State
what is meant by the hardness of an X-ray beam,
how the hardness of an X-ray beam from an X-ray tube is increased.
The same parallel beam of X-ray radiation is incident, separately, on samples of bone and of muscle.
Data for the thickness 
of the samples of bone and of muscle, together with the linear attenuation (absorption) coefficients 
of the radiation in bone and in muscle, are given in the figure below.
Determine the ratio
