Question

Hey friends pls.. give me the answer soon. it's urgent.
convex lens power 5D. concave lens power 7.5D.

pls..... answer me.

Answer 1

Final Answer : a) 12.5D
b) 8 cm
$\bf \: {Combination \: of \: Lens \: } \\ \bf \: {A \: convex \: lens \: of \:\\ power \: 5D and\: concave lens\:\:of Power 7.5D are placed in contact with each other. \: }$
What is the
a) Power of the combination of lenses
b) focal length of the combination of lenses.

Steps and Understanding :
a) To get the Resultant Power, Sum of all individual powers is taken.
P(1) = 5 D
P(2) = 7.5D

P (res) =P(1) + P(2)
=> P (res) = 5 D + 7.5 D = 12.5 D.

b)
$\frac{1}{f(eq)} = \frac{1}{f(1)} + \frac{1}{f(2)} \\ = > \frac{1}{f(eq)} = p(1) + p(2) \\ = > \frac{1}{f(eq)} = 12.5 \\ = > f(eq) = 0.08 \: m \: = 8 \: cm$
Therefore, Resultant focal length is 8 cm.