# 1.an object is placed in front of concave mirror of focal length 20 Cm, the image formed is 3 times the size of the object.find the image distance. 2.A 2.5 cm candle is placed 12 cm away from a convex mirror of focal length 30 cm.find the location of the image and magnification of the image.

2
by dkavyasri13
okay!! done!
i want 2 questions answers from physics
same lesson
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2015-11-01T15:02:15+05:30
1. f = -20cm
m = -3
v = ?

m = -v/u = -3
⇒ v/u = 3
⇒ v = 3u

Using mirror formula 1/v + 1/u = 1/f
⇒ 1/3u + 1/u = 1/-20
⇒ (1+3)/3u = 1/-20
⇒ 4/3u = 1/-20
⇒ u = -(4*20)/3
⇒ u = -80/3 cm
v = 3u = -80/3 * 3 = -80cm

∴ Image distance from mirror is 80cm. It is formed in front of the mirror.

2. h = +2.5cm
u = -12cm
f = +30cm
v = ?
m = ?

1/v + 1/u = 1/f
⇒ 1/v = 1/f - 1/u = 1/30 - 1/-12
⇒ 1/v = 1/30 + 1/12
⇒ 1/v = (2+5)/60 = 7/60
⇒ v = 60/7 cm

m = -v/u = -(60/7)/(-12) = 60/(7*12) = 5/7

∴ Object distance is 60/7 from mirror. Image is virtual and erect(since v is positive). Magnification is 5/7.
okk
what r those
sir
see my answer for those values..
ok sir
2015-11-01T15:54:51+05:30

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1)
f = -20 cm                m = 3  or  - 3      either of them.

case1)  m = -v/u = -3  =>  v = 3 u          inverted image
1/f = 1/v  + 1/u  =>  1/-20  =  1/v + 3/v = 4/v
=> v = - 80 cm

case 2)  m = -v/u = 3  =>  erect virtual  image.    v = - 3 u
1/-20 = 1/v - 3/v  = - 2/v
=>  v = 40 cm
============================

2)
u = -12 cm      h = 2.5 cm
f = +30 cm

1/f = 1/v + 1/u
1/30  = 1/v -1/12          => v = 60/7  cm     virtual image, behind the mirror.
m = -v/u  = 5/7

virtual image is also formed.... image is always said to be formed... formed means it can be virtual as well as real....
ok sir
thank u so much sir
in a rear view mirror, an image is formed, then it is a virtual image..
ok sir