Question

A concave mirror has focal length of 20 cm. At what distance from the mirror 5 cm tall object be placed so that it forms an image at 15 cm from the mirror.Also calculate the size of the object.​

Answer 1

Given: A concave mirror of radius of curvature 20cm forms image of the sun. The diameter of the sun subtends an angle 1

o

on the earth.

To find the diameter of the image

Solution:

As sun is more far from the earth,

Then we take u = infinite

Radius of curvature, R=−20cm

Focal length, f=

2

R

=

2

−20

=−10cm

From the mirror formula

v

1

+

u

1

=

f

1

⟹

(−10)

1

=

v

1

+

∞

1

⟹v=−10cm

And we also know that

Angle = 1

o

=

180

∘

π

We know,

θ=

f

d

, where θ is the angle subtended on the earth, d is the diameter of the image and f is the focal length of the concave mirror.

d=θ×f

⟹d=

180

∘

π

×10

⟹d=

18

π

cm

is the diameter of the image

hope it helps u☺❤❤