Question

A convex lens has a diameter of 30 mm and a depth of 5 mm at the center. How far from the vertex is the focus?

Answer 1

The formula for determining the distance of the focus from the vertex is as follows,

     f = x² / 4a

where f is focus, x is the radius (half the value of diameter), and a is the depth. Substituting the known values to the given equation,

   f = (30/2 mm)² / (4)(5 mm)

    f = 11.25 mm

ANSWER: 11.25 mm