1. The Compound Microscope

2. The Compound Microscope
A compound microscope uses:
a very short focal length (< 1cm) lens, the objective.
a short focal length lens, the eyepiece, used as a simple magnifier. The image should be formed at infinity to minimize eyestrain.
The image is greatly enlarged by the objective and further magnified is by the eyepiece.

3. Locating images with two lenses
Problem: The microscope uses two lenses How to draw ray diagrams to find the final image of an object when dealing with two or more lenses.

4. Using Ray Diagrams
Final image F1 F2
Object
L1 image L1 L2

5. Using the lens Equation
Rules:
The image of the first lens becomes an object of the second lens (as if the first lens is removed).
The image of the second lens is the final image.
Be careful of the sign of v’s and u’s.

6. Two-lens systems Activity
Draw the ray diagrams for the two-lens systems
Locate the final image of the objects
Find an expression for the magnifications of the lens systems, knowing that the overall magnification M is the product of the magnification M1 by the lens L1 and the magnification M2 of lens L2 :
M = M1* M2
Draw a Venn diagram for the two systems and conclude how would you use these systems and why.

7. Microscope Ray Diagramu fo fe
L =16cm L1 L2
Image at infinity
Object
Objective
Eyepiece

8. The Microscope Ray DiagramL d
rAY
L = 16 cm

9. Microscope Magnification
The eye is relaxed when the final image is focused by the eyepiece at infinity.
L =16 cm is the distance from fo to the image formed by the objective.
Distance D between the lenses:
D = fo + L + fe

10. Microscope Magnification
Lateral magnification Mo produced by the objective is given by :
Mo = -vo/uo = - L/fo = -d/fo
The angular magnification Me of the eyepiece is given by:
Me = 25/fe
The magnification of the microscope M is:

11. Examples
a) A microscope has an objective of focal length=5mm and an eyepiece of focal length=3.5cm. What is its magnification?
b) If the image from the objective is focused at a distance d = 20 cm from the objective, what would be the magnification?
Answers
M = -25(16)/(0.5x3.5) = -229
M = -25(20-0.5)/(0.5x3.5) = -279