A parallel beam of monochromatic light falls on a combination of a convex lens

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A parallel beam of monochromatic light falls on a combination of a convex lens and a concave lens of focal lengths 15 cm & 5 cm respectively.If the light emerges parallel from the concave lens. The distance between two lenses is

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Explanation:

The focus of the lens which is placed d unit away

1/f=1/f₁ +1/f₂ +d/(f₁ f₂ )

For the lenses

f₁=15cm and f₂=-5cm f=∞

⇒1/f=1/15+1/(-5)+d/((15)(-5))

⇒0=1/15-1/5+d/(-75)

⇒d/75=(3-1)/15

⇒d=2/15×(75)

⇒d=10cm

solved 5

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