Scattering and wavelength dependence

[coke]

i don't know why the longer the wavelength, the easier for it to be sattered ?

 

[Danger]

Welcome to PF, Coke.
I have pretty much zero knowledge of optics, but I would suspect that it's simply because longer wavelengths are less energetic. Red light, therefore, would be more likely to be 'diluted' than blue.
There are quite a few guys here who know an awful lot more about the subject, and they'll be with you shortly.

 

[Andy Resnick]

It's not always true that longer wavelengths are scattered more. In fact, the sky appears blue because shorter wavelengths are scattered more efficiently in the atmosphere.

In any case, the question can be simply "How does scattering depend on wavelength"? And the answer to that would fill several books. For atmospheric scattering (particles that are much smaller than a wavelength), the scattering efficiency goes as 1/(wavelength)^4. For spheres of arbitrary size, the scattering goes as 1/x^2, where x = radius/wavelength (Mie scattering).