Lens Formula


 
 
Concept Explanation
 

Lens Formula

If we place an object at a distance u from the optical centre 'O'  and its image is formed at a distance then

 frac{1}{v}-frac{1}{u}=frac{1}{f} 

This relationship is known as lens formula. Here is the focal length of this lens.

Proof:

Consider an object AB placed beyond 2F in front of a convex lens. On the other side of lens an image is formed between F and 2F which is real, inverted and smaller in size. Applying the sign conventions :

Object distance (OB ):= -u

Image distance(OB') := v

Focal length(OF): = +f

In Delta AOB andDelta A^IB^IO 

angle 1 = angle 2                                       ( vertically opposite angle)

angle 3 =angle 4                                         ( each 90^{circ})

therefore angle BAOsim angle B^IA^IO             (By AA similarity)

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Sample Questions
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Question : 1

Which of the following are correct ?

(a) The lens formula u stands for object distance .

(b) The lens formula v stands for image distance .

(c) The lens formula f stands for focal length.

Right Option : D
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Explanation
Question : 2

In the lens f' stands for ._____________.

frac{1}{v}-frac{1}{u}=frac{1}{f}

Right Option : A
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Explanation
Question : 3

Lens formula is given as ?

Right Option : A
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Explanation
 
 
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