A microscope is an instrument that magnifies an object so that it may be seen by the observer. Because cells are usually too small to see with the naked eye, a microscope is an essential tool in the field of biology. In addition to magnification, microscopes also provide resolution, which is the ability to distinguish two nearby objects as separate. A combination of magnification and resolution is necessary to clearly view specimens under the microscope. In this lab, parts of the microscope will be reviewed. Students will learn proper use and care of the microscope and observe samples from pond water.

“Parts of a microscope” by Melissa Hardy, Salt Lake Community College is licensed under CC BY-NC 4.0 / A derivative from the original work

A microscope magnifies the image of an object through a series of lenses. The condenser lens focuses the light from the microscope’s lamp onto the specimen. The light then passes through the object and is refracted by the objective lens. The objective lens is the more powerful lens of a microscope and is closest to the object. The light then travels to the ocular lens , which focuses the image onto the user’s eye. Usually, the power of the ocular lens is fixed for a given microscope.

Magnification

Your microscope has 4 objective lenses. On most of the student microscopes, these are 10x, 40x, 63x, and 100x (Oil Immersion). The number indicates the magnification, e.g., the 10x objective will magnify an object by 10 times. In addition to the objective lenses, the ocular lens (eyepiece) has a magnification.

The total magnification is determined by multiplying the magnification of the ocular and objective lenses.  For instance, if the ocular lens has a magnification of 20x and the objective lens being used has a magnification of 4x, the total magnification will be 80x.

Resolution

The number after the magnification on an objective lens indicates the numerical aperture. This is a measure of the light-gathering ability of the objective lens. It is closely related to resolution. The higher the numerical aperture, the higher the resolution. A higher numerical aperture also means a shallower depth of field. It also requires a shorter working distance (i.e., you must get the objective lens closer to the specimen).

Resolution is also dependent upon the wavelength of light used to illuminate the specimen. Recall that the visible light spectrum (for humans) ranges from about 400 nm to 700 nm.

The visible light spectrum shown from shortest wavelength to longest wavelength. Humans can detect electromagnetic radiation as visible light between about 400 and 700 nanometers. (Figure is in the public domain).