Question

an eye specialist prescribes spectacles having combination of convex lens of focal length 40 cm in contact with a concave lens of focal length 25cm the power of this lens combination in dioptres is

Answer 1

hey !!!! i found ur answer here 100/f because in cms we calculate p in 100/f

Answer 2

Answer:

-1.5 Dioptres

Explanation:

Power of a lens

$p=\frac{1000}{\text{Length in mm}}\\\Rightarrow p=\frac{100}{\text{Length in cm}}$

Convex lens has positive focal length

$p_1=\frac{100}{f_1}\\\Rightarrow p_1=\frac{100}{40}\\\Rightarrow p_1=2.5\ D$

Concave lens has negative focal length

$p_2=\frac{100}{f_2}\\\Rightarrow p_2=-\frac{100}{25}\\\Rightarrow p_1=-4\ D$

Combining the power of the lenses

$p=p_1+p_2\\\Rightarrow p=2.5-4\\\Rightarrow p=-1.5\ D$

Power of the combined lens is -1.5 Dioptres