# focal length of lens

#### shenno

Hi
I am not sure how to do this.
The thin lens equation does not seem useful here.
Does anyone know any useful formula here?

#### Woody

By making the approximation that all the refraction occurs on the centre plane of the lens
The thin lens equation becomes much simpler than a full lens equation
(where the distance between the actual refracting surfaces is considered)

Obviously (as the name implies) the approximation is best for thin lenses
but it can be used with reasonable success for perhaps surprisingly thick lenses.
Just remember that the equation is making an approximation (even for thin lenses)
and that the approximation becomes more approximate the thicker the lens becomes.

Note that the thickness is considered in relation to the lens diameter,
i.e. it is the thickness to diameter ratio that determines how well the "Thin" lens equation approximates.

The multiple choices in your question are quite far apart,
I would have thought that the thin lens approximation would be accurate enough to pick between them.

topsquark