Exploring the Key Drivers of the Opposition Surge Effect in Astronomy

In a continuation of our conversation with Leonardos “Leo” Gkouvelis, who is in the Faculty of Physics at Ludwig Maximilian University (Munich, Germany), we discussed the phenomenon of shadow hiding and coherent back scattering in astronomy, particularly in the context of observing celestial bodies like the moon and exoplanets. Shadow hiding occurs when light hits a surface at an angle, creating shadows that hide when light is horizontal, making the surface appear brighter.

Coherent back scattering is a quantum mechanical effect where light waves constructively interfere, enhancing brightness. These phenomena contribute to the opposition effect, which affects the brightness of objects as viewed from different angles. Our conversation also touches on the mathematical modeling of synthetic spectra to account for these effects, especially in exoplanet science where observations are limited.

This video segment is part of our interview with Gkouvelis. To stay up to date with all the latest video interviews from Spectroscopy, click here.

Will Wetzel, Senior Editor, Spectroscopy: Could you explain in simple terms what the opposition surge is, and why shadow hiding and coherent backscattering are so important in driving this phenomenon?

Gkouvelis: When light hits the surface from a specific angle, some shadows are created, right? Imagine a surface with little dust grains. If you throw light at a high angle, they will create some kind of shadow, right? So, you as an observer will see shadows, right? You will see the reflected light, you will see some brightness, and you will also see shadows.

When this light comes again at the same angle, you will not have shadows because the light will be totally horizontal. That’s why it's called shadow hiding because the shadows are hiding. Then, you will see more reflected light. It will be the surface that will be brighter. Therefore, this is one phenomenon that makes the observation much brighter than if you see it from some angle. Now the other one, coherent backscattering, was actually explained many years ago, and by coincidence. The first time, it was explained by an astronomer called von Schillinger.

Now, many decades later, they found it this phenomenon. It wasn't the only contribution in this opposition surge phenomenon, but there was another one, which is called the coherent back scattering effect, which basically is a quantum mechanical effect. Light is a wave, and when there are two waves, one traveling one direction, and the other one traveling the opposite, then constructive interference occurs.

References

1. Gkouvelis, L. An Analytical Model of Wavelength-dependent Opposition Surge in Emittance and Reflectance Spectroscopy of Airless Rocky Exoplanets. Planet. Sci. J. 2025, 6 (5), 110. DOI: 10.3847/PSJ/adcba8
2. Wetzel, W. Exploring the Opposition Surge Effect When Studying Exoplanets. Spectroscopy. Available at: https://www.spectroscopyonline.com/view/exploring-the-opposition-surge-effect-when-studying-exoplanets (accessed 2025-09-17).