Sarthaks Test
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Two lenses of focal lengths f1 and f2 are placed in contact

1. If the object is at principal axis, draw ray diagram of the image formation by this lens.

2. Obtain a general expression for effective focal length in terms of f1 and f2 .

3. How will you combine a convex lens of focal length f1 and concave lens f2 such that combination acts as

  • Converging
  • diverging
  • plane glass plate

1 Answer

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2. Obtain an expression for the effective focal length of the combination of two thin convex lenses in contact.


  • Keep in a medium of refractive index lower than that of lens.
  • Keep in a medium of refractive index higher than that of lens.
  • Keep in a medium of refractive index equal to refractive index of lens.

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