A concave mirror has a focal length of 20 cm. Find the position or positions of an object for which the image-size is double of the object-size.

10 cm or 30 cm from the mirror

The focal length of the concave mirror f = -20cm


Magnification of the mirror M = = 2


Now, there can be two positions of the object which is to be placed in front of mirror to get the image of double size of the object. The image formed can be real or virtual, depending upon the position of the object.


Using mirror formula



We consider the image formed will be virtual and beyond the mirror. Hence v=-20u





We consider the image formed will be real and inverted. Hence, v=20u





Therefore, the positions of the object should be 10cm and 30cm away from mirror to get the image double in size.


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