This is a simple example of light scattering effects studied with SPRAY. A parallel light beam (sun light) is directed into a region with very small particles - these particles represent density fluctuations in the earth's atmosphere which lead to dipole scattering. SPRAY determines how much light is received behind the scatterers and the fraction of rays scattered to the side. In both cases not only the spectra but also the corresponding colors are computed.
The required scattering properties are computed using an external small program that performs Mie computations (light scattering by spheres). Although water spheres have been used to generate the data, the results are quite universal since the spheres are so small that they have almost only dipole properties. The scattering characteristics (viewed with the tool view_rt.exe) are the following:
Setting the volume fraction of the scattering particles to 0.0001 one gets the following probabilities for scattering (red curve) and absorption (blue) per cm distance:
Clearly blue light (wavenumber 25000 1/cm, or 400 nm wavelength) is scattered more efficiently than red light (700 nm wavelength, corresponding to about 14000 1/cm). The consequences for the propagation of light through a volume filled with dipoles are investigated in a simple SPRAY configuration described in the following.
The parallel light beam created by a circular light source (left object in the picture below) is directed to a large sphere filled with the scattering dipoles (volume fraction: 0.0001). The rays transmitted straightahead through the scattering volume are recorded by the rectangular detector displayed to the right. The large detector in front is used to detect the radiation scattered sideways:
Testing the configuration with some rays (at 500 nm wavelength) gives a qualitative view of the effects:
A certain fraction of the rays remains unscattered and reaches the 'throughput' detector, whereas the scattered fraction is distributed in all directions.
Doing the simulation with 41 spectral points in the range 400 ... 800 nm and 10000 rays per spectral point, the following results are obtained. The transmitted radiation has the following spectral distribution and color (which is computed by SPRAY as well):
The missing rays in the blue are scattered to the side:
The conclusions from this simple simulation are the following: Light travelling through a medium filled with scattering dipoles looses blue light and appears to be red (like sunlight closely before sunset when it has to travel the largest distance through the earth's atmosphere). Watching light scattering dipoles from the side (that's what you do looking at the sky) gives a blue appearance.
