Question

(l) Define focal length of a Spherical lense.

(lll) A divergent lens has a focal length of 30 cm. At what distance should an object of height 5 cm from the optical centre of the lense be placed, so that it's image is formed 15 cm away from the lense? Find the size of the image also.

(lll) Draw a ray diagram to show the formation of image in the above situation.

Answer 1

focal length :-

focal length of lens is the distance between optical centre and principal focus of lens.




now, next question from figure , so look it ..


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Answer 2

HELLO DEAR,

(1) the distance between the principal focus of
the lens to the optical centre of the lens os
called focal length

(2) given that :-

let the height of the image formed  =h1

v= -15cm

f = -30cm

h=5cm

u=?

we know that:-

lens Formula

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


now put the values given above

we get,

.
$\frac{1}{ - 30 } = \frac{1}{ - 15} - \frac{1}{ u} \\ = > \frac{1}{u} = - \frac{1}{15} + \frac{1}{ 30} \\ = > \frac{1}{u} = \frac{ - 30 + 15}{15 \times 30} \\ = > \frac{1}{u} = \frac{ - 15}{15 \times 30} \\ = > \frac{1}{u} = - \frac{1}{30} \\ = > u = - 30cm$


hence the object placed at a distance of 30cm from the lens

and magnification is

.
$m = \frac{v}{u} = \frac{ - 15}{ - 30} \\ = > m = \frac{1}{2} = 0.5$


we know that:-
.
$m = \frac{v}{u} = \frac{ h_{1}}{h} \\ = > 0.5 = \frac{ h_{1}}{5} \\ = > h_{1} = 2.5cm$
hence,

height of the image formed is 2.5 cm


I HOPE ITS HELP YOU DEAR,
THANKS