Parabolic mirrors do not suffer from spherical aberration (spherical mirrors cannot focus all incoming, on-axis, light onto a point), nor chromatic aberration (single lens refracting telescopes focus light of different colors at different points).The parabolic mirrors will be achromatic across all wavelengths as the rays are focused by a mirror that has no chromatic aberration. We have been using a parabolic mirror for concentrating solar energy and have never observed chromatic aberration at its focal spot.
But, we got the following comment from a reviewer of our manuscript: “The author performs detailed numerical calculation. I have recognized author's numerous efforts. However, focusing of the solar light, the author suggests that no aberration occurs in the solar focal system because the solar light does not pass through the material. It is not correct. In the case of the solar reflection, solar light contains different wide wavelength components. Each component propagates in different directions. In the experiment, we can observe the color separation. Many efforts (thinking ideas and real studies) to concentrate the solar light with high density have been performed by Israel or NASA (JPL) groups for around fifty years. We hear only the ideas, and no experimental data were shown.”
Please kindly inform me who is right?
Dear Dawei,
I agree with you as the geometric optics principles are applicable to the reflection of optical rays from the metal surfaces, then according to the Snell law your findings is right and i agree with you. Geometrically the paraboloid has single point focus contrary to the spherical mirror.
However there are some practical considerations that may cause the enlargement of the focal point such as the roughness of the surface of the mirror which must be much smaller than the wavelength. The other issue is the angle of the incidence of the solar radiation that may vary slightly around the parallel beam incidence to the mirror axis.
For the evidence of the unique focus point of the ideal praboloid please refer to the link: http://scipp.ucsc.edu/~haber/ph5B/parabolic09.pdf
Estimated Professor Abdelhalim Zekry
Many thanks for your very instructive answers.
All my best regards
Yours Sincerely
dawei liang
Dawei Liang - You is true for mirror system. But Your 's reviewer supposes lense focussing system.
Hi Dawei,
the CNRS solar furnaces are glass mirrors backside coated: the light goes inside the glass by ~10mm, hence there should be some chromatic aberrations.
With the Bastion 2m diameter parabola with f=900mm, I calculate a max slope of 0.55 on the outer rim, or 28.8°: not extremely high (which would explain why we this is not an obvious observation), but not neglectable either.
As we are still wondering the exact glass material, I can't assume a Abbe value to further evaluate the chromatic aberration. It may with the suitable contents to be low dispersion, as it is very clear and rather far from flint glass.
So for me, as the parabolic mirror used in Odeillo is reflectively coated on its back, light goes through the glass hence we can have chromatic aberration, in theory. We don't observe it:
- Badges
- Science topic