Question

Two thin lenses of power 3D and -2D areaced in contact then power and focal length of the lens combination is

(A) 2D, +100 cm
(B)+1D,+100 cm
(C) +5D,+20 cm
(D) +1D,-100 cm​

Answer 1

Given

✭ Power of two lenses are 3D and -2D

To Find

◈ Power & the focal length?

Solution

Concept

We shall first find the power of the combined and then use the formula P = 1/f to find the focal length and hence solved!!

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So we know that,

P = P¹+P²

• P¹ = +3D
• P² = -2D

Substituting the given values,

➝ P = P¹+P²

➝ P = 3+(-2)

➝ P = +1D

Now the focal length can be found with the help of,

P = 1/f

• P = +1D

Substituting the values,

➳ P = 1/f

➳ f = 1/P

➳ f = 1/1

➳ f = 1 m

So on converting to cm,

»» 1 m = 100 cm

∴ Power is +1D & Focal Length is +100 cm (Option B)

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Answer 2

Step-by-step explanation:

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♡ ↪OPTION B IS CORRECT

♡

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