There is an X-ray source. The emitted X-rays have a wavelength λ . The X-rays are allowed to be incident on a graphite target. They are scattered in many directions after interacting with the target. The intensity of the scattered radiation is measured at various angles (with respect to the direction of incident radiation) for different wavelengths present in the scattered radiation. At every angle, it is found that there are two wavelengths for which maximum intensity is there in the scattered radiation. One of the wavelengths in each case is the same as the incident wavelength. The second wavelength changes as a function of the angle. What could this second wavelength correspond to?
A.H. Compton in 1923 repoted this effect in his paper titled "The Spectrum of Scattered X-Rays" in Physical Review.