Determine Cell Size with a Slide Projector

This is one of my favorite lab activities. Onion cells are visualized using a slide projector. Using an internal reference mark, the students can calculate the actual size of onion cells. It does not require the use of microscopic equipment and can be conducted in the normal classroom (lab not required).


Slide Projector (an overhead projector will not work!), slide frames with glass, onion, ruler, marker.


  1. Cut out a piece of onion skin of about 1 cm² size. The onion skin is the membrane which can be found between the layers of the onion.
  2. Using a ruler, draw a 1 cm long line on the inside glass of the projector slide. This is our internal reference.
  3. Place the onion skin flat into the slide.
  4. Project the skin on the wall and measure the length of 10 cells using a ruler. The students then calculate an average. Also measure the length of the 1cm reference line.
  5. We now can calculate the magnification by dividing the length of the projected line by the original size: Magnification = length of projected line / length of original line E.g. if the projected line is 30 cm, then the magnification is 30 cm / 1 cm = 30x.
  6. Now divide the size of the projected cell by the magnification to obtain the real cell size.