DIY Atmospheric Optics Experiments
Demonstration experiments for atmospheric optics phenomena exists abundantly. Nonetheless, the web also proliferates myths and at times does not lay emphasis on a correct physical explanation in favour of vivid expositions of phenomena such as rainbows. Diffraction experiments (e.g. using a CD-R) or simple dispersion by a wedge of water often substitute true physical analog experiments. By contrast, there exist also beautiful and physically more faithful experiments which demonstrate for instance rainbows or halos using only very simple means. Below you will find setup schematics for a few selected long-known experiments (i.e. Florensce's rainbow) as well as new experiments such as the analog refraction halo experiments. The experiments are analyzed quantitatively in the following two publications:
- "Revisiting the round bottom flask rainbow experiment", M. Selmke and S. Selmke, submitted, available on arXiv.org
"Artificial Circumzenithal and Circumhorizontal Arcs", M. Selmke and S. Selmke, accepted at the American Journal of Physics (Am. J. Phys.), available on arXiv.org
Florence's rainbow experiment refers to the century-old model raindrop experiment in which a water-filled globe (spherical flask, i.e. a Florence flask) is illuminated by white & parallel light through a hole in a screen. Such experiments were already made in the 14th century by Theodoric of Freiburg and later by Descartes (Lés Météores, 1637). If the screen is far enough away, even Alexander's dark band may be observed. If the screen is rather close to the flask the secondary rainbow will still be inside the primary bow. Splitting of rainbow orders may be observed and is due to the glass wall.
Using a long cylindrical water-filled vessel (e.g. a vase), one may experimentally reproduce the first and second order rainbows fairly easily. In addition, Alexander's dark band may be visualized more easily by inclining the incident light direction (whereby the effective index of refraction is tuned, according to A. Bravais' theory of refraction of inclined rays).
Some special ice halos may be reproduced faithfully using water-filled glasses as well. For instance, illuminating a cylindrical water-filled glass or a V-shaped cocktail glass (Martini glass) one may reproduce the circumzenithal arc (CZA) and circumhorizontal arc (CHA) or the suncave Parry arc, see below. These experiments produce pure spectra, provided that the distance of the projection surface is far enough away from the glasses. For the CZA the illumination angle must be shallower than 32°, whereas for the CHA the illumination must be very steep from below.
Using a wine glass of the specific form as shown, and using different filling levels, one may tune an artificial projected halo from the circumzenithal arc (a circle segment around the point below the glass) to a suncave (with respect to the glass shadow) upper Parry halo. The reason being that the rays which contribute to the halo will predominantly experience a second refraction which will have the right inclination angle of the interface, being somewhere between a vertical second interface or an inclined interface as for the Martini cocktail glass. I.e., this experiment combines the CZA and the Parry halo experiments without requiring two different glasses.
A few fun experiments I have done in the past years are summarized below. On the image you see me working on the custum-built confocal microscope setup which was used in one way or another for almost all experiments I did on photothermal physics. The upper microscope objective collects the light transmitted through the sample (metallic square plate mounts the sample glass-slides on the piezo table fixed in the upper stage breadboard) and focuses it on the two photodiodes.
Optical Tweezer Setup
The following setup was designed to trap and observe microscopic particles.
Photonic Rutherford scattering
This macroscopic demonstration experiment visualizes the concept of photothermal microscopy.
The experiment and its analysis is described in detail in my article "Photonic Rutherford scattering: A classical and quantum mechanical analogy in ray and wave optics", M. Selmke and F. Cichos, American Journal of Physics, June 2013, Volume 81, Issue 6, pp. 405.
The physical principle which lies behind photothermal microscopy is a lens-like effect of the refrative index gradient that s created around a heated particle. In this macroscopic demonstration experiment, a large (1cm) metal sphere was embedded inside a transparent resin cube (5cm x 5cm x 5cm). The thermal expansion that accompanies the laser heating of the sphere by a 2W focused green laser beam into a small hole of the sphere results in a refractive index profile which has a characteristic 1/r dependence on the distance r to the center of the sphere. The same type of refractive index profile is generated in photothermal microscopy around an absorbing nanoparticle of a few nanometers size, i.e. 10^(-9) m. One may then observe either a deflection of a probing beam (red ray in the image), or see the lens-like symmetric warping around the sphere (imgage below).
Similar to the mirage effect which reflects grazing rays of light through the heated air above an asphalt surface on a hot day, the inhomogeneous optical density of the medium around the sphere acts like a lens. Thereby, a characteristic warping of anything viewed through the cube can be seen. In ths experiment, a digital camera took a picture of a square lattice pattern positioned about 20cm behind the thermal lens medium, while the camera itself was positioned about 30cm in front of the cube. The resulting images of the pattern as taken by the camera through the medium are shown below. Without heating (blue) and a few seconds after 10 minutes of heating with 2W.
The Green Flash
Inspired by the wonderful article "A simple experiment that demonstrates the green flash" by Johannes Courtial in the American Journal of Physics, this experiment recreated the elusive and rare so-called "green flash", an optical atmospheric phenomenon in which the setting sun sometimes appears green just before it disappears behind the horizon. A fishtank filled with water simulates the Rayleigh-scattering of the atmosphere while the wedge-like shape of the tank acts like a prism whereby the dispersion of light by the air of the atmosphere is also simulated. In combination, a white light source can appear green seen through the tank. The image taken seen below taken with a digital camera shows an array of high-power LEDs seen through the tank. The further right the columns get, the more greenish the left-most part of the dispersed LED spectrum becomes. The milk effectively scatters out the blue part first, leaving the green to remain.