When x-rays hit a material, some are absorbed and others pass through. Generally, the higher the energy the more x-rays will pass through. Due to this penetrating power, hard x-rays are widely used to image the inside of visually opaque objects. The most often seen applications are in medical radiography. In such applications, soft x-rays are unwanted, since they are totally absorbed by the body, increasing the radiation dose without contributing to the image.
Because bones and teeth are dense, they absorb more x-rays than skin and softer tissue. X-ray machines are specifically designed to take advantage of the absorption difference between bone and soft tissue, allowing physicians to examine structures in the human body.
Another practical application of x-ray technology is x-ray crystallography. Since the wavelengths of hard x-rays are similar to the size of atoms, they are useful in determining the structure of proteins and other biological molecules.
Many things in space emit x-rays. Among them are stars, the Sun, comets, pulsars, and black holes - celestial objects that are millions of degrees celsius. When something is heated to over a million degrees, it will give off
x-rays!
Since the Earth's atmosphere blocks
x-ray radiation, telescopes with x-ray detectors positioned above the Earth's absorbing atmosphere have been used to observe and study astronomical phenomena such as supernovae remnants and the Earth’s x-ray aurora.