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Light Reflection and Refraction
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03/12/2020 12:19 pm
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List the sign conventions for reflection of light by spherical mirrors. Draw a diagram and apply these conventions in the determination of focal length of a spherical mirror which forms three times magnified real image of an object placed 16 cm in front of it.
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03/12/2020 12:23 pm
Sign conventions for reflection of light by spherical mirror are:
- The object is always placed to the left of the mirror.
- All the distances parallel to the principal axis are always measured from the pole of the spherical mirror.
- All the distances measured along the direction of incident light (along +ve x-axis), are considered to be positive.
- Those distances measured opposite to the direction of incidence light (i.e. along -ve x-axis), are taken as negative.
- The distances measured in upward direction, i.e. perpendicular to and above the principal axis (along +ve y-axis), are taken as positive.
- The distances measured in the downward direction, (along -ve y-axis), i.e. perpendicular to and below the principal axis are taken as negative.
From the question
u = -16 cm, m = -3 for real image
But m = - v/u = -3
v = 3u = 3(-16) = -48 cm
Using mirror formula,
1/f = 1/v + 1/u
We get, 1/f = 1/-48 + 1/-16
= 1/-48 - 1/16 = -1-3/48
= -4/48 = -1/12
f = -12 cm
So, focal length of spherical mirror is 12 cm. Negative sign of focal length indicates that mirror is concave in nature.