Collision between a Photon and an Electron
In this scenario, a photon of initial momentum p1 collides with an electron of rest mass m1 and relativistic momentum p and energy e. The collision results in a change of wavelength after scattering through an angle @. Let's break down the explanation into key points:

Photon-Electron Collision:
- A photon is a fundamental particle of light with no rest mass and carries momentum.
- An electron, on the other hand, has rest mass m1 and moves with relativistic momentum p and energy e.

Change in Wavelength:
- When the photon collides with the electron, it transfers some of its energy and momentum to the electron.
- This transfer results in a change in the wavelength of the photon after scattering through an angle @.
- The change in wavelength can be calculated using the principles of conservation of energy and momentum in the collision.

Explanation:
- The change in wavelength can be determined by analyzing the final momentum and energy of the photon-electron system after the collision.
- By applying the laws of conservation of energy and momentum, the change in wavelength can be expressed in terms of the initial and final momenta and energies of the particles involved.
In conclusion, the collision between a photon and an electron leads to a change in wavelength of the photon after scattering through an angle @. This change can be calculated using the principles of conservation of energy and momentum in the collision scenario.