BioEd Online

Teacher Background

Scientific progress is often tied to the development of new tools and technologies. For example, until magnifying lenses were developed, people were able to see only as much of the world as their naked eyes would allow. The development of high-quality magnifiers and microscopes opened up the world of cells and microorganisms for exploration by generations of scientists.

Lenses are made of transparent materials and have one or two curved surfaces. They work by refraction, which means that the pathway of light is altered as it passes from one clear material, such as air, into another, such as glass. Magnifying glasses are single lenses that are convex on both sides. The compound light (or optical) microscope uses two magnifying lenses in series to make things appear much larger to the eye than would be possible with a single lens. The simplest compound microscopes consist of tubes with lenses at each end. Objects can be magnified up to 2,000 times using a high-quality compound microscope.

The invention of the transmission electron microscope (TEM) in the mid-20th century made it possible to view objects even tinier than cells, such as viruses. This type of microscope magnifies objects up to two million times by passing a beam of electrons through a very thin specimen and recording changes in the electron beam.

Zacharias Janssen is credited with developing the first compound microscope around 1595. But in 1665, scientist Robert Hooke was the first to use such an instrument to observe the division of plant tissues into tiny compartments, which he termed “cellulae,” or cells. Inspired by the work of Hooke, Anton van Leeuwenhoek, a Dutch inventor, used simple (one lens) microscopes to describe bacteria and protozoa. Van Leeuwenhoek’s well-made microscopes magnified objects more than 200 times and allowed him to make very detailed observations.

Objectives and Standards

Inquiry

• Use appropriate tools and techniques to gather, analyze, and interpret data.

Science and Technology

• Science and technology are reciprocal. Technology provides instruments and techniques that enable observations of objects and phenomena that are otherwise unobservable.

Materials and Setup

Materials per Group of Students (see Setup)

• 4 hand lenses or magnifiers

• 4 index cards (or similarly sized sections of cardstock)

• 4 pairs of scissors

• 4 pieces of newsprint, about 2 cm x 10 cm each. Select pieces that have newsprint on one side only so that print will not show through under the microscope.

• 4 pipettes or droppers

• 4 plastic cover slips

• 4 plastic or glass microscope slides

• 4 rulers (measurements in cm)

• microscope (any kind)

• other objects to observe (e.g., a leaf, coin, or dollar bill)

• set of colored pencils or markers

• sheet of wax paper (6 cm in length)

• small container of tap water

• transparent tape

Materials per Student

• copies of the student sheets

Setup

1. Make copies of the student sheets. Place materials for each group on trays in a central location.

2. Before allowing students to carry microscopes to their work areas, demonstrate how to hold a microscope by placing one hand on the microscope stand (arm) and the other under the base (foot).

3. Have students work in groups of 4.

Procedure and Extensions

Part 1. Lenses & Magnification

1. Hold up a magnifier or hand lens. Ask students, How many of you have used something like this? What can it be used for? (Lenses can be used to focus light on a single point and also are used in eyeglasses, cameras, etc.) Tell students they will be using hand lenses to make observations.

2. Distribute materials to each team of students. Direct students to observe each item on the tray using the hand lens. Ask, Did you observe anything on any of the objects that you have never noticed before? Let students report their observations informally.

3. Tell students to use a hand lens to observe the newsprint and draw what they observe on their student sheets.

4. Next, have each student make a magnifier following these steps:

   • Fold index cards in half lengthwise.

   • Cut an opening about 2 cm long and 1 cm wide in the center of the card. Open the card (opening will be 2 cm x 2 cm).

   • Cut out a 3-cm x 3-cm square of wax paper.

   • Place wax paper over the opening and secure it with tape.

5. Direct students to observe the newsprint through the wax paper window. Next, have students place a single drop of water (using a pipette or dropper) in the center of the wax paper window, and observe the newsprint through the water drop. Have students draw their observations as before.

6. Discuss students’ observations. Ask, What happened when you looked at the newsprint through wax paper? (No change.) Through the water drop? (Print was magnified.) Are there any similarities between the magnifier and the drop? (CE.g., clear, transparent, curved surface.) Help students understand that the magnifier lens and the water drop shared similar characteristics. If students need additional clarification, have them observe the newsprint through a glass or plastic slide, which is flat. The slide will not magnify (or shrink) the image, because the surface is not curved.

Part 2. Microscopes

1. Ask, What could we use to magnify the materials further? Distribute the microscopes and allow groups to examine them for a few minutes. Then ask, Where is the lens? Is there more than one lens? (Yes, in the eyepiece and at the bases of the objectives.) Ask, What do you notice about the lenses? Students should note the curvature of the lens and the “X” markings on the sides of the eyepiece and objective. Ask, What does “X” usually mean in mathematics? (Multiplication, or “times.”) Explain that the bottom lens number (on the objective in use) and the top lens number (on the eyepiece) are multiplied to indicate the total number of times a specimen is magnified when observed. For example, an eyepiece of 10x with an objective of 4x will magnify an image 40 times (10 x 4 = 40).

2. If students are not familiar with microscopes, help them locate the basic parts. For example, tell students, One part of the microscope is called the stage. It is similar to a stage for a performance. Can you find it? What about the arm? Have students use "The Compound Microscope" sheet to find the eyepiece, objectives, coarse and fine focus knobs, arm, stage, and light source of their microscopes.
   
   Many microscopes also have a condenser to intensify the light and a diaphragm aperture to adjust the amount of light passing from the light source up through the object. Encourage students to examine the microscope and propose the function of each part.

3. Finally, have students create a temporary slide called a “wet mount.” Instruct students to cut out a 1-cm x 1-cm piece of newsprint, and to put the piece of newsprint in the center of a clean microscope slide. Have students place a drop of water on the paper, cover the drop gently with a coverslip, and then place the slide on the microscope stage. If the stage has clips, have students place the clips over the slide to hold it in place.
   
   If the microscope has a light source, make sure the light is aimed up through the paper. Initially, the diaphragm should be adjusted to its largest opening. If the image is too bright (seems “washed out”) when viewed, help students reduce the amount of light by partially closing the diaphragm.
   
   Note: Low-power “dissecting-type” microscopes may not have a light source below the stage.

4. Direct students to move the lowest power objective into place above the print material (not all microscopes have multiple objectives) and to use the coarse focus knob to lower the tip of the objective until it is just above the coverslip. Students should look through the eyepiece and use the coarse focus knob (depending on the microscope) to move the objective gradually upward until the printing on the paper comes into view.
   
   Remind students that the object sample will come into focus when the objective is very close to the stage. Tell students to use the fine focus knob to sharpen the appearance of the image and use caution not to break the coverslip. Each student should have an opportunity to adjust and focus the microscope.

5. Have students draw their observations of the newsprint on the "Magnification Observations" sheet. Some students may wish to study the newsprint at a higher magnification by first centering the object in the field of view, then gently rotating the middle objective into position and adjusting the focus using the fine focus knob only.

6. Discuss students’ observations or have them answer the following questions in their science notebooks. Ask, Which tool provided the greatest magnification? What did all of the tools have in common? What were the differences between each of the tools?

Extension

Allow students to bring in their own samples of materials to observe using wet mounts under the microscope.

Handouts and Downloads