Analysing Glass and Plastic

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Glass and plastic materials can be analysed by measuring the refractive . When light enters one transparent medium from another, the of the light changes.


The light is bent, or , as the light travels from air to glass. The speed also changes when light leaves the glass. The refractive index is a measure of how much the speed of light changes, or how much the light is refracted. To determing the refractive index, two mesurements are needed. The angle b, called the angle of , is measured using a . Angle c, the angle of , is also measured. These angles are measured from a line called the . The normal is at an angle of 90 degrees to the boundary between the two transparent materials. The refractive index is calculated by dividing the sine of the angle of , by the sine of the angle of .

Forensic scientists can measure the refractive index from small fragments of glass or plastic found at the scene of the and on the suspect. Different types of glass and plastics can have slightly refractive indices. This can used be to match samples of glass and plastic found in different locations.