An object of height 4.0 cm is placed at a distance of 30 cm from the optical centre 'O' of a convexlens offocal length 20 cm. Draw a ray diagramto find theposition and size of the image formed.Mark opticalcentre 'O' and principal focus 'F' on the diagram. Also find the approximate ratio of size of the image to the sizeof the object.

An object of height 4.0 cm is placed at a distance of 30 cm from the o

1.	An object of height 4.0 cm is placed at a distance of 30 cm from the optical centre 'O' of a convexlens offocal length 20 cm. Draw a ray diagramto find theposition and size of the image formed.Mark opticalcentre 'O' and principal focus 'F' on the diagram. Also find the approximate ratio of size of the image to the sizeof the object.
Answer» Solution :Given : `h_(1) =+44, u = - 30 cm, f = +20 cm` When theobject is placed between F and 2F, a real, INVERTED and magnifiedimage is formed. Using lens formula , `1/f = 1/v - 1/u rArr 1/20= 1/v - 1/(-30)` `1/v = 1/20 - 1/30 = (3-2)/(60) = 1/60` `v = +60 cm` Now, `(h_(2))/(h_(1)) = v/u = 60/(-30) = - 2 rArr (h_(2))/(h_(1))= 2/1` or `h_(2) : h_(1) = 2 : 1` `:.` Ratio of size of IMAGE to sizeof object is `2:1`