The problem may be that you you are thinking about a string. It may also be that you are mixing classical and quantum views of physics. Never the less, let's try some visual images.
Let's move to 2 dimensions. A classical electric field at a point tells you which about the force on a test charge at that point. You can represent that as an arrow. If you put an arrow on every point of a plane, you might get something resembling a field of wheat. An electric field fills space. So you really have to imagine stack of wheat fields. But 2D is useful to get the idea.
Now imagine a gusty wind blowing through the field, disturbing the arrows this way and that. This gives the idea of an electromagnetic wave. Textbooks talk mostly about waves that oscillate regularly back and forth because they are the simplest to analyze and because waves often are like that. But they don't have to be regular.
For an electromagnetic wave, the "wind" is really changing electric and magnetic fields generated by moving charges. Most moving charges are electrons, and most electrons are found in atoms. You need quantum mechanics to describe how electrons in atoms behave, particularly if you want to think about a single photon. A simple image of a vibrating particle doesn't really work.
Likewise, the idea of an electromagnetic wave changes in quantum mechanics. A single atom can emit a photon. Sometime later, a distant atom might be affected by the photon. The atom next to it won't feel anything. In quantum mechanics, waves describe probabilities. They tell you where atoms are likely to be affected and where they are likely to not to be.
The classical electric field works when you have lots of photons. Regions where the probability is high receive more of photons. Test charges there feel a larger force. The intensity of the electromagnetic wave is high. The intensity of light is high. Regions where the probability is low are darker.
Going back to the gusty wind, one might ask how quickly the wind can shift, so that wheat blows left then right then left again. A related question is how far apart are stalks bent left and bent right. These questions correspond to the frequency and wavelength of the electromagnetic wave.
One of the rules of quantum mechanics is that high frequencies require high energies to generate. I can't give any better reason for it than that is how the universe works.
Microwaves are generated by low energy processes. They have low frequencies and long wavelengths. Visible light are higher energy, higher frequency, and shorter wavelength.
Going back to a classical picture, the grating on a microwave oven is made of strips of electrical conductor with insulator in between. Electrons can flow freely in a conductor, but are held in place in an insulator.
When an electromagnetic wave hits a conductor, the forces on the free electrons cause them to vibrate. The wave is absorbed when this happens. The vibrating electrons emit a new electromagnetic wave.
In a smooth mirror, the emitted wave is just like the incident wave, except that it goes in a different direction.
If the mirror has holes in it, you have to add up parts of the wave from all parts of the mirror and holes to find out much is transmitted and reflected. When adding waves together, keep in mind that they reinforce when in phase and cancel when out of phase.
For gratings with holes smaller than the wavelength, it works out that very little gets through the grating. Everything is cancelled.