Water is placed in a reservoir. When a pencil is dipped into the water, the tip appears bent. Now put concentrated sugar water into a cup and try the same thing. The tip of the pencil should appear even more bent. This is the phenomenon of light refraction. Refractometers are measuring instruments in which this phenomenon of light refraction is put to practical use. They are based on the principal that as the density of a substance (e.g. when sugar is dissolved in water), it's refractive index rises proportionately.

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When a straw is placed into a glass of water, the straw appears bent. Now if a straw is placed in a glass with water containing dissolved sugar, the straw should appear even more bent (see illustrations). This phenomenon is known as the principle of light refraction. Refractometers are measuring instruments which put this phenomenon of light refraction to practical use. They are based on the principle that as the density of a substance increases (e.g. when sugar is dissolved in water), its refractive index (how much the straw appears bent) rises proportionately. Refractometers were devised by Dr. Ernst Abbe, a German/Austrian scientist in the early 20th century.

The prism in refractometers