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Solution


Q.

An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be               [2018]
1 30 cm away from the mirror  
2 36 cm away from the mirror  
3 30 cm towards the mirror  
4 36 cm towards the mirror  

Ans.

(2)

Using mirror formula,

1f=1v1+1u1;  -115=1v1-140⇒1v1=1-15+140

v1=-24cm

When object is displaced by 20 cm towards mirror.

Now, u2=-20cm

1f=1v2+1u2;  1-15=1v2-120⇒1v2=120-115;

v2=-60cm

So, the image will be shift away from mirror by (60 - 24) cm = 36 cm.