Outcome

Distinguish between magnification and resolution power of a microscope.
Use the compound light microscope to observe cells.
Demonstrate the ability to prepare a wet mount of a specimen, including staining techniques.

Materials

Compound light microscope, iodine dye, dropper pipette, silk threads, microscope slides cover slips, newspaper, scissors, and onion.

Prepared slides

Paramecium (unicellular organism) and bacteria (prokaryotic organisms).

Teaching suggestions

Demonstration, short lecture and group work

Starter Activity (5 minutes)

The teacher may introduce the lesson by posing focus questions as like:-

Can we see tiny organisms like bacteria, protists etc. with our naked eye?
How can we observe very small objects?
Can you mention a powerful tool that enables us to observe very tiny objects and cells?

The teacher may expect responses from students that may elaborate the concept of microscopy.

The basic concept to be discussed here is the role of microscope in viewing tiny organisms that could not be viewed with naked eyes. The microscope's invention and improvement in the seventeenth century led to the discovery and study of cells. In 1665, Robert Hooke, after studying a cork cell under light microscope, introduced the term cell. A microscope enables biologists to investigate living things and objects that are too small to be seen with the naked eye. The microscope is able to magnify these tiny specimens by means of lenses located in the eyepiece and objectives. The microscope is also capable of revealing fine details. This ability to reveal fine entity is known as resolving power.

Specimens that are viewed under the microscope are mounted on one of two types of glass slides. Prepared slides are slides that are for permanent use. Mount slides are for temporary use. Most of the slides that we use in biology will be wet mounts.

Main activity as Exploration(25 minutes)

Demonstration of objects under a compound light microscope is required. Charts and diagrams that clarify those objects to be observed under the microscope should be posted in the corner of the laboratory room. This enables the students to identify the parts and compare their observations with actual diagram or picture of the objects or specimens.

The teacher is expected to demonstrate the cells under a microscope so that students can get the opportunity to observe them.

The students can begin working with familiar objects (e.g. a printed letter such as “e” or “f” to illustrate inversion properties of the microscope), and work up to easily viewed objects (e.g. permanent slides selected by the teacher or teacher prepared slides), and then to a variety of objects they select and prepare by themselves. A simple and familiar cell for microscopic observation might be onion epidermal cell. Therefore, it is highly recommended to observe onion epidermal cells using compound microscope.

The teacher should guide students how they can properly use a microscope. Therefore, in this investigation, the students should get the opportunity to:

Practice proper handling of the compound microscope.
List and describe parts of the compound microscope.
Acquire skills in using the microscope by carefully following all directions.
Observe objects under low and high power magnifications.
Learn what the position of an object is when viewed through the microscope in relation to its position on the microscope stage.
Practice how to prepare a wet mount.

Using compound Microscope

Turn on the lamp.
Turn and adjust the lower power objective lens so that it is directly over the stage opening.
Look through the eyepiece of the microscope. A circle of bright light should now be visible. This is called the field of view.
Adjust the diaphragm to make the field of view as bright as possible.
Look to the side of the microscope. Slowly turn the coarse adjustment wheel back and forth. Note the movement of the stage in relation to the primary objective.
In which direction does the stage move as you turn the coarse adjustment wheel towards you?
In which direction does the stage move as you turn the coarse adjustment wheel away from you?

Looking at an Object under the Microscope

Adjust the lower objective lens into viewing position.

NOTE. Always locate an object first with lower magnification even if a higher magnification is desired.

Adjust the diaphragm for the best light.
Place a prepared slide of silk fibers on the stage of your microscope. Position the slide on the stage so that the threads cross directly over the center of the stage opening.
Secure the slide in place with the stage clips.
While looking through the eyepiece, slowly turn the coarse adjustment wheel until the silk fibers become visible. Bring the silk fibers into sharp focus by turning the fine adjustment wheel.

Increasing the Magnification of the Microscope

Any object to be viewed under low or high power magnification is always located first with the primary objective. Move the primary objective out of viewing position and adjust the low power objective into place.
Look through the eyepiece. The silk fibers should be visible. If the image needs to be focused, use only the fine adjustment wheel to sharpen the focus.
For high power, again move the (low power) objective out of the viewing position and adjust the high power objective into place.

