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.

The image of a candle flame placed at a distance of 45 cm from a spher

1.	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.
Answer» Solution :As the image is being formed on a screen, the image MUST be a real image and the lens used must be a convex lens. As per question with sign convention : U = - 45 CM, V = + 90 cm and h = + 2 cm. `therefore""(1)/(f) = (1)/(v) - (1)/(u) = (1)/((+90)) - (1)/((-45)) = (1)/(90) + (1)/(45) = (1 + 2)/(90) = (3)/(90) = (1)/(30)` `rArr""f = + 30` cm If height of the image be h., then `(h.)/(h) = (v)/(u)` `rArr""h. = (v)/(u).h = ((+90))/((-45)) XX 2 = - 4` cm