Question

answers to the questions plz​

Answer 1

Given :

In concave lens,

Focal length = - 30 cm

Image distance = - 20 cm

Note : In concave lens focal length and image distance is denoted by negative sign.

To find :

The object distance from the lens

Solution :

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,

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

$\dashrightarrow \sf \dfrac{1}{( - 20)} - \dfrac{1}{u}= \dfrac{1}{( - 30)}$

$\dashrightarrow \sf - \dfrac{1}{u}= \dfrac{1}{( - 30)} + \dfrac{1}{20}$

$\dashrightarrow \sf - \dfrac{1}{u}= \dfrac{ - 2 + 3}{60}$

$\dashrightarrow \sf - \dfrac{1}{u}= \dfrac{1}{60}$

$\dashrightarrow \boxed{ \sf u = - 60 \: cm}$

Thus, the object distance is -60 cm.