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CBSE Class 10 Answered

an object is placed in front of a concave mirror of focal length 20 cm the image formed is 3 times the size, of object calcualte two possible dostances of object from the mirror
Asked by priyankakachawar79 | 19 Nov, 2019, 06:28: PM
Expert Answer
Given:
Image must be 3 times the size of an object. 
m = -3 or +3 
f = - 20 cm 
 
We know, 
m = - v/u
 
For m = + 3
Thus, 
v = -3u 
 
 
For this, 
v = 40 cm
This means the image will be formed behind the mirror. And thus, the image formed will be virtual, erect and magnified i.e. 3 times the size of an object.
 
For m = -3 
v = 3u 
 
 
For this, image distance, v = -80 cm 
 
Thus, when object is placed at 26.6 cm in front of this concave mirror, image formed in front of concave mirror. And the negative sign of the magnification indicates that the image formed is real and inverted. 
 
Thus, when the object is placed at 13.3 cm ( between focus and pole) or at 26.6 cm ( between focus and centre of curvature) the image is three times the size of an object. 
 
 
Answered by Shiwani Sawant | 20 Nov, 2019, 05:16: PM

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