Question

The following figure represents two biconvex lenses $L_1$ and $L_2$ having focal lengths 10 cm and 15 cm, respectively. The distance between $L_1$ and $L_2$ is:

• A. 10 cm
• B. 15 cm
• C. 25 cm
• D. 35 cm

Answer 1

Answer: (C)

25 cm

Answer 2

The distance between two lenses, when both lenses are separated by a distance $D$, depends on their focal lengths and how they interact. For this setup with two biconvex lenses, the total distance between them $D$ is the sum of their individual focal lengths:

$D = f_1 + f_2 = 10 \, \text{cm} + 15 \, \text{cm} = 25 \, \text{cm}.$

This configuration ensures that parallel rays entering the system pass through the first lens's focus and exit the second lens as parallel rays, which is a key condition for this lens arrangement.