Question

Two thin lenses of power +6D and -2D are in contact . What is focal length of the combination

Answer 1

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So, here is your answer
Power of combination = 6 D +(-2 D )
       = 6 D -2 D = 4 D
      power = 1/focal length(m) 
      4 = 1/f
       f = 1/4
       f = 0.25 metres
       f of combination = 0.25 * 100 = 25 cm
                          

Answer 2

Hey dear,

◆ Answer-
f = +25 cm

◆ Explaination-
# Given-
P1 = + 6 D
P2 = - 2 D
P = ?
f = ?

# Solution-
Let f1, f2 & f be focal length of 1st lens, 2nd lens and combination, and P1, P2 & P be their respective powers.

Power of combination of lenses is calculated by-
P = P1 + P2
P = 6 - 2
P = + 4 D

Focal length of combination is reciprocal of power,
f = 1/P
f = 1/4
f = 0.25 m
f = + 25 cm

Therefore, focal length of the lens combination is +25 cm and it will act as convex lens.

Hope this helps you...