Compound Light Microscope
TOTAL MAGNIFICATION
• Power of the eyepiece (10X) multiplied by
objective lenses determines total magnification.
Magnification
Objective Ocular
Total
Eyepiece Magnification
Low Power 4 x
10 x
40 x
Medium
Power
10 x
10 x
100 x
High Power 40 x
10 x
400 x
Field of View
• Field of View (FV) is the
illuminated circle that
you see when looking
through the ocular
eyepiece.
• If we know the diameter
of the FV then we can
estimate the size of our
microorganisms.
Field of View
• With our microscopes the
diameter of the FV under low
power is 4 mm
• FV is measured in micrometers
or microns.
• 1 mm = 1000 microns
• Therefore, our FV under low
power is 4000 microns
Using the Field of View to Estimate
Microscopic Measurements
If an organism takes up ½ of the
FV under low power, it must be
about 2000 microns in length
Making cheek cell and onion cells
slides
• Cheek
– Scrape the inside of your cheek with a toothpick
– Rub toothpick onto the slide and add a drop of
methylene blue
– Place a cover slip onto the slide. Observe
• Onion
–
–
–
–
Peel off inner epidermis of onion using forceps
Place on slide
Put a drop of iodine and a cover slip onto your slide
Observe
How does the FV change as Magnification
goes Up??
• As magnification goes up, the size of the FV
gets smaller.
• If magnification increases 2x, the FV is divided
by 2x, or 2x smaller.
• If we switch from low power (4X) to medium
power (10X), the increase in magnification is
2.5 times (10X/4X).
• The FV under medium power will be 1600
microns (4000/2.5)
Calculating the Diameter of the Field of View
• Step 1 Calculate the Increase in Magnification.
New Objective
•
Old Objective
• Step 2 Divide the old F of V by the increase in
magnification calculated in Step 1
•
Old F of V (Microns)
•
Increase in Mag
•
To Calculate the Changing FV
• Low – 5x; Med – 10x; High – 50x
• Low FV = 5mm (1000um x 5mm= 5000um)
• Step 1 – Calculate the increase in
Magnification
• Ex – 10x/5x = 2
• Step 2 – Calculate the reduction of the FV
• 5000um/2 = 2500um