A concave mirror forms a real image at the same position of the object, if the object is at 20cm away from the mirror then, (a) find the focal length of the mirro r(b) If the object is moved 5cm towards the , what will be the position of the image

-90cm

Explanation:

u=−30cm and magnification m=(−v/u)=−3   so   v=(3u)image is real so m should be negative  v should be negative so v=−90cm

putting the above values of v and u in the Mirror equation, 

f1=u1+v1

 we get focal length f=−22.5cm and image position v=−90cm

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