NCERT Solution for Class 10 Science Chapter 10
Heredity and Evolution
NCERT Books for Session 2022-2023
CBSE Board and UP Board
Exercises Questions
Page No-186
Questions No-15
An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.
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Object distance, u = −27 cm
Object height, h = 7 cm
Focal length, f = −18 cm
According to the mirror formula,
1/u + 1/v = 1/f
1/v = 1/f – 1/u = -1/18 + 1/27 = -1/54
V = -54 cm
The screen should be placed at a distance of 54 cm in front of the given mirror.
Magnification, m = – Image Distance/Object Distance = -54/27 = -2
The negative value of magnification indicates that the image formed is real.
Magnification, m = Height of the image / Height of the object = h’/h
h‘ = 7 x (-2) = -14 cm
The negative value of image height indicates that the image formed is inverted.
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