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A photon of wavelength 
is incident on an isolated stationary electron, as illustrated in the figure below.
The photon is deflected elastically by the electron of mass 
. The wavelength of the deflected photon is 
.
Calculate, for the incident photon,
its momentum,
its energy.
The angle 
through which the photon is deflected is given by the expression
where 
is the change in wavelength of the photon, 
is the Planck constant, and 
is the speed of light in free space.
Calculate the angle .
Use energy considerations to suggest why must always be positive.
.
.
.
