Q. 304.3( 6 Votes )

A concave lens has focal length of 20cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at a distance of 15 cm from the lens? Also calculate the size of the image formed.

Answer :

Given,

Focal length of concave lens f = -20 cm


Height of object h1= 5 cm


Image distance v = -15 cm


Object distance u =???


Height of image h2 =???


We know that lens formula is:



Substituting the given values in the formula:







u = -60 cm





Substituting the value object height here




Height or size of the image = 1.25 cm


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