Microscopes and Magnification
One of the tools that biologist use is the microscope. It function is to view a world that the normal eye cannot see. Stop for a moment and Imagine the first time some one saw moving matter under the lens. If you were that person, would you be afraid or full of wonder? This is a loaded question simply because now it is common to see magnified images of virus and bacteria in TV commercials. Perhaps the novelty is gone but the usage of the microscope remains a basic skill of any one studying biology.
Concept One: Power
“You’ve got the power…”, nope-wrong idea, but here is the scope. Power means the ability to make something bigger. End of story. The larger the power, the smaller the object you can view. The smaller the power, then you are already looking at organisms that are relatively large. The compound microscope enlarges images through a series of lens and mirrors. By illuminating the image, a reflection of that image passes through the lens to the eye. Starting with the eye is the ocular lens. This lens is used for viewing and is the lens that is adjusted to focus on an object. The objective lens is next to the object and remains stationary while viewing. So how much bigger is the object? Well if you take the ocular lens magnification (on the side of the lens) and multiply that number times the objective lens magnification ( on the side of the lens) you have the total magnification or power that the microscope enlarges the object. Here is the catch. Magnification in this context, is how many times larger is the object your are looking at. For example, at low power on a microscope, the ocular lens is let’s say (10x). This means that the lens will make the actual object 10 times larger. The objective lens is perhaps 5X. So the actual object will now appear ( 5×10) or 50 times larger than it is in real life.
So what!
Well if we are looking at objects under the microscope, we have to realize that the tool, the microscope, is altering what is actualy occurring under the lens. All that we view is now larger than life and just to keep things interesting, all images are inverted and upside down. So if you are looking at an organism swimming to the left of your field of view, it is actually upside down and swimming the opposite way. This may be helpful to remember the next time you are trying to draw a moving organism.
So how do we draw these critters? Well let’s use the worksheet to explain…
Prior to answering questions, lets come up with a strategy to organize our work. This will make life and marking a lot easier.
How to lay out your work: (save this as a template!)
Record facts Do work or calculations Answer
here here
Write formulas
here
Problems..
1.
Record facts Do work or calculations Answer
three lens 5 x 2 = 10 low power
2x 5 x 20 = 100 medium power
20x 5 x 200 = 1000 high power
200x
ocular=5x
Write formulas
ocular times objective = total power
2.
Record facts Do work or calculations Answer
field of diameter = 10 mm 10 mm/ 4 = 2.5 mm
Write formulas
field of diameter/ # of object = actual size
3.
Record facts Do work or calculations Answer
none use micrometer slide to measure
field of view
remember on average
low power field diameter
Write formulas
4.
Record facts Do work or calculations Answer
high power diameter .45 mm / 20 seconds = mm/sec
equals .45 mm always include units
Write formulas
field diameter (distance)
divided by time equals speed
5.
the source of light is actually quite good and can be focused to level of magnification, as you increase power, you need more light. The amount of light can be adjusted by using the diaphragm.
the organism may or may not be dead. If alive and you are using a slide with a concave depression in the slide, the critter is going to move up and down through the water. So you need to adjust focus as the critter moves. Correct answer
Usually the microscope remains at the same level of magnification. This may change not with time but with who is looking down the lens. Always remember to start with low power, switch to the next objective power and slowly adjust the fine tuning knob.
6.
Trick question…it depends on the type and size of cells. At low power, you would be able to see the largest field of view, so more distance. This is the obvious answer. However you may not be able to focus on small images and so you may have to move up to the next power to see these images. Don’t worry I don’t like trick questions either.
Questions
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