# If a Concave Mirror Has a Focal Length of 10 Cm, Find the Two Positions Where an Object Can Be Placed to Give, in Each Case, an Image Twice the Height of the Object. - Science

If a concave mirror has a focal length of 10 cm, find the two positions where an object can be placed to give, in each case, an image twice the height of the object.

#### Solution

Given,
Focal length (f) of the concave mirror = -10 cm
Case-1
The image is real, and its magnification (m) is -2.
Using the magnification formula, we get
1-10=12u+1u

m=(-v)/u

-2=(-v)/u

v=2u

Now, using the mirror formula, we get

1/f=1/v+1/u

1/-10=1/2u+1/u

1/-10=1/(2u)+2/(2u)=3/2u

u=(3xx(-10))/2=-15 cm

The object should be placed at a distance of 15cm from the concave mirror.

Case-2
The image is virtual and has a magnification 'm' of 2.
Using the magnification formula, we get

m=-v/u

2=-v/u

v=-2u
Now, using the mirror formula, we get

1/f=1/v+1/u

1/-10=1/(-2u)+1/u

1/-10=-1/(2u)+2/(2u)=1/(2u)

u=(-10)/2=-5 cm `

Thus, the object should be placed at a distance of 5 cm in front of the concave mirror.

Concept: Concave Mirror
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#### APPEARS IN

Lakhmir Singh Class 10 Physics (Science)
Chapter 4 Reflection of Light
Q 44 | Page 200