Question

convex lens object is placed at a distance 7.5 CM its image is found at 10 cm from the lens the focal length of the lens will be _______

a)10 cm.
b)20cm.
c)30 cm.
d)40 cm. ​

Answer 1

and

v= -10cm

f= ?

u= - 7.5 cm

1/v -1/u = 1/f

1/-10 - 1/-7.5 = 1/ f

1/ -10 + 1/ 7.5 = 1/ f

1/-10 + 10/ 75 = 1/f

-15 + 20/150 = 1/f

5 /150 = 1/ f

f = 150 / 5

f= 30 cm

Answer 2

Answer:

c)30 cm.

Explanation:

Given that

Lens is convex lens

Object distance u = - 7.5 cm  (negative sign because it is sign convention)

Distance of image formed v = - 10 cm

We know that relationship between u ,v and f for convex lens

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

Where f is the focus length

Now by putting the values of u and v

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

$\dfrac{1}{f}=\dfrac{1}{-10}-\dfrac{1}{-7.5}$

$\dfrac{1}{f}=-\dfrac{1}{10}+\dfrac{1}{7.5}$

f= 30 cm

So option c is correct.

c)30 cm.