Microscopes and lenses

Although cells vary in size, they’re generally quite small. For instance, the diameter of a typical human red blood cell is about eight micrometers (0.008 millimeters). To give you some context, the head of a pin of is about one millimeter in diameter, so about 125 red blood cells could be lined up in a row across the head of a pin. With a few exceptions, individual cells cannot be seen with the naked eye, so scientists must instead use microscopes (micro- = “small”; -scope = “to look at”) to study them. A microscope is an instrument that magnifies objects otherwise too small to be seen, producing an image in which the object appears larger. Most photographs of cells are taken using a microscope, and these pictures can also be called micrographs.

From the definition above, it might sound like a microscope is just a kind of magnifying glass. In fact, magnifying glasses do qualify as microscopes; since they have just one lens, they are called simple microscopes. The fancier instruments that we typically think of as microscopes are compound microscopes, meaning that they have multiple lenses. Because of the way these lenses are arranged, they can bend light to produce a much more magnified image than that of a magnifying glass.

What separates a basic microscope from a powerful machine used in a research lab? Two parameters are especially important in microscopy: magnification and resolution.

Magnification is a measure of how much larger a microscope (or set of lenses within a microscope) causes an object to appear. For instance, the light microscopes typically used in high schools and colleges magnify up to about 400 times actual size. So, something that was 1 mm wide in real life would be 400 mm wide in the microscope image.

The resolution of a microscope or lens is the smallest distance by which two points can be separated and still be distinguished as separate objects. The smaller this value, the higher the resolving power of the microscope and the better the clarity and detail of the image. If two bacterial cells were very close together on a slide, they might look like a single, blurry dot on a microscope with low resolving power, but could be told apart as separate on a microscope with high resolving power.

Both magnification and resolution are important if you want a clear picture of something very tiny. For example, if a microscope has high magnification but low resolution, all you’ll get is a bigger version of a blurry image. Different types of microscopes differ in their magnification and resolution.

Light microscopes

Most student microscopes are classified as light microscopes. In a light microscope, visible light passes through the specimen (the biological sample you are looking at) and is bent through the lens system, allowing the user to see a magnified image. Student lab microscopes tend to be brightfield microscopes, meaning that visible light is passed through the sample and used to form an image directly, without any modifications. A benefit of light microscopy is that it can often be performed on living cells, so it’s possible to watch cells carrying out their normal behaviors (e.g., migrating or dividing) under the microscope.

MICROSCOPE PARTS & SPECIFICATIONS

Historians credit the invention of the compound microscope to the Dutch spectacle maker, Zacharias Janssen, around the year 1590. The compound microscope uses lenses and light to enlarge the image and is also called an optical or light microscope. The simplest optical microscope is the magnifying glass and is good to about ten times (10x) magnification. The compound microscope has two systems of lenses for greater magnification:

1. Ocular eyepiece lens to look through. 2. Objective lens, closest to the object.

Functions of each part of the microscope.

1. Eyepiece Lens: the lens at the top that you look through, usually 10x or 15x power. 2. Tube: Connects the eyepiece to the objective lenses. 3. Arm: Supports the tube and connects it to the base. ALWAYS hold by arm and base 4. Base: The bottom of the microscope, used for support. 5. Illuminator: A steady light source (110 volts). 6. Stage: The flat platform where you place your slides. Stage clips hold the slides in place. If your microscope has a mechanical stage, you will be able to move the slide around by turning two knobs. One moves it left and right, the other moves it up and down. 7. Revolving Nosepiece or Turret: This is the part that holds two or more objective lenses and can be rotated to easily change power. 8. Objective Lenses: Usually you will find 3 objective lenses on a microscope. They almost always consist of 4x, 10x, and 40x powers. When coupled with a 10x eyepiece lens, total magnification is 40x (4x times 10x), 100x (10x times 10x), 400x (40x times 10x). The shortest lens is the lowest power, the longest one is the lens with the greatest power. 9. Diaphragm or Iris: Many microscopes have a rotating disk under the stage. This diaphragm has different sized holes and is used to vary the intensity and size of the cone of light that is projected upward into the slide. There is no set rule regarding which setting to use for a particular power. Rather, the setting is a function of the transparency of the specimen, the degree of contrast you desire and the particular objective lens in use.

How to Focus Your Microscope: The proper way to focus a microscope is to start with the lowest power objective lens first and while looking from the side, crank the lens down as close to the specimen as possible without touching it. Now, look through the eyepiece lens and focus upward only until the image is sharp. If you can't get it in focus, repeat the process again. Once the image is sharp with the low power lens, you should be able to simply click in the next power lens and do minor adjustments with the focus knob. If your microscope has a fine focus adjustment, turning it a bit should be all that's necessary. Continue with subsequent objective lenses and fine focus each time.