A converging lens of focal length 15 centimetre and a converging mirror of focal length 10 centimetre are placed 50 cm apart with the common principal Axis .a point source is placed in between lens and the mirror at a distance of 40 cm from the lens. find the location of the final image.

focal length = 15cm
converging mirror=10cm
distance between lens and mirror= 50cm
A point source is placed in between the lens and the mirror at a distance of 40 cm from the lens
point source is focus of mirror
Thus two images will be formed :
a) one due to direct  transmission of light through lens
b) one due to reflection and then transmission of rays through lens
case 1:
u= -40cm
f=15cm
using lens formula :
1/f=1/v-1/u
1/v= 1/15-1/40
= 40-15/40x15
v= 40x15/25
v=24cm
Case ii:
Since  object is placed at focus, the rays become parallel to lens after reflection 
u= ∞
f=15cm  
1/v-1/u=1/15
1/v-1/ ∞=1/15
v=15cm