A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. Calculate the focal length of this mirror.

Given,

Height of object h1= 10 mm


Height of image h2= 5 mm


Image distance v= -30 cm


To find focal length f =??



Substituting values given in this formula:



Magnification = 0.5


So, the given information signifies that image formed by mirror is of small size than that of object. Also,



Substituting values in this,




Object distance = 60 cm


Since object is placed on left of mirror so


u = -60 cm


We know that mirror formula is:







f = -20 cm


Focal length of the concave mirror is -20 cm.


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