Compound microscope

Class practical

Making a model microscope and measuring its magnification.

Apparatus and Materials

• For each student or group of students
• Telescope mount or metre rule with Plasticine or Blu-Tack
• Plano-convex lens, + 7D (14D if possible)
• Plano-convex lens, + 13D (20D if possible)
• Graph paper (2 mm), sheet
• Wooden strip or tongue depressor
• Retort stands and bosses, 2
• Greaseproof paper
• Microscopes, selection of

Health & Safety and Technical Notes

Read our standard health & safety guidance

Graph paper fixed to a holder or wooden strip makes a good object provided it is very well illuminated. A translucent scale illuminated from behind is better. (This could be a transparent ruler or greaseproof paper with a scale marked on it.)

Procedure

1. Set the support bar or rule at head height for comfort in manipulating the lens positions.
2. Place the object near the far end of the support rod. Fix the + 13D objective lens about 10 cm from the object.
3. Pick up a real image of the object on a piece of greaseproof paper, placed about 30 cm from the objective on the observer's side, by moving the object.
4. Place the + 7D eyepiece lens near to the image on the greaseproof paper, so that it acts as a magnifying glass for this image. When the greaseproof paper, is removed the final virtual image can be clearly focused by moving the eyepiece slightly backwards and forwards.
5. Move the lamp around to the front of the object so that it illuminates the object from the front. With both eyes wide open, look directly at the object with one eye, whilst the other eye looks through the microscope. Concentrate on the naked eye, and move the eyepiece so that the image is still clearly in focus with the image sitting on top of the object. The magnification can be measured by comparing the graph paper scales of the object and the image.
6. The student usually tries to place their eye close to the eyepiece but, in fact, as they move their head faster back from the eyepiece, their field of view increases until it reaches a maximum. Students should find the eye position for the largest field of view. This is in fact the exit pupil or eye ring and may be about 10 cm from the eyepiece.

Teaching Notes

• In a microscope, the objective lens makes a large, real image of a near object, whereas a telescope objective makes a small real image of a distant object. An eyepiece is then used as a magnifying glass to magnify this real image.
• With the distance between the lenses about 50 cm, a magnification of about 6 is obtained. A useful applet for calculating the likely magnification is provided on the Hyperphysics site.
• After students have made this model microscope, have them use a selection of professional microscopes. They should be able to adjust them so that the image is at their near point, near to the object, so that there is no eye strain. Knowledge of microscope design might help at this point.

This experiment was safety-tested in January 2007