Question

A concave lens has focal length 20cm.An object of size 5 cm is placed infront of it at a distance of 40 cm.Find the nature,size and position of image formed

Answer 1

f= 20cm
u = -40cm
v= ?
1/f=1v -1/u
1/20 = 1/v - (-1/u)
1/20 - 1/40 = 1/v
2-1/40 = 1/v
1/40= 1/v
v=40 cm
h1/h2=v/u

h1/5=40/-40
h1/5=-1
h1= -5cm


image is real and inverted
image is formed 40 cm towards right side of mirror
image is of same size as tha object.

Answer 2

object distance (u) =-40cm
focal length (f) =-20cm
Image distance (v) =?
since 1/f=1/v-1/u
or, 1/f+1/u=1/v
1/v=1/-20-1/40==>1/v=-2-1/40
v=40/-3==>-13.3
the image formed is in front of lens negative sign shows that it is virtual and erect .
m=h1/h2=v/u
h1=13.3/40*5
h1=13.3/8
h1=1.6