Question

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 90 cm from the lens calculate its focal length identify the type of lens if the height of the flame is 2 cm find the height of its image​

Answer 1

u = -45 cm
v = 90 cm (Formed on a screen, therefore real image, and hence convex lens)

1/f = 1/v  - 1/u
1/f = 1/90 - (-1/45)
= 1/90 + 1/45
= 3/90 = 1/30

Therefore f = 30 cm. 

magnification M = v/u = height of image / height of object
v / u = 90/-45 = -2
-2 = hi / ho 
-2 = hi / 2
hi = -4 cm (Negative, located below the principal axis)

Therefore it is a convex lens and a converging lens. It's focal length is 30 cm. And the height of the image is 4 cm and is located below the principal axis, hence negative.

Answer 2

Answer:

hope this helps you a lot