Question

an object placed 30cm from a lens from an image on screen placed 60cm on of other side of the lens . identify the type of lens and determine it's focal length​

Answer 1

Answer:

Given: needle is placed at 30cm from the lens, it forms an image on a screen placed at 60cm on other side of the lens.

To find the focal length and type of lens used

Solution:

According to the given criteria,

Object distance, u=−30cm

Image distance, v=60cm

Since image is formed on other side of the lens, it is a convex lens.

Applying lens formula,

f1=v1−u1⟹f1=601−−301⟹1f=601+2⟹f=20cm

is the focal length of the lens.

Answer 2

Given :

Object distance : 30 cm

Image distance : 60 cm

To find :

The focal length of the image

Solution :

Here, image is formed on other side of the lens so, it is a convex lens.

using lens formula that is,

» The formula which gives the relationship between image distance, object distance and focal length of a lens is known as the lens formula.

The lens formula can be written as :

$\boxed{ \bf \dfrac{1}{v} - \dfrac{1}{u}= \dfrac{1}{f}}$

where,

• v denotes image distance
• u denotes object distance
• f denotes focal length

by substituting all the given values in the formula,

$\dashrightarrow \: \sf \dfrac{1}{v} - \dfrac{1}{u}= \dfrac{1}{f}$

$\dashrightarrow \: \sf \dfrac{1}{60} - \dfrac{1}{ - 30}= \dfrac{1}{f}$

$\dashrightarrow \: \sf \dfrac{1}{60} + \dfrac{1}{30}= \dfrac{1}{f}$

$\dashrightarrow \: \sf \dfrac{1 + 2}{60} = \dfrac{1}{f}$

$\dashrightarrow \: \sf \dfrac{3}{60} = \dfrac{1}{f}$

$\dashrightarrow \: \sf f = 20 \: cm$

Thus, the focal length of the image is 20 cm.