The equivalent focal length of this combination-lenses should be +50 cm.
Given:
Focal length of convex lens, f₁ = +25 cm
Focal length of concave lens, f₂ = -50 cm
Calculation:
We know that the power of a lens is given as:
P = 1 / f
where, f is the focal length of lens
Now, the power of convex lens can be calculated as:
P₁ = 1/f₁
= 1/(25×10⁻² m)
= 100/25
= +4 D
Similarly, the power of concave lens can be calculated as:
P₂ = 1/f₂
= (1/-50×10⁻² m)
= -100/50
= -2 D
Thus, the power of the combined lenses can be calculated as:
P₍₁₊₂₎ = P₁ + P₂
= +4 D - 2 D
= +2 D
Now, the combined focal length of the lens can be calculated as:
f₍₁₊₂₎ = 1 / P₍₁₊₂₎
= 100 / 2 D
= +50 cm
Therefore, the equivalent focal length of this combination-lenses will be +50 cm.
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