Question

a convex lens has the convex radii of 80cm and 120cm.Its refraction index is n=1.5.An object is 2m from the lens. The position of the image is?

Answer 1

Effective focal length of a thin lens is given by:
\[\frac{ 1 }{ f} = (n-1) \left( \frac{ 1 }{ R1} - \frac{ 1 }{ R2}\right)\]
n:refractive index
R1: radius of first surface (positive for convex lens)
R2:radius of second surface (negative for convex lens)

So 1/f = (1.5 - 1)(1/80 - 1/(-120))
1/f = 5/480
f = 96 cm
Now lens formula:
\[\frac{ 1 }{ f} = \frac{ 1 }{ u} + \frac{ 1 }{ v}\]
u : distance of the object from the lens (negative)
v: distance of the image from the lens
So,1/96 = -1/200 + 1/v
v = + 64.86 cm