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Light Reflection and Refraction
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23/08/2021 3:48 pm
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An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed.
(a) State which type of lens is used.
(b) Calculate focal length of the lens.
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23/08/2021 3:53 pm
(a) Since the image is formed on a screen, it must be a real image.
Hence, the lens should be a convex lens.
(b) m = -3 (Real and inverted image)
-u + v = 80 cm
m = \(\frac{v}{u}\)
-3 = \(\frac{80 + u}{u}\)
-3u = 80 + u
u = -20 cm
v = 80 + u = 80 + (-20)
= 60 cm
\(\frac{1}{v}\) - \(\frac{1}{u}\) = \(\frac{1}{f}\)
\(\frac{1}{60}\) - \(\frac{1}{-20}\) = \(\frac{1}{f}\)
\(\frac{1}{f}\) = \(\frac{4}{60}\)
f = +15 cm