Two thin lenses of focal lengths f₁ and f₂ are in contact and coaxial.

Two thin lenses of focal lengths f₁ and f₂ are in contact and coaxial. The power of the combination is:

(a) √ f₁ / f₂
(b) √ f₂ / f₁
(c) f₁ + f₂ / 2
(d) f₁ + f₂ / f₁f₂

f₁ + f₂ / f₁f₂

The focal length of the combination
1/f = 1/f₁ + 1/f₂
Power of the combinations,
P = f₁ + f₂ / f₁ f₂ ( P = 1 / f )

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