Question

A candle placed at a distance of 40 cm from convex lens forms image on a screen placed at

a distance of 40 cm on the other side of it. Find (i) the focal length of the lens,

(ii) the magnification produced by the lens. Mention the nature of the image.​

Answer 1

Given:

A candle placed at a distance of 40 cm from convex lens forms image on a screen placed at a distance of 40 cm on the other side of it.

To find:

• Focal length of lens
• Magnification
• Nature of image

Calculation:

Applying Len's Formula:

$\rm \: \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$\rm \implies \: \dfrac{1}{f} = \dfrac{1}{40} - \dfrac{1}{( - 40)}$

$\rm \implies \: \dfrac{1}{f} = \dfrac{1}{40} + \dfrac{1}{40}$

$\rm \implies \: \dfrac{1}{f} = \dfrac{2}{40}$

$\rm \implies \: \dfrac{1}{f} = \dfrac{1}{20}$

$\rm \implies \: f = 20 \: cm$

So , focal length of lens is 20 cm.

$\rm \: magnification = \dfrac{v}{u}$

$\rm \implies\: magnification = \dfrac{40}{( - 40)}$

$\rm \implies\: magnification = - 1$

So , magnification is -1.

Image characteristics:

• Image is real, inverted and is of the same size as that of the object.

Hope It Helps.

Answer 2

Explanation:

So , focal length of lens is 20 cm.

\rm \: magnification = \dfrac{v}{u}magnification=

u

v

\rm \implies\: magnification = \dfrac{40}{( - 40)}⟹magnification=

(−40)

40

\rm \implies\: magnification = - 1⟹magnification=−1

So , magnification is -1.