Question

There is an equinox lens of focal length of 40 cm. if the lens is cut into two equal parts perpendicular to the principle axis, the focal length of each part will be:

• A. 20 cm
• B. 10 cm
• C. 40 cm
• D. 80 cm

Answer 1

The correct option is D 80 cm

The focal length for each turns out to be double the original focal length.

$1/f=1/f1+1/f2.$

Where the original focal length is f, and the focal length of each half is f1 and f2.

Since the two halves are indentical, then $f1=f2.$ So the equation becomes:

$1/f=1/f1+1/f1=2/f1.orf=f1/2$

Solving for one of the lens half:$f1=2f.$

$2\times 40=80$