Magnification and Resolution

Information

The reason for using a microscope is to magnify features to the point where new details can be resolved.

Magnification is the factor by which an image appears to be enlarged. It will be a whole number greater than 1 and is usually followed by an “x”, as in 10x magnification.

When you look through microscope eyepieces, you are seeing a virtual image because in reality, what you are looking at is not as large as it appears through the eyepieces, and because there can be some distortion of the image.

Resolution is the shortest distance between two points that can still be visually distinguished as separate. The resolution of a typical unaided human eye is about 200 µm. Using a microscope decreases the resolution to distances as short as 0.2 µm. Resolution is a property of the eye.

Resolving power is the ability of a lens to show two adjacent objects as discrete. Resolving power is a property of a lens.

Each lens in a microscope has a numerical aperture, or NA, value. This has to do with the angles of light that enter and exit a lens. Its applications are beyond the scope of this lab, but numerical aperture does influence the resolution possible with a particular lens, and so the NA value for the lens is usually printed on each objective. It will be a number less than 1.0, and you can ignore it for our purposes.

Each lens in a microscope also has a magnifying factor. This is the degree to which that lens magnifies an image. It will be a number larger than 1.0. For instance a 10x objective magnifies the image ten-fold. The magnifying factor for each objective always printed on it, and the magnifying factor for each eyepiece is usually printed on it. (If the eyepiece is missing a printed magnifying factor, you can usually assume it is 10x.)

The total magnification for any image viewed under a compound microscope is calculated by using the formula:

Total Magnification = eyepiece magnifying factor * objective magnifying factor

So, each time you switch objectives, you change the total magnification. Total magnification does not have units, but is usually indicated by an “x”, as in “total magnification  = 100x.”

 

Lab 4 Exercises 4.2

There is a compound microscope for each student. Carry out the activities listed below and fill in the blanks as you do so.

  1. Write down the magnification factor for the eyepiece lenses on the microscope in front of you.                       
  2. Using the microscope in front of you, write out all the words and numbers written on each objective on your microscope. There are probably three objectives, but some microscopes might have four. Start with the smallest objective and move through them in order of increasing size
    Objective one:

                                                                 

    Objective two:

                                                                 

    Objective three:

                                                               

    Objective four:

                                                                

  3. In the above list, for each objective, circle just the magnification factor for that objective. Remember, the magnifying factor is a whole number, and differs for each different objective.
  4. Write down the total magnification (eyepiece magnifying factor * objective magnifying factor) when using each objective on the microscope in front of you.
    Objective one:

                                                                      

    Objective two:

                                                                      

    Objective three:

                                                                    

    Objective four:

                                                                

  5. If you observed two features on a slide with your naked eye that were 0.5 mm, how far apart would they appear to be if you observed them with the microscope in front of you, using the second objective?