Question

An equiconvex lens is made from $\mu =1.5$ and $r=20cm$. it is $5cm$ thick in the middle. Find the position of the image far away from the lens.

• A. $10cm$
• B. $19.2cm$
• C. $8.72cm$
• D. $9.48cm$
• E. $9.68cm$

Answer 1

Answer: (B)

$19.2cm$

The focal length of a thick lens is given by the Lens' Maker Formula-

$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2}+\frac{(\mu -1)d}{\mu R_1{R}_2})$

$=(1.5-1)(\frac{1}{+20}-\frac{1}{-20}+\frac{0.5\left(5\right)}{1.5\left(20\right)\left(20\right)})$

$⟹f=19.2cm$

Hence the image of object far from lens is formed at its focus, that is, at a distance 19.2cm.