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History and Technique

X-Rays were first produced and detected in 1895 by Wilhelm Roentgen, who received the first Nobel Prize in Physics for his discovery.  X-rays are a form of energy similar to light.  They differ from light in that they are able penetrate opaque substances.  Radiographs are produced by passing X-rays through a patient.  X-rays are attenuated by body tissues of different densities.  Dense structures, such as bone, prevent X-rays from passing through, whereas less dense structure like air, allow X-rays to pass through.  The X-rays that pass through the patient and cause a light producing photochemical reaction on a sheet in a cassette which is in turn stored on a piece of film (like standard photography), a reusable cassette (CR - computed radiography) or an array of electronics like your digital camera (DR - digital radiography).  Each of these can then be transformed into a printed image or an electronic image.

Radiographs display 5 densities ordered from most dense (bright on standard radiography) to least dense (dark): Metal/contrast agents > Bone > Soft tissue > Fat > Air

Wilhelm Roentgen 1
Illustration of a chest radiograph being performed.   X-rays are depicted passing through the patient.  The patient is facing the film detector.  
On this chest radiograph, notice how the bones (ribs, spine and clavicles) are white because they are dense.  The space in between, the lung parenchyma, is dark because it is primarily made up of air. You are able to visualize the cardiac silhouette because it is a different density than the adjacent structures.  
Notice in this radiograph of the skull how the metallic dental fillings are even brighter than the adjacent bone, whereas the paranasal sinuses are relatively dark because they are filled with air.