Question: At what photon energy does the index of refraction start to increase again?
About the article "Telescope", section "Gamma-ray telescopes".

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Higher energy X-ray and Gamma-ray telescopes refrain from focusing completely and use coded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image.X-ray and Gamma-ray telescopes are usually on Earth-orbiting satellites or high-flying balloons since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum.
However, high energy X-rays and gamma-rays do not form an image in the same way as telescopes at visible wavelengths.
An example of this type of telescope is the Fermi Gamma-ray Space Telescope.
The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization.
An example of this type of observatory is VERITAS.
Very high energy gamma-rays are still photons, like visible light, whereas cosmic rays includes particles like electrons, protons, and heavier nuclei.
A discovery in 2012 may allow focusing gamma-ray telescopes.
At photon energies greater than 700 keV, the index of refraction starts to increase again.
An X-ray mirror optic for NuStar space telescope working up 79 keV, was made using multi-layered coatings, computer aided manufacturing, and other techniques.
The mirrors use a Tungsten (W)/Silicon (Si) or Platinum(Pt)/Silicon Carbide(SiC) multi-coating on slumped glass, allowing a Wolter telescope design.
This was thought to be the limit at that time but a discovery in 2012 may allow focusing gamma-ray telescopes.
At photon energies greater than 700 keV, the index of refraction starts to increase again. 
Direct photophoresis is caused by the transfer of photon momentum to a particle by refraction and reflection.
Movement of particles in the forward direction occurs when the particle is transparent and has an index of refraction larger compared to its surrounding medium.
Indirect photophoresis occurs as a result of an increase in the kinetic energy of molecules when particles absorb incident light only on the irradiated side, thus creating a temperature gradient within the particle.
In this situation the surrounding gas layer reaches temperature equilibrium with the surface of the particle.
Molecules with higher kinetic energy in the region of higher gas temperature impinge on the particle with greater momenta than molecules in the cold region; this causes a migration of particles in a direction opposite to the surface temperature gradient.
The component of the photophoretic force responsible for this phenomenon is called the radiometric force.
This comes as a result of uneven distribution of radiant energy (source function within a particle).
Indirect photophoretic force depends on the physical properties of the particle and the surrounding medium. 
Observation of gamma-rays first became possible in the 1960s.
Their observation is much more problematic than that of X-rays or of visible light, because gamma-rays are comparatively rare, even a "bright" source needing an observation time of several minutes before it is even detected, and because gamma-rays are difficult to focus, resulting in a very low resolution.
The most recent generation of gamma-ray telescopes (2000s) have a resolution of the order of 6 arc minutes in the GeV range (seeing the Crab Nebula as a single "pixel"), compared to 0.5 arc seconds seen in the low energy X-ray (1 keV) range by the Chandra X-ray Observatory (1999), and about 1.5 arc minutes in the high energy X-ray (100 keV) range seen by High-Energy Focusing Telescope (2005).
Very energetic gamma rays, with photon energies over ~30 GeV, can also be detected by ground based experiments.
The extremely low photon fluxes at such high energies require detector effective areas that are impractically large for current space-based instruments.
Fortunately such high-energy photons produce extensive showers of secondary particles in the atmosphere that can be observed on the ground, both directly by radiation counters and optically via the Cherenkov light the ultra-relativistic shower particles emit.
The Imaging Atmospheric Cherenkov Telescope technique currently achieves the highest sensitivity.
Gamma radiation in the TeV range emanating from the Crab Nebula was first detected in 1989 by the Whipple Observatory at Mt.
Hopkins, in Arizona in the USA.
Modern Cherenkov telescope experiments like H.E.S.S., VERITAS, MAGIC, and CANGAROO III can detect the Crab Nebula in a few minutes.
The most energetic photons (up to 16 TeV) observed from an extragalactic object originate from the blazar Markarian 501 (Mrk 501).
These measurements were done by the High-Energy-Gamma-Ray Astronomy (HEGRA) air Cherenkov telescopes.
Gamma-ray astronomy observations are still limited by non-gamma ray backgrounds at lower energies, and, at higher energy, by the number of photons that can be detected.
Larger area detectors and better background suppression are essential for progress in the field.
A discovery in 2012 may allow focusing gamma-ray telescopes.
At photon energies greater than 700 keV, the index of refraction starts to increase again. 
The photons of a light beam have a characteristic energy proportional to the frequency of the light.
In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected.
If the photon energy is too low, the electron is unable to escape the material.
Since an increase in the intensity of low-frequency light will only increase the number of low-energy photons sent over a given interval of time, this change in intensity will not create any single photon with enough energy to dislodge an electron.
Thus, the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy (equivalently frequency) of the individual photons.
It is an interaction between the incident photon and the outermost electrons.
Electrons can absorb energy from photons when irradiated, but they usually follow an "all or nothing" principle.
All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or else the energy is re-emitted.
If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's kinetic energy as a free particle.