Question

A candle 4 cm in size is placed at a distance of 25 cm from a concave mirror of focal length 15cm.At what position, the image will be formed? What will be the nature of the image formed?
(class 10 CBSE SAMPLE PAPER 2017-18 SCIENCE)

Answer 1

Solution:

Height of a candle (h1)= 4 cm

Distance of candle (object)'u’=  -25 cm

[Object distance is always negative]

Focal length of concave mirror (f)= -15 cm

[Focal length of a concave mirror is always negative]

Mirror formula:

1/f = 1/v+1/u

1/v = 1/f - 1/u

1/v = (-1/15) - (-1/25)

1/v = -1/15 +1/25

1/v= (-5+3)/75

1/v= -2/75

v= -75/2= -37.5 cm

Image of candle will be formed at 37.5 CM in front of the mirror.

Magnification:

m = h2/h1= -v/u

h2/4 = -(-37.5/-25)

h2/4= -3/2

h2 = (-3×4)/2= -6 cm

h2= 6 cm

The negative sign of v= image formed is real and inverted.

HOPE THIS WILL HELP YOU....

Answer 2

ANSWER
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THE SIZE OF THE OBJECT = + 5.0 cm;
Object-distance, u = (– 20.0 cm;)
Focal length, f = (–15.0 cm;)

FORMULA OF THE MIRROR =   (1/v) + (1/u) = 1/f

THEREFORE
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1/v = (1/f) − (1/u)
      = (−1/15) + (1/20)
      = −1/60
v = − 60 cm.


Magnification m = h'/h = − v/u = 60/(– 20) = – 3

Height of the image h' = mh = (– 3)x5 = − 15 cm
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