Question

We wish to obtain a equal sized inverted image of a candel flame on a screen kept at a distance of 4m from the candle flame a) name the type of lens
B) what should be the focal lenght of the lens and at what distance from the flame the lens should be placed

Answer 1

A) To obtain an inverted image of an object of same size a convex lens is used.

B)

In this question we have given

Distance between candle flame and screen=4

it means distance between candle flame and lens +distance between lens and screen =4m

u+v= 4m

u= v

therefore, 2u= 4m

or, u=-2m              ( u is taken negative because it lies on left side of the lens)

and v=2m

we have to find the focal length of convex lens

$\frac{1}{f}  =  \frac{1}{v}   -  \frac{1}{u}$$\frac{1}{f}  =  \frac{1}{2} - ( -  \frac{1}{2} ) \\  \frac{1}{f}  = \frac{1}{1}\\ f = 1m$

Hence, Focal length of the lens is 1m

and lens should be placed at a distance of 2m from the flame