Training BMO material

General Resources

Presentation

The SD 1 Basic Microscope Optics (BMO).pptx (author: Steffen Dietzel) contains a Powerpoint presentation with slides for a lecture on Basic Microscope Optics.

Web resources

  • Basic Concepts in Optical Microscopy at the Optical Microscopy Primer at the Florida State University.
  • Zeiss brochure "Microscopy from the very beginning" with over 50 pages gives nice introduction for the beginner. The same is available in German, "Mikroskopieren von Anfang an". The link to the pdf changes every once in a while, your favorite search engine will help. You may have to create a free account on the Zeiss website to download.
  • Video Understanding the Light Microscope by Peter Evennett on BioDIP you tube channel
    The tutorial is presented in one piece, so that it can be viewed in logical order, but its components may be viewed separately by reference to the time-points quoted.
  1. Conjugate Planes in the microscope (0 – 21:00 min)
    The optical system of a microscope embodies two series of planes which are conjugate – optically linked, as images one of another. These are the ‘Field’ set of planes consisting of the object and its images, and the ‘Aperture’ set of planes which includes the light source (lamp filament) and it subsequent images. These planes are demonstrated here using a specially assembled microscope and video camera system.
     
  2. Diffraction and the microscope image (21:00 – 50:51 min)
    The most important function of a microscope is resolution – the ability to handle information about fine detail – and this is limited by the phenomenon of diffraction. Two factors are involved – the wavelength of the light, and the Numerical Aperture of the objective lens. The importance of these two factors was demonstrated in 1873 by Ernst Abbe in his ‘Theory of the Microscope’, and illustrated by a series of experiments. These experiments are demonstrated here, again using the specially-adapted microscope. Understanding this section will benefit from having followed section 1.
     
  3. Dark-field Microscopy (51:41 – 59:40 min)
    The second important function of a microscope is to provide contrast in the images of transparent and otherwise invisible objects. Dark-field (or Dark-ground) microscopy offers a simple and inexpensive method of enhancing contrast. It is explained here as a sequel to the Diffraction Experiments, using the same demonstration equipment, and should therefore be viewed following sections 1 & 2.
     
  4. Phase-contrast Microscopy (59:40 min) This section demonstrates the principle of a second system of enhancing visibility of transparent objects, Phase Contrast. Again this is demonstrated using the equipment as in earlier sections, and it should therefore be viewed following these.
    The video was enabled by German Research Foundation (DFG) core facility funding of the Biopolis Dresden Imaging Platform (BioDIP) and has been recorded by Christian Spataro. link to teaching material about basic optical concepts on BioDIP website and has been recorded by Christian Spataro.

Refraction and chromatic aberration

  • Refraction and total internal reflection can be demonstrated nicely with laser pointers and a glass cube or a large plastic cuvette filled with water. Pond water is preferable since the scattering particles allow to follow the beam path from a side view easily.
  • Refraction can also be demonstrated nicely by glueing a coin to the bottom of a cup. Stand such that you just cannot see the coin and slowly add water to the empty cup. Watch the coin appear magically. See linked video below
  • Chromatic aberration can be demonstrated with red and green laser pointers and glass prisms. If you put the arrangement on a normal table, have the students standing, so that the laser beam is not on eye level, to minimize eye injury risk.

Webresource:

The compound microscope: beam path and properties of objectives, oculars and condensers

Building a 'compound microscope' with simple magnifying lenses

What you can show/ explain with this:

  • Intermediate image
  • Roughly calculating the numerical aperture from lens diameter of the "front lens" and the "working distance"
  • Why do you need Köhler alignment? (with modification 1)
  • Chromatic aberration (with modification 2)

What you need:

  • 2 or more magnifying glasses, that can be loupes, microscope oculars, linen testers etc. A relatively strong lens will act as objective, a weaker one as eye piece.
  • a light emitting object, that can be a low power halogen bulb, an LED or - if compliant with your safety regulations - a candle
  • 2 "screens", can be normal white paper, better tracing paper, frosted glass etc.; surface has to be flat
  • a room that can at least partially be darkened

Step-by-step:

  1. Place the light emitting object on the table.
  2. Put a magnifying glass (the 'objective') and a screen on the table such that you get a focused, magnified image of your object on the screen, the future intermediate image.
    Image: intermediate image (author: Christian Feldhaus)

    The object must have a distance between 1 and 2 focal lengths from the objective to allow a magnified image. If the focal length is written on the lens holder, a ruler may help to construct the beam path.

    Demonstrate that by changing the distance between object and objective, the size as well as the position of the focused intermediate image changes (the screen must be repositioned to find the focused image).

  3. Position the second magnifying glass, the eye piece. Use either a second screen to magnify the intermediate image from the first screen as a real image on the second screen. This beam path would resemble the one for a camera behind an eye piece. Or, without a second screen, have a student using the eye piece as a magnifying lens so that s/he can see a magnified, virtual image of the intermediate image. This beam path would represent the one for the compound microscope with a human observer. You may have to move the whole construction at this point, so that the eye piece is close to the table edge, to allow for human anatomy.
  4. Now remove the first screen to demonstrate that the screen showing the intermediate image is not required to produce the image on the second screen or in the student's eye.
    Image: Final image on the 'second screen' (author: Christian Feldhaus)

Modifications:

  1. With a 3rd lens you can build a "condenser"; this requires a real sample and comparably strong light source
  2. The screen showing the final image can be replaced with a lensless digital camera; for that you can get a cheap webcam and take out the lens or use an ocular camera