Some food or medical items, like rasberry juice concentrate I just bought, have a "protect from light" warning on the label. How can light influence products like milk and what does this warning mean in practice?
The photons of light, especially ultra violet photons from sunlight, carry energy that can break chemical bonds.
The bonds in chemicals that make up colours, ie. dye molecules, are most affected because they create the colour effect by absorbing some photons (of one colour) and not others.
Since they have chemical bonds of the correct energy to efficiently absorb light, these molecules will eventually be destroyed by strong light and the color will fade - which is what happens to clothes left in strong light. In the case of the fruit juice it will probably lose some of it's strong colour, it may also destroy some of the vitamins - but it's unlikely to make the juice poisonous!