Answer:
The focal length is [tex]f = -4.721 \ m[/tex]
Explanation:
From the question we are told that
The image distance is [tex]v = 4.7 + (2.1 *10^{-2}) = - 4.721 \ m[/tex] the negative means image is formed behind the lens
Generally the object distance is [tex]u = \infty[/tex] this mean the light ray coming from the object are parallel to each other
So from lens equation
[tex]\frac{1}{\infty } - \frac{1}{4.721} = \frac{1}{f}[/tex]
=> [tex]f = -4.721 \ m[/tex]
