Mr. T's Science Scene

Microscopes

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The compound light microscope is the instrument that you will be using to view microscopic things in class.  The compound light microscope is pictured to the left.

This is called a light microscope because light passes through an object, through lenses and up to the eye.  The eyepiece is one of the lenses and, generally, has a power of 10X.  This means that it alone will magnify an object to make it look tens times bigger than it is.  So a one foot ruler would look 10 feet (it wouldn't actually BE ten feet)!  

The nosepiece has three other lenses on it called objectives; low 4X, medium 10X, and high 40X.  So when the image passes through BOTH an objective AND the eyepiece we have to calculate TOTAL MAGNIFICATION.

Total magnigication = the eyepiece X the objective being used.  So, low power is 10X * 4X = 40X, medium power, 10X * 10X = 100X, and high power 10X * 40X = 400X.  Therefore, you microscopes can magnify an object 40 times, 100 times, or 400 times its ACTUAL size.

In addition there is something called the field of view.  The field of view (FOV) is the circle of light you see when you look into the microscope.  That is, it is everything you can see when you look into the eyepiece.  The FOV gets smaller the more magnifcation you apply or the higher the objective you are using.  This is because, while the actual, physical circle doesn't get smaller, the higher objectives get closer to the slide or object.  Therefore, the amount you can see in the FOV is less.  For all sakes and purposes the FOV shrinks from the outer ring in.  So, in order to see someting from low power to high power the object must be in the middle of your FOV when you are using low power.

LOW Power AND LARGE FOV

Under these conditions you can see a LOT of the thing you're looking at BUT you can't really see much detail.

MEDIUM Power AND SMALLER FOV

Under these conditions you can see LESS of the thing you're looking at BUT the details are easier to see.

HIGH Power AND SMALL FOV

Under these conditions you can see a small part of the thing you're looking at BUT you can see great detail.