Write Newton's formula for refraction of Light through a lens. Explain the meaning of symbols used.

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Solution

If the distance of object and image are not measured from optical centre (C), but from first and second principal foci respectively, then newton’s formula states that,
$f_{1} f_{2} = x_{1} x_{2}$
where,
$x_{1} = F_{1} A =$ distance of object from first focus $F_{1}$ .
$x_{2} = F_{2} A^{'} =$ distance of object from second focus $F_{2}$ .
If medium on either side of the lens is same, then $f_{2} = - f_{1} = f$
so the newtons formula becomes,
$x_{1} x_{2} = - f^{2}$

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