Question

Q-An object of height 1.2m is placed before a concave mirror of focal length 20cm so that a real image is formed at a distance of 60cm from it. Find the position of an object. What will be the height of the image formed?

Answer 1

h=1.2m=120cm
f=-20cm
v=-60cm
1/f=1/v +1/u
-1/20 = -1/60 +1/u
u=-20
now,m=-v/u=height of image/ height of object
h/120=60/-30
h=-2.4 m

Answer 2

OBJECT IS ALWAYS PLACE TO THE LEFT SIDE OF A MIRROR THEREFORE THE OBJECT DIATANCE (U) IS ALWAYS NEGATIVE.

IF THE IMAGE FORMED IS FONT OF THE MIRROR ( REAL IMAGE) ON THE LEFT SIDE THEN THE IMAGE DISTANCE (V) WILL BE NEGATIVE.

THE FOCUS OF CONCAVE MIRROR IS IN FRONT OF THE MIRROR OF THE LEFT SIDE SO THE FOCAL LENGTH OF THE CONCAVE MIRROR IS CONSIDERED NEGATIVE.

GIVEN,

HEIGHT OF THE OBJECT (H1) = 1.2cm
IMAGE DISTANCE (V) = -60CM (REAL IMAGE )
FOCAL LENGTH (F)= -20CM

MIRROR FORMULA
1/V +1/U=1/F
(-1/60) + (1/u)= (-1/20)
1/u = -1/20 +1/60
1/u = (-3+1)/60 =-2/60 =-1/30
1/u =-1/30
u = -30

OBJECT DISTANCE (U) = -30
MAGNIFICATION (M) = -v/u

m=(-60)/-30=60/-30=2
m=h2/ho
-2×1.2=h2
h2= -2.4cm

HEIGHT OF REAL IMAGE (h2)= -2.4cm

REAL IMAGE IS FORMED BELOW THE AXIS. SO THE HEIGHT OF A REAL IMAGE IS NEGATIVE.

HENCE, THE POSITIVE OF AN OBJECT IS -30 CM ( TO THE LEFT OF MIRROR ) AND THE HEIGHT OF THE REAL IMAGE IS FORMED = -2.4CM

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HOPE THIS WILL HELP YOU.......