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CBSE Class 10 Answered

Define: a) Diffraction b) Relation between focal length and radius of curvature and also proof it? c) between which two points related to a concave mirror ,should an object be placed to obtain on a screen an image twice the size of the object d) lateral displacements and its calculation.    
Asked by spankaj.ekt | 30 Apr, 2020, 07:53: AM
Expert Answer
a) Diffraction is the phenomenon of bending of light around the corners of small apertures or obstacles and its spreading into the region of the geometrical shadow. 

The relationship between the focal length f and radius of curvature r is r = 2f.


Consider a ray of light AB, parallel to the principal axis and incident on a spherical mirror at point B. The normal to the surface at point B is CB and CP = CB = R, is the radius of curvature. The ray AB, after reflection from mirror, will pass through F (concave mirror) or will appear to diverge from F (convex mirror) and obeys the law of reflection i.e. i = r.

From the geometry of the figure,

∠BCP = θ = i

In D CBF, θ = r

∴BF = FC (because i = r)

If the aperture of the mirror is small, B lies close to P, and therefore BF = PF

Or FC = FP = PF

Or PC = PF + FC = PF + PF

Or R = 2 PF = 2f

Or f = R/2

Similar relation holds for convex mirror also. In deriving this relation, we have assumed that the aperture of the mirror is small.

c) object must be placed between C and F 

A ray of light travels in a certain straight line path. After refraction through a glass slab, it emerges in a direction parallel to the original direction but slightly displaced from the line. This is called the lateral displacement.

Please ask single question per post hereafter.  Only one question can be answered in each post. 
Answered by Shiwani Sawant | 30 Apr, 2020, 01:01: PM

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