Question

two convex lenses of focal length 10cm and 20cm are placed apart in air calculate the focal length of equivalent lens.​

Answer 1

Explanation:

the formula is

f(effective)=f1+f2/f1+f2-d

but there is no value of d so question is inappropriate

Answer 2

Explanation:

for lens-1,f1=10 cm

for lens-2,f2=20 cm

given distance=5 cm(in question f1 and f2 are placed 5cm apart in air)

1/f(equivalent)=1/f1+1/f2-d/f1f2=f2+f1-d/f1f2=f1+f2-d/f1f2

f(equivalent)=f1f2/f1+f2-d

f(equivalent)=10×20/10+20-5=200/25=8cm