Introduction

Scientists use microscopes to study material and organisms too small to observe with the naked eye. The first microscope was built by Anton van Leeuwenhoek in the 17th century. Containing only one lens, it was called the Simple Light Microscope. In order to illuminate the object under study, it required a mirror to reflect visible light from the sun or a candle flame.

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Parts of a Compound Light Microscope

The type of microscope used today in most secondary schools, the Compound Light Microscope, uses visible light to illuminate the specimen. It contains both an ocular lens and multiple objective lenses, allowing for a significant increase in the magnification of a specimen (up to a maximum of 2000 times). A built-in electric lamp is used to illuminate the specimen from below, permitting viewing in all light conditions.

Watch this video to learn about how to use a Light Microscope:

Magnification of a Compound Light Microscope

Magnification is the ability of a lens to enlarge an image of a specimen. An image magnified 100× will appear 100 times larger than the specimen seems to the unaided eye. To calculate the total magnification of an object, multiply the ocular lens (eyepiece) magnification by the objective lens magnification.

Example

You are examining an animal cell under the microscope. If the ocular lens is 10× (sometimes also called “10 times” or “10 power”) and the objective lens is 40× (“40 times” or “40 power”),

then the total magnification = 10 × 40 = 400×.

Your virtual microscope will only go up to a maximum of 400× magnification.

You usually start examining a slide image under the lowest magnification on a microscope because this helps you locate the slide image and begin focusing. Then you gradually switch to higher magnifications to view more details.

Field Diameter

The field diameter is the diameter of the field of view when you are examining through the ocular lens. This value is useful for estimating cell size.

To determine the field diameter under low power, a millimeter ruler is placed on the microscope stage and examined through the eyepiece at the lowest magnification. The field diameter is equivalent to the number of millimeters visible on the ruler. For the microscope described above, the field diameter at 40× is 2mm.

To calculate the field diameter under higher magnification, use this simple calculation:

1) Calculate the total magnifications for each objective lens:

Total magnification using the 4× objective lens = ocular power × objective power = 10 × 4 = 40×

Total magnification using the 25× objective lens = ocular power × objective power = 10 × 25 = 250×

2) Calculate field diameter using the 25× objective lens:

$Field diameter (at 25x) = \frac{2mm}{250} = × 40 = 0.32mm$

Therefore, the field diameter using a 25× objective lens is 0.32 mm. This makes sense: you have zoomed in on the slide by examining through a higher power objective lens, so the field diameter ought to be smaller.

Example 2: If the ocular lens has a magnification of 10x and the objective lens has a magnification of 60x, what is the total magnification?

Solution:

10 x 60 = 600x

Cell Size

It is often helpful to estimate the size of the cell you are viewing through the microscope. This gives you a better understanding of the specimen you are working with and can help classify specimens.

A fast way to estimate cell size is to divide the field diameter by the number of times you think the cell would fit across the field of view.

Cell size estimate = Field diameter ÷ Number of cells that fit

This makes intuitive sense: if the field diameter were 0.20mm and you think 10 cells could fit across the field of view, then each cell would have to be about 0.020 mm in width.

Making Biological Drawings

When viewing objects under the microscope, you often need to hand draw a biological drawing to make proper observations of the specimen.
Here are some tips for drawing biological drawings by hand.

Use a pencil for drawings, including labels, names, and other information.
The title of the drawing is simply the name of the object you are examining—for example, “Human Bone Cell.”
Use unlined white paper.
Make the drawing as large as possible. It should occupy at least a third of the page and should be kept to the left of the center of the page.
Print all labels, and line them up on the right-hand side of the drawing where possible. Use a ruler to label lines.
Each cell is different. The drawing should be an outline of what you observe. Do not include additional structures just because you think you should observe them.
Do not shade or fill in areas with color. All lines should be solid and complete.
All drawings are titled. The printed and underlined title appears immediately above the drawing, against the left-hand margin. The magnification of the object drawn follows the title and is in parentheses. Example: Blood Cell (300X).

Here are examples of correct and incorrect biological drawings. Note the arrangement of the labels and the magnification of the object.

Untitled Document

Virtual Microscope

Explore topics on usage, care, terminology and then interact with a fully functional, virtual microscope from the website posted below.
This website provides a virtual microscope which is a great practice tool to prepare you for working in a science lab.

When you are ready, challenge your knowledge in the testing section to see what you have learned.

Virtual Microscope- Interactive Tool

Click on "Launch" and follow the prompts/ instructions on the screen.

ePortfolio Submission

Download the Microscope Diagram worksheet below, and complete the assignment. Save your completed work as a pdf file and add it to your ePortfolio.

Microscope Diagram.docx