light from one medium into another medium will produce refraction phenomenon, and the ratio of the incidence angle sine is constant, this ratio is called refractive index. soluble solid content in fruit and vegetable juice and refractive index under certain conditions (the same temperature, pressure) is directly proportional, so determine the refractive index of fruit and vegetable juice, can find the concentration of fruit and vegetable juice (how much sugar content). commonly used instruments are handheld refractometer, also known as sugar mirror, handheld sugar meter, through the determination of soluble solid content of fruits and vegetables (sugar content), you can understand the quality of fruits and vegetables, about the estimated maturity of the fruit.