Question

The above image shows the formation of an image with an optical instrument.

1. Identify the optical instrument (shown schematically as a rectangle) in the image.
2. What type of image is formed in this case?
3. Based on the measurements given in the image, calculate the focal length of the instrument.

Answer 1

A. The optical instrument shown in the figure is a concave lens, as the rays diverge after passing through it and appear to come from a point on the same side as the object. 

B. The image formed is a virtual, erect and diminished image.

C. Given, object distance (u) = −20 cm

Image distance (v) = −10 cm

Using lens formula,

`1/f = 1/v - 1/u`

= `1/-10 - 1/-20`

= `(-1)/20`

f = −20 cm