Question 92: Ray Optics

Question

A convex lens of focal length 30 cm is placed in contact with a concave lens of focal length 20 cm. An object is placed at 20 cm to the left of this lens system. The distance of the image from the lens in cm is ________.

Accepted Answer

Equivalent focal length
$$\begin{aligned}
& \frac{1}{\mathrm{f}}=\frac{1}{\mathrm{f}_1}+\frac{1}{\mathrm{f}_2} \
& =\frac{1}{30}+\frac{1}{-20}=\frac{2-3}{60}=-\frac{1}{60} \
& \mathrm{f}=-60 \mathrm{~cm}
\end{aligned}$$
Lens formula
$$\begin{aligned}
& \frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \
& \frac{1}{\mathrm{v}}-\frac{1}{-20}=\frac{1}{-60} \
& \mathrm{v}=-15 \mathrm{~cm}
\end{aligned}$$

Answer

$ \frac{60}{7} $

Answer

15

Answer

45

Answer

30

Ray Optics

A convex lens of focal length 30 cm is placed in contact with a concave lens of focal length 20 cm. An object is placed at 20 cm to the left of this lens system. The distance of the image from the lens in cm is ________.

Choose the correct answer: