Calculating the transmission of "optically clear" microscope slides

I blogged recently about turning your smartphone into a colorimeter to determine the concentration of an unknown sample using the excellent Physics Toolbox Suite from and that got me thinking about other uses for a smartphone to measure incident light.

There used to a physics practical that A-level (post 16) students carried out with microscope slides. Measure the light transmitted by a range of stacked slides, plot a graph and determine the number of slides in a "bundle" without counting them. All standard stuff - the problem with that practical was that it was really "physics for the sake of it" - the practical itself had no real intrinsic value. But what if we could measure something that we could look up on a data sheet? Could we use the sensor as a portable QA device?

According to the data sheet for my microscope slides the optical transmission of a single 1mm slide was 91.2% ±1% - could my students quality assure that figure?

In a similar manner to the post on colorimeters (here) microscope slides were progressively stacked over the light sensor and Physics Tooolbox Suite was used to measure the light transmitted by the stack. A graph was plotted from 0 to 15 slides and Excel was used to calculate a 3rd order polynomial line of best fit (R2 = 0.9998 - so a pretty excellent fit). From this, the equation of the line was used to calculate the transmission of 1 slide - in this case 2646.435 lux. Using the fitted value for 0 slides (2884.8) the ratio T1/T0 was calculated - 2646.435/2884.8 = 91.74%

This is well within the ±1% of 91.2% - my slides are in tolerance and pass the QA test.

Feedback from the students was positive - they particularly liked the fact that they were solving a "real" problem, one that felt authentic to them and not contrived by the teacher to demonstrate a technique. It was particularly satisfying to see a range of mobile phones being used for this experiment - providing 1-to-1 access to sensors that would have been impossible had we used the equipment available in the school laboratory.

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