Q20 of 36 Page 379

A convex lens has a focal length of 10 cm. At what distance from the lens should the object be placed so that it forms a real and inverted image 20 cm away from the lens? What would be the size of the image formed if the object is 2 cm high? With the help of a ray diagram show the formation of the image by the lens in this case.

According to the question;

Object distance = u;


Image distance (v) = 20cm;


Focal length = 10cm


By lens formula;







⇒ u = -20cm.


Therefore, object is placed at 20 cm in front of lens.


Now;


Height of object h1= 2cm;


Magnification = =


Putting values of v and u


Magnification =


⇒


⇒


Height of image is 2 cm.


Negative sign means image is inverted.


Thus, the image is real, inverted and same as size of object.


Diagram below shows the image formation.



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