Question

Two lenses are in contact a converging of focal length 30cm and a diverging one of focal length -10cm , what is focal length of the combination A. -5cm
B.-10cm
C. -15cm
D. -20cm
E. -25cm

Answer 1

Answer:

Answer - (c)

Explanation:

Because 1/f = 1/ f1+ 1/f2

so, 1/f= f2+f1/ f1f2

1/f= -10+30/30 *(-10)

1/f= 20/ (-300)

f= -300/20

f=-15cm