- Why does the photon lose energy during Compton scattering?
- The photon transfers some of its energy and momentum to the recoiling electron to conserve both quantities in the collision. Since the photon's energy is inversely proportional to its wavelength (E = hc/λ), this energy loss results in an increase, or shift, in its wavelength. The greater the scattering angle, the greater the energy transfer and thus the larger the wavelength shift.
- Is the electron really 'free and at rest' in a real material?
- This is a key simplification of the basic model. In actual experiments with atomic targets, electrons are bound with varying energies. For photons with energy much greater than the electron's binding energy (e.g., X-rays or gamma rays), the approximation is excellent. For lower-energy photons, the binding energy cannot be ignored, leading to more complex 'incoherent' or bound-electron scattering.
- What is the significance of the Compton wavelength (λ_C)?
- The Compton wavelength (λ_C = h/(m_e c) ≈ 2.43 pm) sets the natural scale for the scattering effect. It represents the maximum possible wavelength shift, which occurs when the photon is backscattered (θ = 180°). It is a fundamental constant combining quantum mechanics (h), relativity (c), and the property of a specific particle (m_e).
- How does Compton scattering prove light is made of particles?
- The classical wave theory of light cannot explain the wavelength shift's dependence on angle or the existence of a scattered electron. The successful derivation of the Compton formula requires treating the photon as a discrete particle with energy E = hf and momentum p = h/λ, which collides with an electron like a billiard ball, conserving energy and momentum. The precise agreement with experiment was pivotal evidence for the photon concept.