Q38 of 38 Page 84

A student 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 the lens as under Position of candle = 12.0 cm Position of convex lens = 50.0 cm Position of the screen = 88.0 cm

(i) What is the focal length of the convex lens?


(ii) Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm?


(iii) What will be the nature of the image formed if he further shifts the candle towards the lens?


(iv) Draw a ray diagram to show the formation of the image in case


(iii) as said above.

(i)object distance u = position of convex lens – position of candle


u = - (50 -12)= -38 cm


image distance v = position of screen –position of convex lens


v= 88-50 cm


v= 38 cm


Applying the lens formula:





on calculating focal length willbe f = 19 cm


(ii) When the candle is shifted towards the lens at a position of 31.0 cm:


Object distance u =-(50-31)


u = -19 cm


Applying the lens formula





Therefore, v is infinity


(iii) If the candle is further shifted towards the lens, then the nature of the image formed will be virtual, erect and magnified


(iv) The ray diagram will be as follows:



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