Why does the sun....?

Question

Why does the sun bleach things. Say if you leave something in the window why does it change colour and go lighter?

Answer 1

Photons, even though they don't have mass, do have momentum.  They can be absorbed by  molecules that reflect a certain color, and modify those molecules.  Y'know how a rubber band, when new, is stretchy.....?  Get two rubber bands - place one in the cold darkness of your fridge, and another in a window sill that gets direct sunlight.

After a week or two, the one in the sill gets hard & brittle, due to the sunlight accelerating it's aging process - while the one in the fridge stays stretchy.  The one in the window has undergone some changes, accelerated by the sunlight - high densities of photons have been impacting the rubber band, and being being absorbed by it - changing the chemical properties of the rubber band.  

Answer 2

Colors in things are made by substances known as pigments. Some pigments naturally react to light, such as chlorophyll - the green pigment in many plants that they use to capture sunlight and convert its energy into sugars for their own energy.

If the pigment in your poster, for instance, reacts with the sunlight, it is being converted into other substances as it reacts, which uses up all of the pigment, and therefore has no more of the color.

Answer 3

It's mostly the ultraviolet in sunlight that affects things. Those are the more energetic photons and are the ones that can cause chemical changes if stuff they hit.