Question

A single bi-convex lens (a lens with two convex surfaces) made of glass (index of refraction n = 1.55) has surfaces with radii of curvature r₁ = 27.0 cm and r₂ = -27.0 cm. a)What is the focal length of the lens in air?
b)If an object is placed at p = 39.7 cm from the lens, where is the image? (Use plus sign for a real image, and minus sign for a virtual image.)
c)If the object has a height of h = 1.20 cm, how large is the image? (Use plus sign for an upright image, and minus sign for an inverted image.)

Answer 1

PART A:

The focal length of a thin lens is given by

Putting the values in the equation

So, the focal length of the lens is 24.54 cm.

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PART B:

We know, for thin lenses

Putting the values

(object distance is negative)

So, the image distance is +64.3 cm (real image).

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PART C:

The magnification is given by

And

So, the image height is -1.94 cm (inverted).