Question

An illuminated object and a screen are 90 cm apart. What is the focal length and nature of the lens required to produce a clear image of the object on the screen which is twice the size of the object. The formula used are

Answer 1

Answer:

Since the image is formed on the screen, it is a real image. Let the distance of the object from the lens be x. Then, distance of the image from the lens be (90-x)

magnification, m=

u

v

According to new Cartesian coordinate system,

u=−x,v=+(90−x) and m=−2

∴ −2=

−x

90−x

⇒ x=30cm

∴ u=−30cm and v=+(90−30)=60cm

Also,−

u

1

+

v

1

=

f

1

−

−30

1

+

60

1

=

f

1

f

1

=

30

1

+

60

1

=

20

1

⇒ F=20cm (convex lens). follow me and like this answer you

Answer 2

Answer:

Explanation:

Given u + v = 90cm …(i)  

By sign convention u = -30cm, v = 60cm