Workshop on optical experiments
Saturday 19th November 2016
Dennis Fullwood introduced this afternoon workshop, held in the PA 135 Meeting Room in the Natural History Museum and attended by a good crowd of members.
Dennis brought along lots of items, including prisms, diffraction gratings, laser pointers, calcite crystals and an optical bench that he had made from aluminium sections.
Jacky McPherson with the optical bench
David Linstead (left), John Tolliday and Dennis Fullwood
Fleur White (left) and Jacky McPherson
Dennis also set up an experiment to measure the wavelength of light, using 2 metre rules, a diffraction grating, a pencil, an LED torch and some coloured filters.
Mary Morris, Pam H., Chris Thomas and Dannae Haskath measuring the wavelength of light
Tony Dutton showed 2 objectives, a Watson ×40 and a Zeiss ×100, from which the late Don Thomson had cut away 90° of the metalwork with a fine jewellers saw, leaving the lenses intact.
Watson ×40 Holos objective
Tony used an LED torch shining into the rear of the ×100 objective to illuminate the edges of the lenses and make them glow.
Zeiss ×100 apochromatic oil-immersion objective
Anne Dawson, one of Don Thomson’s daughters, has very kindly donated the two cut-away objectives to the Club.
Tony also showed an intact objective in which parts of the cement between lenses had become opaque. Shining a laser pointer through the rear objective projected an image onto a white card that showed the opaque areas as shadows.
Pam H. had a number of demonstrations related to microscopes, resolution and numerical aperture.
One of Pam’s exhibits showed how the two lenses in a compound microscope work, with the first lens (the objective) forming a magnified aerial image that is then further magnified by the second lens (the eyepiece).
Two lenses forming a compound microscope
Pam also brought some apertometers, used for measuring the numerical aperture (NA) of objectives. One was a swinging apertometer that Phil Robinson had made, with the body tube of a Watson Kima microscope mounted on a semicircular board. With an objective mounted (but no eyepiece), you turn the microscope until the light source disappears from view and note the angle engraved on the board. Then turn the microscope in the opposite direction until the light source disappears again, and note the angle. Knowing the angle between the two markings (α), you can calculate the NA from the formula:
NA = n sin α
where n is 1, the refractive index of air.
One of Pam’s references was Andrew D. Booth (1965) The Measurement of Numerical Aperture.
Pam also brought 3 Cheshire’s apertometers that are used on the stage of a microscope. You place a 25 mm cube on top of the apertometer and focus on its top surface. Then remove the cube and the eyepiece, look through the eye tube and you can read off the numerical aperture on the printed scale. A phase telescope makes it easier to read the scale.
Cheshire’s apertometer on a Lomo microscope
Two Cheshire’s apertometers
In another demonstration related to NA, Pam provided a laser pointer and 3 diffraction gratings (2,400, 7,500 and 15,000 lines per inch) to show that at higher densities the projected spots of light are further apart.
Diffraction by fine details
For bright-field imaging, the cone of light (described by the NA) entering the objective must include the 0 and 1 orders of diffracted light, and the resolution is described by:
d = nλ/2NA
where λ is the wavelength of light and n is the refractive index of the medium (1 for air)
Pam also showed sheets of red and green fluorescent plastic, with notes explaining why if light passes through the green sheet first then both sheets fluoresce, but if light passes through the red sheet first then the green sheet does not fluoresce.
Paul Smith showed 3 spectroscopes, all of which consist of a tube, a slit aperture and a diffraction grating. Paul had made one with a cardboard tube, and the others were commercial, one cheap plastic and the other by Ealing.
Paul Smith with his home-made spectroscope
Paul also showed a new afocal adapter he had made for use with his Canon EOS M3 mirrorless digital camera. The camera was fitted with an EOS/EOS M adapter, a 50 mm lens, a 52–28 mm step-down adapter, a 28–30 mm step-up adapter, and a short length of aluminium tube. The 52–28 adapter was fitted with 2 small knobs to make it easy to fit and remove. The combination of step-up and step-down adapters provided a ledge to prevent the microscope eyepiece touching the camera lens.
Canon EOS M3 equipped for afocal photomicrography
Stephen Parker brought along 4 items. Two of them were Rowland replica diffraction gratings that were used in spectrometers. Another was a box containing 2 circular diffraction gratings, the purpose of which was unknown. The fourth was a metal and glass contraption, also of unknown function, marked “A. A. PEARSON, LEEDS”. The glass disc was marked with dozens of small optical targets and graticules arranged in a spiral. Turning a knob on a thread rotated the disc and also moved it slowly from side to side, so that each engraving in turn lined up with a small rectangular aperture.
Laser pointer shining through a diffraction grating [by Chris Thomas]
Kit Brownlee showed an Optics box from the University of Oxford smart box series, aimed mainly at children and containing a wide assortment of items.
Kit Brownlee’s exhibit
Norman Chapman with Kit Brownlee’s exhibit
Alan Wood used his laptop computer and the Museum’s free WiFi to show the Club’s new group on Facebook, where members can discuss interesting specimens, equipment and techniques, and ask for help with using equipment or improving photographs.
QuekettMicro Facebook group
Chris Thomas posted photographs while the workshop was in progress.
Report and photographs by Alan Wood