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Light Reflection and Refraction
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03/12/2020 11:57 am
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The image of a candle flame placed at a distance of 45 cm from a spherical lens is formed on a screen placed at a distance of 90 cm from the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 2 cm, find the height of its image.
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03/12/2020 12:02 pm
Nature of spherical lens = convex lens
Given: u = -45 cm, v = +90 cm
h1 = +2 cm
Using lens formula,
1/f = 1/v - 1/u
= 1/90 - 1/-45 = 1/90 + 1/45
= 1 + 2/90 = 3/90 = 1/30
∴ f = + 30 cm
So, focal length of convex lens is 30 cm.
Again,
m = h2/h1 = v/u
h2 = v/u x h1 = 90/-45 x 2
= -4 cm
So, height of image is 4 cm. Negative sign indicates that it is formed below the principal axis.