Measuring with a Microscope YOU NEED TO KNOW THIS STUFF!

Total Magnification In order to find total magnification you need to multiple the ocular lens by the objective. Total Mag = Ocular Lens x Objective Ex.) Ocular = 10x, Objective = 40x Total Mag = 400x

Practice Ocular = 15x, Objective = 30x, Total Mag =? Ocular = ?, Objective = 40x, Total Mag = 400x Ocular = 10x, Objective = ?, Total Mag =200x Answers: 450x 10x 20x

Field of View The area of the slide that you see when you look through a microscope is called the "Field of View". If you know how wide your field of view is, you can estimate the size of things you see in the field of view.

Field of View By carefully placing a thin metric ruler on the stage (where a slide would usually go) and focusing under low power, we can measure the field of view in millimeters. 

Field of View - Conversion Now millimeters is a nice metric unit, but when we use a MICROscope we tend to use MICROmeters.  To convert from millimeters to micrometers, move the decimal 3 places to the right.  Ex.) 1.3 mm becomes 1300 micrometers Once you get the field of view, you can then estimate the size of specimens. If the FOV was 1300 micrometers and something took up ½ the FOV, we could estimate it’s size to be 650 micrometers

Converting millimeters to micrometers Multiply by 1000 Move the decimal point 3 places to the right Video

Calculating Specimen Size Because the high power objective is so close to the stage, we can't measure the width of the field of view under high power directly. We can use the width of the field of view under low power and the relationship between the low & high power  magnifications to mathematically calculate the width of the field of view under high power

Calculating Specimen Size First of all memorize this : When switching from low to high power, the area in the  field of view gets smaller & darker.  The fraction of the area seen under high power is the same as the ratio of the low & high power magnifications. For example : if the low  power objective is 20x and the high power objective is 40x, then under high power we will see 20/40 or 1/2 of the area of the slide we saw under low power

When switching from low to high power… You will see LESS of your sample…it’s like zooming in on a picture You will see MORE detail The diameter of the field of view DECREASES The field of view gets DARKER

Example #1 ocular power = 10x low power objective = 20x high power objective = 50x a) What is the highest magnification you could get using this microscope ? b) If the diameter of the low power field is 2 mm, what is the diameter of the high power field of view in mm? in micrometers ? c) If 10 cells can fit end to end in the low power field of view, how many of those cells would you see under high power ?

Answer to A ocular power = 10x low power objective = 20x high power objective = 50x a) What is the highest magnification you could get using this microscope ? 500x Ocular x high power = 10 x 50 = 500. (We can only use 2 lenses at a time, not all three.)

Answer to B b) If the diameter of the low power field is 2 mm, what is the diameter of the high power field of view in mm ? .8 mm The ratio of low to high power is 20/50. So at high power you will see 2/5 of the low power field of view (2 mm). 2/5 x 2 = 4/5 = .8 mm in micrometers ? 800 micrometers To convert mm to micrometers, move the decimal 3 places to the right (multiply by 1000). .8 mm x 1000 = 800 micrometers

Answer to C c) If 10 cells can fit end to end in the low power field of view, how many of those cells would you see under high power ? 4 cells. We can answer this question the same way we go about "b" above.  At high power we would see 2/5 of the low field.  2/5 x 10 cells = 4 cells would be seen under high power.

Example #2 ocular power = 10x low power objective = 10x high power objective = 40x The diagram shows the edge of a millimeter ruler viewed under the microscope with the lenses listed above.  The field shown is the low power field of view. a) What is the approximate width of the field of view in micrometers ? b) What would be the width of the field of view under high power ? c) If 5 cells fit across the high power field of view, what is the approximate size of each cell ?

Answer to A a) What is the approximate width of the field of view in micrometers ? 3500 - 3800 micrometers Each white space is 1 mm. We can see approximately 3 1/2 (or so) white spaces.  That is equivalent to 3.5 mm, which converts to 3500 micrometers. Any answer in the range above would be OK.

Answer to B b) What would be the width of the field of view under high power ? 875 micrometers The ratio of low to high power for this microscope is 10/40 or 1/4.  So, under high power we will see 1/4 of the low power field of view.  1/4 x 3500 micrometers (from "a" above) = 875 micrometers.

Answer to C c) If 5 cells fit across the high power field of view, what is the approximate size of each cell ? 175 micrometers If 5 cells fit in the high power field of view (which we determined is 875 micrometers in "b"), then the size of 1 cell = 875/5 = 175 micrometers.

Review Questions! 1.) How do you carry one of those things ? With two hands, one holding the arm & the other under the base.  Kinda like a football. (They're expensive, we don't want to drop 'em.) 2.) What about focusing ? How do you do that ? Start on low power and turn the COARSE FOCUS knob until the slide looks clear Once it is focused under low power you can move to the next objective and refocus if necessary Under high power only use the FINE FOCUS knob to focus in on your specimen

What Do The Parts Do? 1.the lens you look through, magnifies the specimen ocular (eyepiece) 2. supports the microscope base 3. holds objective lenses nosepiece 4. magnify the specimen (2) high power objective lens low power objective lens 5. supports upper parts of the microscope, used to carry the microscope Arm

What Do The Parts Do? 6. used to focus when using the high power objective fine focus knob 7. where the slide is placed stage 8. regulates the amount of light reaching the objective lens diaphragm 9. used to focus when using the low power objective coarse focus knob 10. provides light light source 11. hold slide in place on the stage stage clips