Spherical aberration in spherical mirrors is a defect which is due to dependence of focal length ‘f’ on angle of incidence ‘ q ’ as shown in figure is given by f equals R minus fraction numerator K over denominator 2 end fraction s e c invisible function application theta where R is radius of curvature of mirror and q is the angle of incidence The rays which are closed to principal axis are called paraxial rays and the rays far away from principal axis are called marginal rays As a result of above dependence different rays are brought to focus at different points and the image of a point object is on a point For paraxial rays, focal length approximately is

  1. R    
  2. R/2    
  3. 2R    
  4. none of these    

The correct answer is: R/2

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