A students plots a graph between inverse of magnification 1/m produced by a convex thin lens and the object distance u as shown in figure. What was the focal length of the lens used?

Question

A students plots a graph between inverse of magnification 1/m produced by a convex thin lens and the object distance u as shown in figure. What was the focal length of the lens used? (A) (b/ca) (B) (bc/a) (C) (c/b) (D) (b/c)

Tardigrade Admin · Accepted Answer

Lens formula,(1/v)-(1/u)=(1/f) or (u/v)-1=(u/f) ⇒ (1/m)-1=(u/f) or (1/m)((1/f))u+1 this is the equation of a straight line whose slope (1/f)=(b/c) ∴ f = (c/b)

Q. A students plots a graph between inverse of magnification 1/ $m$ produced by a convex thin lens and the object distance $u$ as shown in figure. What was the focal length of the lens used?

$\frac{b}{c a}$

$\frac{b c}{a}$

$\frac{c}{b}$

$\frac{b}{c}$

Lens formula, $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
or $\frac{u}{v} - 1 = \frac{u}{f}$
$\Rightarrow \frac{1}{m} - 1 = \frac{u}{f}$
or $\frac{1}{m} (\frac{1}{f}) u + 1$
this is the equation of a straight line whose slope
$\frac{1}{f} = \frac{b}{c}$
$∴ f = \frac{c}{b}$

Q. A students plots a graph between inverse of magnification 1/m produced by a convex thin lens and the object distance u as shown in figure. What was the focal length of the lens used?

cab​

abc​

bc​

cb​