Solve this:
Rohit focused the Image of a candle flame on a white screen using a convex lens. He noted down the position of the candle, screen and lens as under .
Position Of candle          =   26.0 cm
Position Of convex lens =   50.0 cm
Position Of screen          =   74.0 cm
(I) What is the focal length Of the convex lens?
(II) Where will the image be formed it he shifts the candle towards the at a position Of 38 cm 
(III) Draw a ray diagram to show the formation Of the image in case (ii) as said above.

Dear Student
Here, we will measure all distance with respect to the position of convex lens.
 

so,

the object distance will be, u = 26 cm - 50 cm = -24 cm

the image distance will be, v = 74.0 cm - 50 cm = 24 cm

(1)

so, the focal length will be calculated using the lens formula

1/f = 1/v - 1/u

or

1/f = (2 / 24)

thus, the focal length fot he lens is 

f = 12 cm

(2)

here,

the image distance will be, u = 38 cm - 50 cm = -12cm

and focal length, f = 12 cm

so,

1/v = 1/f + 1/u

or

1/v = 1/12 - 1/12

so,

v = infinity, the image will be formed at infinity as the object is kept on the point of focus of the lens.

(c)