Question

An object is placed 36 cm from a convex lens. A real image is formed 24 cm from the lens. Calculate the focal length of the lens.

• A. 20 cm
• B. 14.0 cm
• C. 14.4 cm
• D. 28 cm

Answer 1

The correct option is C

14.4 cm

According to the sign convention, the object is placed on the left-hand side of the lens. So object distance (u) is negative. The real image is formed on the other side of the lens. So the image distance (n) is positive. Thus, u = -36 cm, n = +24 cm, f = ?

Using Len's formula, $\frac{1}{v}$ - $\frac{1}{u}$ = $\frac{1}{f}$

we get, $\frac{1}{+24}$ - $\frac{1}{-36}$ = $\frac{1}{f}$

$\frac{1}{f}$ = $\frac{1}{24}$ + $\frac{1}{36}$ = $\frac{5}{72}$

f = 14.4 cm