Everything Science Grade 11

CHAPTER 13. GEOMETRICAL OPTICS 13.5

Example 5: The Compound Microscope

QUESTION

A compound microscope consists of two convexlenses. The eyepiece has a focal length of

10 cm. The objective lens has a focal length of6 cm. The two lenses are 30 cm apart. A 2

cm-tall object is placed8 cm from the objectivelens.

1. Where is the final image?


2. Is the final image real or virtual?

SOLUTION

We can use ray tracing to follow light rays through the microscope, onelens at a time.

Step 1 : Set up the system

To prepare to trace the light rays, make a diagram. In the diagram here, we

place the image on the left side of the microscope. Since the light will pass

through the objective lens first, we’ll call this Lens 1. The eyepiece willbe

called Lens 2. Be sure toinclude the focal pointsof both lenses in your diagram.

Object

Lens 1 (Objective) Lens 2 (Eyepiece)

f 1 f 1 f 2 f 2

� � � � � �

30 cm

8 cm

6cm 6cm 10 cm 10 cm

Step 2 : Find the image for theobjective lens.

Object

Image

f 2 f 2

f 1

� � � � � �

30 cm