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CBSE Class 12-science Answered

Derive lens formula for convex lens.
Asked by polyrelation | 14 Mar, 2019, 09:16: PM
answered-by-expert Expert Answer
 
Figure 1 shows the geometry of formation of image I of an object O on the principal axis of a spherical surface with centre of curvature C,
and radius of curvature R. The rays are incident from a medium of refractive index n1, to another of refractive index n2.
As before, we take the aperture (or the lateral size) of the surface to be small compared to other distances involved, so that small
angle approximation can be made. In particular, NM will be taken to be nearly equal to the length of the perpendicular from the point N
on the principal axis. We have, for small angles,

begin mathsize 12px style tan space angle N O M space equals space fraction numerator M N over denominator O M end fraction space space space comma space space tan angle N C M space equals space fraction numerator M N over denominator M C end fraction space space comma space space space tan angle N I M space equals space fraction numerator M N over denominator M I end fraction end stylebegin mathsize 12px style tan space angle N O M space equals space fraction numerator M N over denominator O M end fraction space space space comma space space tan angle N C M space equals space fraction numerator M N over denominator M C end fraction space space comma space space space tan angle N I M space equals space fraction numerator M N over denominator M I end fraction end style
Now for ΔNOC, i is exterior angle. Therefore   i = begin mathsize 12px style angle end styleNOM + begin mathsize 12px style angle end styleNCM
begin mathsize 12px style i space equals space fraction numerator M N over denominator O M end fraction plus fraction numerator M N over denominator M C end fraction space space................................. left parenthesis 1 right parenthesis
S i m i l a r l y comma space space r space equals space angle N C M space minus space angle N I M space space space comma space space i. e. comma space r space equals space fraction numerator M N over denominator M C end fraction minus fraction numerator M N over denominator M I end fraction space......................... left parenthesis 2 right parenthesis end style
 
By Snell's law,  n1 sin i = n2 sin r   or  for small angles ,   n1 i = n2 r  ........................(3)
 
substituting for i and r from (1) and (2), we simplify and rewrite eqn.(3) as  begin mathsize 12px style fraction numerator n subscript 1 over denominator O M end fraction space plus space fraction numerator n subscript 2 over denominator M I end fraction space equals space fraction numerator n subscript 2 minus n subscript 1 over denominator M C end fraction space space space.............................. left parenthesis 4 right parenthesis end style
Equation (4) gives us a relation between object and image distance in terms of refractive index of the medium and the radius of
curvature of the curved spherical surface. It holds for any curved spherical surface.
 
 
 
Figure 2 shows the geometry of image formation by a double convex lens. The image formation can be seen in terms of two steps:
(i) The first refracting surface forms the image I1 of the object O [ figure. 3]. The image I1 acts as a virtual object for the second surface
that forms the image at I [ figure 4. ]. Applying Eq. ( 4 ) to the first interface ABC, we get

begin mathsize 12px style fraction numerator n subscript 1 over denominator O B end fraction space plus space fraction numerator n subscript 2 over denominator B I subscript 1 end fraction space equals space fraction numerator n subscript 2 minus n subscript 1 over denominator B C subscript 1 end fraction space space space......................... left parenthesis 5 right parenthesis end style
A similar procedure applied to the second interface ADC gives,
 
begin mathsize 12px style fraction numerator negative n subscript 2 over denominator D I subscript 1 end fraction space plus space fraction numerator n subscript 1 over denominator D I end fraction space equals space fraction numerator n subscript 2 space minus space n subscript 1 over denominator D C subscript 2 end fraction space space....................... left parenthesis 6 right parenthesis end style
For a thin lens, BI1 = DI1. Adding Eqs. (5) and (6), we get  begin mathsize 12px style fraction numerator n subscript 1 over denominator O B end fraction space plus space fraction numerator n subscript 1 over denominator D I end fraction space equals space open parentheses n subscript 2 space minus space n subscript 1 close parentheses open square brackets fraction numerator 1 over denominator B C subscript 1 end fraction space plus space fraction numerator 1 over denominator D C subscript 2 end fraction close square brackets end style  .............................(6)
Suppose the object is at infinity, i.e.,  OB → ∞ and DI = f, Eq. (6 ) gives begin mathsize 12px style n subscript 1 over f space equals space open parentheses n subscript 2 space minus space n subscript 1 close parentheses open parentheses fraction numerator 1 over denominator B C subscript 1 end fraction space plus space fraction numerator 1 over denominator D C subscript 2 end fraction close parentheses space space.................... left parenthesis 7 right parenthesis end style
The point where image of an object placed at infinity is formed is called the focus F, of the lens and the distance f gives
its focal length. A lens has two foci, F and F′, on either side of it . By the sign convention, BC1 = + R1, DC2 = –R2
So Eq. (7) can be written as  begin mathsize 12px style 1 over f space equals space open parentheses n subscript 21 space minus space 1 close parentheses space open parentheses 1 over R subscript 1 space minus space 1 over R subscript 2 subscript blank end subscript close parentheses end style  ..........................(8)
because n21 = n2 / n1 .Equation (8) is known as the lens maker’s formula. It relates the focal length of given lens to the
radii of curvature of lens and also the refractive index of material of lens. It is useful to design lenses of desired focal length
using surfaces of suitable radii of curvature. Note that the formula is true for a concave lens also. In that case R1is negative,
R2 positive and therefore, f is negative.

From Eqs. (5 ) and (6), we get  begin mathsize 12px style fraction numerator n subscript 1 over denominator O B end fraction space plus space fraction numerator n subscript 1 over denominator D I end fraction space equals space n subscript 1 over f end style   ...............................(9)
Again, in the thin lens approximation, B and D are both close to the optical centre of the lens. Applying the sign convention,
BO = – u, DI = +v, we get begin mathsize 12px style 1 over v space minus space 1 over u space equals space 1 over f end style  .............................(10)
Equation (10) is the familiar thin lens formula. Though we derived it for a real image formed by a convex lens, the formula is valid for both
convex as well as concave lenses and for both real and virtual images.
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