The position of fical image formed by the given lens combination from the third lens will be at a distance of `f_1=+10cm,f_2=-10cm,f_3=+30`

Options

A. 15 cm, concave
B. infinity
C. 45 cm
D. 30 cm

Answer

D

Explanation

Image by first lens:
`(1)/(v_1)-(1)/(-30)=(1)/(10)`
`(1)/(v_1)=(1)/(10)-(1)/(30)=(2)/(30)`
`v_1=15cm`
Image by second lens `u=(15-5)=10cm`
`(1)/(v_2)-(1)/(10)=(1)/(-10)impliesv_2=infty`
Image by third lens `u=infty`
`(1)/(v_3)-(1)/(infty)=(1)/(30)impliesv_3=30cm`