The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 4m away by means of a large convex lens. What is the max. possiblefocal length of the lens required for this purpose.

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Solution

Condition for real image to be formed on the wall by satisfying lens equation:

Maximum focal length, $f = \frac{x}{4}$

Here, x is the distance between object and screen.

Hence,

Maximum focal length, $f = \frac{x}{4}$
$= \frac{4 m}{4} = 1 m$

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The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 4m away by means of a large convex lens. What is the max. possiblefocal length of the lens required for this purpose.

Condition for real image to be formed on the wall by satisfying lens equation: Maximum focal length, f=x4 Here, x is the distance between object and screen. Hence, Maximum focal length, f=x4 =4 m4=1 m

Condition for real image to be formed on the wall by satisfying lens equation:

Maximum focal length, $f = \frac{x}{4}$

Here, x is the distance between object and screen.

Hence,

Maximum focal length, $f = \frac{x}{4}$
$= \frac{4 m}{4} = 1 m$