Question

A convex lens has a focal length of 10 cm. Where should the object be place to form a virtual image at a distance 30 cm from the lens? ​

Answer 1

Answer:

Explanation:

In a convex lens virtual image is formed when the object is placed between pole and focus of the lens . So to obtain a virtual image at a distance of 30 cm from the lens the object must be placed between pole and focus of the lens.

Answer 2

Given,

The focal length of lens, f = + 10cm (convex lens have a + focal length)

The distance of image from the lens, v = + 30cm (real image)

To find,

The distance at which the object should be placed.

Solution,

In order to solve this question quickly and correctly, we can follow the given steps.

From what knowledge we have of this chapter in Physics we know that,

According to the convex lens equation, the lens formula is

$\frac{1}{f} = \frac{1}{v}- \frac{1}{u}$

Where,

f = focal length of lens

v = distance of image

u = distance of object

In order to calculate the distance of the object we have,

$\frac{1}{u} = \frac{1}{v}- \frac{1}{f}$

= $\frac{1}{+30} - \frac{1}{+10}$

= $\frac{-2}{30}$

= $\frac{-1}{15}$

So we have,

u = - 15cm

The minus sign shows that the object is placed to the left of the mirror as compared to the image which is formed on the right-hand side.

Hence, the object should be placed at 15 cm on the left-hand side of the mirror.