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Light Reflection and Refraction
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03/12/2020 12:38 pm
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List the sign conventions that are followed in case of refraction of light through spherical lenses. Draw a diagram and apply these conventions in determining the nature and focal length of a spherical lens which forms three times magnified real image of an object placed 16 cm from the lens.
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03/12/2020 12:41 pm
Sign conventions for refraction of light through spherical lens are:
- The object is always placed to the left of the lens so that incident light moves from left to right.
- All distances are to be measured from the optical centre of the lens.
- The distances measured in the direction of incident light (along +ve x-axis) will be taken as positive. while those measured to the left of the origin (along -ve x-axis) will be taken as negative.
- All measurements of heights above the principal axis (along +ve y-axis) will be considered as positive while below it (along -ve y-axis) will be taken as negative.
According to question
u = -16 cm, m = -3 (real image)
But m = v/u
= -3
v = -3u
= -3 x (-16)
= 48 cm
Using lens formula
1/f = 1/v - 1/u
= 1/48 - 1/-16 = 1/48 + 1/16
= 1+3/48 = 4/48 = 1/12
f = +12 cm
So, focal length of the given spherical lens is 12 cm. The positive sign of focal length shows that the nature of spherical lens is convex.
Ray diagram: