Question

Long Answer Type Questions:
Marks : 3/4
A concave lense of focal length 10cm forms an image on a screen 50cm away
from it.
Find out the position and size of the object if the height of the image is 10cm.
​

Answer 1

Answer:

Since m=2

focal length =10cm

v

1

−

u

1

=

f

1

→

v

1

−

u

1

=

f

1

Multiply by u on both side;

v

u

−1=

f

u

2

1

=

m

1

=

v

u

=

f

u+f

2=

f+u

f

⇒u=−15 cm

Answer 2

Dear student  

The focal length f =20cm  

height of image is 10cm  

as screen is 50cm away so the image distance v = 50cm

using lens formula  

1v-1u=1f150-1u=120150-120=1u0.02-0.05=1u-0.03=1uu=-33.3cm

magnification of the lens  

hiho=-vuho=-hi×uvho=6.66cm

diminished virtual and erect.