Question

Q.5) A concave lens has focal length of 20 cm. At what distance from the lens should a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed. What is the character of Image?​

Answer 1

Answer:

object should be placed 60 cm to the left of the lens.

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Answer 2

Answer:

$\huge\bold\red{Solution:}$

$\bold{Given:}$

f = -20cm

h1 = 5 cm

v = -15 cm (Concave lens forms virtual image)

$\bold{Formula Used:}$

1/v – 1/u = 1/f

$\bold{By Putting Values:}$

1/-15 – 1/u = 1/-20

1/u = -(1/15) + 1/20

1/u = -1/60

$\bold{u= -60}$

Object should be placed 60 cm to the left of the lens.

$\bold{Now,}$

m = v/u = h2/h1

-15/-60 = h2/5

$\bold{h2 = 1.25 cm}$

Hence,Image formed is 1.25 cm high.

Thus, the height of the image is 1.25 cm and the positive sign means the image is virtual and erect.