Question

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

Answer 1

1. 

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

3. 

• 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.