an important role in determining the limit of resolution. For the two stars to be just resolved `f Deltatheta=r_(0)=(0.61lambdaf)/(a)` `Deltatheta=(0.61lambda)/(a).....(1)` Thus, `Deltatheta` will be small, if the diameter (2a) of the objective is large. This imlies that telescope will have better resolveing power if a is large. In case of microscope, the object is placed slightly beyond f. The corresponding minimum seperation `(d_(Min))` between the object and the objective lens is given by `d_("Min")=(1.22lambda)/(2musin beta)` `"Where "mu="Refractive index"` `mu sin beta`= Numerical aperture. 
|
|