Explain how the hardness of an X-ray beam is controlled by the accelerating voltage in the X-ray tube.
The attenuation of a parallel beam of X-ray radiation is given by the expression
where 
is the linear attenuation (absorption) coefficient and 
is the thickness of the material through which the beam passes.
State
what is meant by attenuation,
why the expression applies only to a parallel beam.
The linear attenuation coefficients for X-rays in bone and in soft tissue are 
and 
respectively. Calculate, for a parallel X-ray beam, the ratio
