Part A
The focal length of a mirror can be determined
entirely from the shape of the mirror. In contrast, to
determine the focal length of a lens we must know
both the shape of the lens and its index of
refraction - and the index of refraction of the
surrounding medium. For instance, when a thin
lens is immersed in a liquid we must modify the
thin-lens equation to take into account the refractive
properties of the surrounding liquid:
$\frac{1}{f} = \left(\frac{n}{n_{liq}} - 1\right)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$
where $n_{liq}$ is the index of refraction of the liquid
and $n$ is the index of refraction of the glass.
If you place a glass lens ($n = 1.5$), which has a focal length of 0.5 meters in air, into a tank of water ($n = 1.33$), what will
happen to its focal length?
Nothing will happen.
The focal length of the lens will be reduced.
The focal length of the lens will be increased.
There is not enough information to answer the question.
