an object kept at 20 cm from a mirror forms an image at 20 cm on the same side of object for a mirror, then the value of focal length will be
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Question ❓
➡️ An object kept at 20 cm from a mirror forms an image at 20 cm on the same side of object for a mirror, then the value of focal length will be?
Answer✔
➡️ It is given that,
Focal length
f=−20cm
h
i
=3h
o
The image is three times the size of the object. So there are two cases.
Case 1. For real image
m=−
u
v
=−3
v=3u
Using mirror’s formula
f
1
=
v
1
+
u
1
−20
1
=
3u
1
+
u
1
u=
3
−80
cm
Case 2. For virtual image
m=
u
−v
=+3
v=−3u
Again using mirror’s formula
−20
1
=
−3u
1
+
u
1
u=
3
−40
cm
The possible values of u are
3
−80
cmand
3
−40
cm.
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