How big is that object in the microscope?How big is that object in the microscope?

Calculating total Calculating total magnificationmagnification

REVIEWREVIEWIf two lenses are always magnifying the If two lenses are always magnifying the

specimen, how do you figure out the total specimen, how do you figure out the total magnification being used ? magnification being used ?

Total Magnification = ocular x objectiveTotal Magnification = ocular x objective

= 10 x 4 (low power)= 10 x 4 (low power)

= 40 (low power)= 40 (low power)

Try this one!If the ocular is 10x and the low power If the ocular is 10x and the low power

objective is 40x what is the total objective is 40x what is the total magnification?magnification?

Then the total magnification under low power Then the total magnification under low power is 10 x 40 = 400xis 10 x 40 = 400x

Learning GoalsLearning GoalsLearn to calibrate a Learn to calibrate a

microscope.microscope.Use your microscope Use your microscope

calibration to estimate the calibration to estimate the size of objects. size of objects.

Compound light Compound light microscopemicroscope

The microscopes at The microscopes at Auburn Drive have three Auburn Drive have three objectives. objectives. Low Power (4x)Low Power (4x) Medium Power (10x)Medium Power (10x) High Power (40x)High Power (40x)

Field of ViewWhen you look into a microscope, the “field of

view” is the visible circular area.If you change the objective, you change the

magnification of the specimen.By knowing the size of the field of view (diameter),

you can measure the size of objects in the microscope.

Because the size of objects in the field of view is different at each magnification you have to calculate the diameters of the fields of view at each magnification.

This process is called “calibrating your microscope”

Calibration Steps1. Place a ruler onto the microscope stage2. Record the diameter of the field of view in

mm3. Change the objective to medium power, and

record the measurement to the nearest mm4. Increase the magnification again. Notice that

when you use the 40x magnification, the scale on the ruler is no longer visible. So how can we estimate the size of the field of view at 40x?

Calibrating with a rulerCalibrating with a ruler

My Calibration ChartObjectiv

eMeasurem

ent(mm)

4 x10 x40 x

Micrometers

(µm)

Estimating Specimen Estimating Specimen SizeSizeThe area of the slide that you see when The area of the slide that you see when

you look through a microscope is called the you look through a microscope is called the "Field of View".  "Field of View". 

If you know how wide your field of view is, If you know how wide your field of view is, you can estimate the size of things you see you can estimate the size of things you see in the field of view. in the field of view.

Figuring out the width of the field of view Figuring out the width of the field of view is easy - all you need is a thin metric ruler.is easy - all you need is a thin metric ruler.

IMPORTANT FORMULA!Object Size = field of view (in mm) • 1000 number of “fits”

Object Size = ? µm

** Remember that the field of view changes with each objective! If you are using objective 10 you must be sure to use a field of view of 2 mm.

Unit ConversionsUnit ConversionsMillimeters is a nice metric unit, but when we Millimeters is a nice metric unit, but when we

use a use a MICROMICROscope we tend to use scope we tend to use MICROMICROmeters.  meters. 

Remember that there are 1000 micrometers Remember that there are 1000 micrometers ((µm) in one millimeter. µm) in one millimeter.

1000 1000 µm = 1 micron µm = 1 micron

To convert from millimeters to micrometers, To convert from millimeters to micrometers, move the decimal 3 places to the right.  move the decimal 3 places to the right. 

Let’s try it out!