Question

Two lenses if power +5d and -10d are seperated by 20cm. What is equivalent focal length

Answer 1

Answer:

F = -0.2cm where the '-'ve sign indicates a diverging lens.

Explanation:

We known that Power = 1/f

where f is the focal length.

Given that P₁ = +5d

Therefore, f₁ = 1/5cm

Similarly, f₂ = -1/10cm

According to the formula, it is known that

$\frac{1}{F}$ = $\frac{1}{f_{1} }$ + $\frac{1}{f_{2} }$

where F is the equivalent focal length

f₁ is the focal length of first lens

f₂ is the focal length of second lens

=> $\frac{1}{F}$ = $\frac{1}{\frac{1}{5} }$ + $\frac{1}{\frac{1}{-10} }$

=> $\frac{1}{F}$ = 5 + (-10)

=> $\frac{1}{F}$ = -5

=> F = -1/5

=> F = -0.2cm

The '-'ve sign indicates a diverging lens.