Microscope Activities, 10: True Magnification

In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.

EXPERIMENT 10: True Magnification

Goal

To determine the true magnification of a microscope system.

Level

Basic

Materials Needed

Stage micrometer, metric rule, sheet of blank white paper

Procedure

1. Using each objective in turn, focus on a stage micrometer.
2. Turn the light up to maximum intensity, and open the aperture diaphragm.
3. In a darkened room, place a blank piece of paper right on top of the eyepiece, and notice that the disc of light on the paper has a diameter of about 9 mm. Now slowly raise the paper, and note that the diameter of the lighted disc becomes smaller and smaller, until, at about 10 mm above the eyepiece, the light comes to a focus as a point of light—this is the eyepoint, which you measured in Experiment 6. Next, continue to raise the paper, and notice that the image of the stage micrometer now becomes larger and larger the farther away you raise the paper above the eyepoint.
4. Hold the paper 25 cm (250 mm; 10 inches) above the eyepoint, and measure the length of the projected image of the 1 mm long stage micrometer using your metric rule. If there is not enough light at 10 inches, hold the paper at 5 inches (125 mm; 12.5 cm) above the eyepoint, measure the projected scale using the metric rule, and multiply by 2; Figure 10-1 is a view of this step using the 10X objective, and holding the piece of paper at 5 inches above the eyepoint.

Figure 10-2 is a close up view, with the paper being held five inches away from the eyepoint, showing that the 1 mm scale of the stage micrometer measures 50 mm long, or a magnification of 50X; multiplying by two, we obtain a true magnification of 100X, which is what we expect using a 10X objective and 10X eyepiece.

Discussion

In the past, magnifications indicated on objectives were nominal values; an objective engraved 10X could be 9X, 10X, 11.1X, etc. With the advent of computer-assisted design, engraved magnifications tend to be their actual magnification.

Older objectives were marked with their focal length instead of their magnification; their true magnification may be determined as described here. Also, zoom eyepieces and intermediate magnifying systems affect final magnification, and their true magnification may also be determined this way.

The reason this method works is that the standard viewing distance is taken to be 250 mm (10 inches; 25 cm) by the objective lens designers when calculating the lens formulas. Pocket magnifiers, reading glasses, and all correction lenses are referred to this standard distance.

You saw how the magnification of the final image may be changed by changing the projection distance. This is the principle of the bellows photomicrographic camera. In the metallurgical field, certain fixed magnifications (50X, 100X, 200X, 500X, 1000X) are standard for publication. Before a photomicrograph is made, a stage micrometer is placed on the stage, focused, and the bellows length (projection distance) is adjusted, while holding a metric rule on the ground glass focusing screen, until the desired true magnification is obtained.

Finally, it is vital to realize that when your own eye lens is located at the eyepiece eyepoint, the final virtual image you see is located 250 mm (10 inches) away. Look at any sample in your microscope and try to say exactly where the image is located; it will be 250 mm away, assuming your own eyes are focused at infinity.

Task

Determine the true magnification of your 10X or 40X objective using the method described in the Procedure section.