The image of an object place at 60 cm in front of a lens is obtained on a screen at a distance of 120 cm from it. Find the focal length of the lens. What would be the lens? What would be the height of the image? If the object is 5 cm high?
According to the question;
Object distance (u) = -60cm;
Image distance (v) = 120cm;
Focal length = f
By lens formula;
⇒ f = 40cm.
Therefore Focal length of lens is 40 cm.
Since the focal length is positive. Therefore the lens is convex lens of focal length 40cm.
Now;
Height of object h1= 5cm;
Magnification = 
= 
Putting values of v and u
Magnification = 
⇒ 
⇒ h2 = -2 × 5= -10 cm
Height of image is 10 cm.
Negative sign means image is real and inverted..
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