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Light Reflection and Refraction
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20/08/2021 1:23 pm
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A concave lens has a focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also, calculate the size of the image formed.
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20/08/2021 1:28 pm
f = -20 cm
\(h_1\) = 5 cm
v = -15 cm (concave lens forms virtual image)
\(\frac{1}{v}\) - \(\frac{1}{u}\) = \(\frac{1}{f}\)
\(\frac{1}{-15}\) - \(\frac{1}{u}\) = \(\frac{1}{-20}\)
\(\frac{1}{u}\) = -\(\frac{1}{15}\) + \(\frac{1}{20}\)
\(\frac{1}{u}\) = \(\frac{-1}{60}\)
u = -60 cm
Object should be placed 60 cm to the left of the lens.
m = \(\frac{v}{u}\) = \(\frac{h_2}{h_1}\)
\(\frac{-15}{-60}\) = \(\frac{h_2}{5}\)
\(h_2\) = 1.25 cm
Image formed is 1.25 cm high.