Question

an object is kept in front of a concave mirror of focal length 20 centimetre the image formed is 3 times the size of the object calculate the possible distances of the object from the mirror​

Answer 1

The possible distance of the object from the mirror are - 13.3 cm and - 26.67 cm respectively.

Explanation:

• The magnification is given below:
• Focal length of mirror = 20 cm

m = +3 ; m = -3

• 1st case:

When m=+3

m = -v/u

v = -3 u

• Using mirror formula.

1/f = 1/v + 1/u

-1/20 = -1/3 u+1/u

1/u = 1/3 u-1/20  

-3 u=40

u = - 40 / 3

u = - 13.33 cm

• Second Case:

when m = -3

m = -v/u

-3 = -v/u

v = 3 u

• Now using mirror formula.

1/f = 1/v + 1/u

1/ -20 = 1/3 u + 1/u = 4/3 u

OR

u = - 80 / 3 = - 26.67 cm

Thus the possible distance of the object from the mirror are - 13.3 cm and - 26.67 cm respectively.

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