Electrons in a dilute gas cell are used as effective "free" targets for scattering…
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Modern Physics · Modern Physics
Problem
Electrons in a dilute gas cell are used as effective "free" targets for scattering experiments. A student aligns a beam of ultraviolet photons of wavelength 2.47×10−8m along the +x axis and records both the scattered photons and recoil electrons.
During analysis, the student notices that whenever a photon is detected at 90∘ above the +x axis (i.e., along +y), the associated electron is found at an angle slightly below the −y direction.
(a) Describe qualitatively, in words, why the electron cannot be detected exactly opposite the scattered photon’s direction (i.e., at −y) when the photon scatters at +y. Your explanation should reference fundamental conservation principles.
(b) Using vector arguments, explain how measurement of the electron’s angle allows one to determine whether the initial photon wavelength is shorter or longer than a critical value. Outline the reasoning without performing a calculation.