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Whats First and second principal focal length?

First principal focus: It is a point on the principal axis of the lens such that the rays of light starting from that point in case of a convex lens becomes parallel after refraction and in case of a concave lens a parallel beam of light appear to meet at that point after refraction.

Second principal focus: It is a point on the principal axis of the lens such that rays of light parallel to the principal axis meet at this point after refraction in the case of a convex lens and in the case of a concave lens the rays of light coming towards this focus becomes parallel after refraction.

 

 

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 Derive it also.

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    First Principal- It's the point on the principal axis of the lens,the rays starting from which appear to                                                       converge,become parallel to principal axis after refraction.

Second Principal-It's the point on the principal axis  at which the rays coming paralle to the principal axis                                               appears to converge or diverge after refraction from the lens.

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     it's a diagram.....google it...!

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on d right side of anyray diag. there is first focal length...n on left side there is second principal foca;l length

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I am asking about this thing,

f1 = -n1R/n2-n1

f2 = -n2R/n2-n1

Derive these fomulas....

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