Q:

What are high energy photons?

I have often read the term High Energy photons, does this mean that there do exist photons with low energy? Aren't they supposed to have constant energy according to $E= h\nu$?

A:

Each photon has a different wavelength $\lambda$, which determines the energy of the photon. For example, blue photons you observe with your eyes have a wavelength of $400 nm$. Using that $\nu = \frac{c}{\lambda}$ and $E= h\nu$ we can calculate it's energy: $3.1 eV$. A gamma-ray has a much smaller wavelength, smaller then $1 pm$ ($10^{-12}m$). That corresponds with an energy of $1.24 \dot{} 10^6 eV$, a factor $10^6$ higher. Photons with a energy in the order of this, or higher, are considered High Energy Photons

A:

High energy photons mean the photons whose frequency is very high.(relatively)
 If the frequency is high then from this equation 
$$E=hf$$
(where $E$ is energy
and 'h' is Planck's constant.)
energy is also high.
Thats what we really mean when we talk about high energy photons.
By high energy photons we ''usually'' mean gamma photons or X-ray photons .
Frequency ,wavelenght,and energy of infrared radiation:
Frequencies: .003 - 4 x 1014Hz
Wavelengths: 1 mm - 750 nm
Quantum energies: 0.0012 - 1.65 eV
similarly for visible light.
Frequencies: 4 - 7.5 x 1014Hz
Wavelengths: 750 - 400 nm
Quantum energies: 1.65 - 3.1 eV
Ultraviolet radiation:
Frequencies: 7.5 x 1014- 3 x 1016Hz
Wavelengths: 400 nm - 10 nm
Quantum energies: 3.1 - 124 eV
X-rays:
Frequencies: 3 x 1016Hz upward
Wavelengths: 10 nm - > downward
Quantum energies: 124 eV -> upward
Gamma-Rays:
Frequencies: typically >1020Hz
Wavelengths: typically < 10-12m
Quantum energies: typically >1 MeV
here you can see the photons whose frequencies are high energies are also high.
So we say they are high energy photons.(relatively)
source:http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html

