State and explain how, in an X-ray tube, the hardness of the X-ray beam is controlled.
A parallel beam of X-rays has intensity 
and is incident on a medium having a linear absorption (attenuation) coefficient 
.
State an equation for the variation of the intensity 
with the thickness 
of the medium.
Data for the linear absorption (attenuation) coefficient for an X-ray beam in blood and in muscle is shown in the figure below.
Suggest why, if this X-ray beam is used to image blood vessels in muscle, contrast on the image would be poor.
