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Solar rays are a source of ultraviolet radiation which are divided into three groups A, B and C depending on their wavelength. Their intensity is strongest in the morning at sunrise when there are prevailing the waves of type A spectrum. Then there occurs the most harmful radiation. At sun set the situation is the same. Most useful are the waves of type B which are characteristic for solar illumination in the period from 10:00 to 16:00.

Exposure to the sun's rays is very useful to the organism because of the need for vitamin D which is produced in the body from cholesterol and is used for the absorption of calcium. Therefore, this vitamin is called calciferol (literally translated from Greek language - "moving calcium"). The adequate everyday duration of sun exposure in order to achieve an optimal level of vitamin D in the body is 15 to 30 minutes.

Prolonged exposure to very strong sunlight even during the hours of 10:00 to 16:00 however, is harmful, especially when UV-index of solar radiation is high. Every smart phone can provide you with information in seconds for this index. When its value is more than 3 stay at sunlight for not more than 30 minutes a day.

When you're on the beach the danger of rapid burning is even greater. The water reflects the sun's rays thus increasing the total amount of light directed to your skin and the droplets of water fell on it due to their hemispherical shape become magnifying glasses that concentrate the rays in even more harmful way. Must be considered that beach sand is silica (glass) which has highly reflective properties and to the other sources of solar radiation it adds more solar radiation.

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