The image of a candle flame placed at a distance of 45 cm from a spherical lens is formed on a screen placed at a distance of 90 cm from the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 2 cm, find the height of its image.
According to the question;
Object distance (u) = -45cm;
Image distance (v) = 90cm;
By lens formula;
⇒ f = 30cm.
Since f = 30cm which is positive hence it is a convex lens of focal length 30cm.
Now;
Height of Flame h1= 2cm;
Magnification = 
= 
Putting values of v and u
Magnification = 
⇒ 
⇒ 
Height of image is 4 cm.
Negative sign means image is inverted.
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