I have a hard time understanding why light waves of different wavelengths diffract in a different manner. According to Huygens' principle, every point on the wavefront is a source of a secondary wave. So if we have a white light going through, say, a single slit (light rays parallel to each other and perpendicular to slit's plane), all what's supposed to happen is a plain diffraction, just like of any other wave. That is, the wave will progress spherically, but it will still be a white light. Why instead we get a splitting of different wavelengths? In other words, how does light color affect diffraction geometrically?
[Physics] White light diffraction
diffractionelectromagnetic-radiationhuygens-principleinterferenceoptics
Related Solutions
It's difficult to differentiate between the two phenomena because they are fundamentally the same phenomenon. It's common to use the term "interference" in a more general sense, where the interfering optical fields may not have the same source, or where a beam was split and is interfering with itself after being recombined. Diffraction is more commonly used when talking about the way a single optical field evolves as it propagates. Ultimately, we use the exact same mathematical tools to describe both, because they are really just different examples of wavelike behavior.
Let's examine more attentively what says Huygens' principle: that each point on the wave-surface that gets inside the slit, becomes a source by itself, emitting around itself a spherical wave. As see that you work in the 2D geometry, but 3D is better (I got once an explanation why but I don't remember right now). Anyway, each point on the wave-surface inside the slits emits a spherical wave, and your drawings only show the envelope, of all these spherical waves. See in the picture that I inserted the pink envelope.
So, from each such spherical wave, a ray goes to the point $P_0$, another ray to the point $P_1$, still another to $P_2$, etc. In short, each point on the screen gets a ray from each point of the wave inside the slit.
Best Answer
Diffraction effects depend on the wavelength of the light. Considering a single narrow slit with monochromatic light, light with wavelengths much larger than the slit will not be transmitted and light with wavelengths much shorter than the slit will be transmitted without significant diffraction effects, but light with wavelengths comparable to the slit will show significant diffraction effects.
The reason that diffraction effects are able to split white light into its different colors is because white light is composed of an incoherent combination of many different wavelengths of light. The different wavelengths get diffracted by different amounts, and the effect you see is that the white light gets split into its spectrum of colors. Additionally, since the light is incoherent, you don't see dark and bright spots like you would with monochromatic light.
How do we understand from Huygen's principle that light with wavelengths much shorter than the slit do not diffract very much? This is because points near the middle of the slit and points near the edges of the slit, which are both emitting spherical waves will interfere destructively except for in the direction straight ahead.