Question

an object is placed at 5 cm in front of lens and an image is formed at 10 cm behind the lens . what type of lens is it and what is the value of its focal length

Answer 1

given u=-5cm
v=10
focal length will be
1/f=1/v-1/u
put values of u and v
1/10-(-1/5)=1/10+1/5
3/10=1/f
f=10/3

Answer 2

Concept:

Focal length is defined as the distance between the center of the lens or mirror and its focal point.

Given:

The object is placed $5cm$ in front of the lens.

The image is formed behind the lens is at $10cm$.

Find:

The focal length of the lens and the type of length.

Solution:

The object is placed in front of the lens, $u=-5cm$.

The image is formed behind the lens, $v=10cm$.

Now, since the image formed behind the mirror and the image formed at a distance double the object placed. The lens is a convex lens.

Using the lens formula,

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

$\frac{1}{f}=\frac{1}{10}-\frac{1}{-5}$

$\frac{1}{f}=\frac{3}{10}$

$f=\frac{10}{3}$

The focal length of the lens is $\frac{10}{3} cm$.

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