Diffraction is the distortion a wave undergoes whenever it hits an object. A simple example of this is the way waves in a water tank will attempt to 'bend around' the corners of a hole in their path.
In the case of light waves diffraction is more difficult to spot because the wavelength of visible light is so small, but if you remember how blurred the world looks when you view it through a fine gauze then you have an example of light diffracting.
In a simple microscope when visible light shines at the object on the slide the this light is diffracted. It is then the job of the objective lens of the microscope to collect that diffracted light so that it can be seen. However, in order for an object to diffract a wave then it must be at least as big as it's wavelength (l). It is for this reason that you have to use the smaller wavelengths of x-rays in order to visualise things at the molecular level.
Unfortunately there are also two other major obstacles to overcome before you get pictures of molecules.
This means that this signal must be amplified by having an organised matrix of molecules which diffract in the same way. In the case of a muscle fibre then this array occurs naturally, but the level of the organization is such that your can only obtain a very low resolution. The way that high resolution pictures of macromolecules such as proteins are obtained is to create a regular array in the form of a crystal. It is the regulatiry in the way that the molecules are arranged in a crystal that allows the signals brought about by diffraction to be amplified and thus a picture can be formed.
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An
historical look at muscle contraction