CAUTION:Never use the coarse adjustment wheel for focusing with high power. Damage to the lens and slide may result if the coarse adjustment wheel is used.

Students should be able to practice wet-mount and staining techniques, focusing techniques, and effective presentation of their observations to others. Diagrams of the specimens viewed should include field of view and magnification, should accurately show what they have seen.

Students should view a variety of plant and animal cells under the microscope, and use diagrams, models, or charts to illustrate and distinguish the structures of plant and animal cells.

Making Wet Mount

Use a pair of scissors to cut out the letter “e” from a piece of newspaper. Cut out small letter “e”. Position the letter “e” upright on the center of a clean microscope or you can write a letter on a piece of paper.
Use a dropper pipette to place one drop of water on the cut piece of newspaper.
Hold a clean cover slip by the edges and slowly lower the cover slip onto the wet newspaper. Slowly lowering the cover slip prevents air bubbles from being trapped between the slide and the cover slip. The type of slide you have just made is called a wet mount.
Observe the letter “e” using low power on your microscope. Focus the “e” with the fine adjustment wheel.

Question

What is the position of the “e” viewed with the microscope compared to
Its position on the stage?
While looking through the eyepiece, move the slide slowly from left to right,
In what position does the letter move as seen through the microscope?
While looking through the eyepiece, move the slide slowly from right to left,
In what direction does the letter move as seen through the microscope?
While looking through the eyepiece, move the slide slowly toward you,
In what direction does the letter move as seen through the microscope?

The following basic points should be noted.

Visible light is focused on a specimen with a condenser lens.
Light passing through the specimen is refracted with an objective lensand an ocular lens. The specimen's image is thus magnified and inverted for the observer.

Two important concepts in microscopy are magnificationand resolving power.

Magnificationrefers tohow much larger an object could be when compared to its real size.
Resolving powerrefers tominimum distance between two points that can still be distinguished as two separate points. Resolution is usually expressed in terms of the minimum distance observable between two objects. Resolution of a light microscope is limited by the wavelength of visible light. Highest magnification in a light microscope with maximum resolution is about 2000 times. The Resolution power of light microscope is limited by the physical characteristics of light (At powers of magnification beyond about 2000X, the image of the specimen becomes blurry.)

By the early 1900s, optics in light microscopes was good enough to achieve the best resolution, so improvements since then have focused on improving contrast. In the 1950s, researchers began to use the electron microscopewhich far surpassed the resolving power of the light microscope.

Electron Microscopes

They are used to view extremely small objects.
They use beam of electrons rather than light and produces an enlarged image.
Electron microscopes have high magnification and resolution power than light microscopes.
There are different types of electron microscopes; transmission electron microscopes, and scanning electron microscopes.
Let the students explain the difference between transmission electron microscope and scanning electron microscope by searching from library books.

Resolving power is inversely related to wavelength. Modern electron microscopes have a practical resolving power of about 2 nm. Enhanced resolution and magnification allowed researchers to clearly identify sub cellular organelles and to study cell ultra structure using electron microscopes.

Concluding activity

The teacher is supposed to summarize the lesson.

The students can match the following listed terms with their appropriate part of a microscope indicated with numbers and describe the use of each part.

Eyepiece or ocular, Body tube , Fine adjustment knob , Nosepiece, High power objective lens , Low power objective, Diaphragm , Mirror , Base , Coarse adjustment, Arm , Stage clips, Inclination joint, Middle objective.

Fig.1.1: Parts of Compound Microscope.

Assessment Questions

Let the students draw a diagram of a single onion cell as it appears under high power objective from their practical observations.
Distinguish between light and electron microscope.
What is the total magnification power of a microscope with a 10 x ocular lens and a 25× objective lens?
What is the difference between magnification and resolution power of a microscope?
Which knob is used to get a clear image when focusing on low power objective lens?
Ask students to identify parts of a microscope and describe the use of each.