In the double slit experiment with electrons why the single slit case is not diffracting as photons would have? The famous double slit experiment used in books to describe the duality of particles nature usually presents a case where only one slit is open and there is no diffraction.
Aren't we supposed to see diffraction from single slit using electrons ? They are also waves after all. If we use instead photons they will diffract from single slit.

 A: According to the Quantum Mechanics, one cannot talk about a particle's position but about the probability of the position of the particle. A wave function is a function whose product with its conjugate gives the probability distribution of the position of the particle. Since the particle is known to be located somewhere in the range, the wave function should be normalizable. The wave function of a particle is determined from the famous Schrödinger Equation.
The diffraction of photons can be explained by the particle-wave duality but when it comes to a particle like an electron it is needed to introduce the concept of the wave function. So when electrons are sprayed through a single slit their wave functions diffract. The probability of the position of an electron is in a wave form and when passing through the slit the fashion of the probability of the position of the electron changes. That is why even if you send electrons one by one through a single or double slit you observe a diffraction and interference. However, there is an efect of the observer.
Earlier in the 20th century there was a famous discussion about the position of the particle just before the measurement is done. There were three suggested answers: agnostic, realistic and orthodox. For more details on this subject you can refer the textbook on quantum mechanics by Griffiths. As a result of a well-prepared experiment it is found that the particle was not really anywhere. But, just after the measurement the particle is found to be there where it is measured to be. So as a result of this phenomenon it is observed that the wave function of the particle collapses after the measurement. This should also answer your question. To clarify the effect of the observer some textbooks (like Serway-Jewett at the section of modern physics) give figures where electrons do not diffract after passing through a single slit. This only occurs when the position of the electron is measured before passing through the slit. This measurement results the collapse of its wave function and the electron behaves like a rigid small ball as pictured in classical mechanics. This is also the case in the double slit experiment if you measure the position of the electron. You simply collapse the wave function on this way and somehow force it to be in that position.
A: To answer the question directly I will quote user A. I.: "So when electrons are sprayed through a single slit their wave functions diffract." You should read A. I.'s answer for discussion of the important QM details.
The figure in the question is somewhat misleading because it doesn't show the fringes beyond the central maximum for single slit diffraction. I have been Googling this for a while now, and it seems that most resources don't really care to discuss single slit interference for electrons, or any particle for that matter. My guess is that this is because the higher order fringes are practically negligible compared to the central maximum, so most resources just focus on the exciting, more pronounced result of double slit interference with particles. When they discuss covering up one slit, they treat it as total collapse of the wavefunction and ignore the low intensity, higher order fringes that naturally occur.
