The app controls are labelled in Italian: below, each label is given as it appears in the app, followed by its English meaning.

A wave passing through a single slit

1. Description of the experiment: a plane wave is made to pass through a fairly narrow slit. The wavefronts of the plane wave become "spherical". This is all the more true the narrower the slit is compared with the wavelength of the wave.
2. Instructions
   • Click here to start the 2D Waves app and select at the top left Diffrazione: 1 feritoia (Diffraction: 1 slit);
   • To change the width of the slit:
     • select Stop and Cancella Onde (clear waves). This way only the obstacle and the slit to be modified will be shown;
     • right-click in the display area and select modifica pareti (edit walls); the mouse works in "toggle" mode: clicking where there is an obstacle deletes it, and vice versa;
   • to restart the wave, deselect Stop;
   • to change the wavelength use the Frequenza (frequency) control of source 1;
   • when changing the frequency and the width of the slit, the wave beyond the obstacle may seem to fade away. This can be avoided by increasing its Brillantezza (brightness) with the corresponding control.
3. Remarks:
   • according to Huygens' principle, the individual points of the slit become sources of secondary spherical waves; the envelope is more spherical the fewer the spherical waves that make it up (a number that is obviously proportional to the width of the slit), that is, when the source (in this case the slit) can be considered almost point-like. This happens if the size of the source is much smaller than the wavelength of the generated wave.
4. To find out more: see diffraction, Huygens' principle, superposition principle
5. Other experiments: we also invite you to try diffraction by a double slit by selecting Diffrazione: 2 feritoie (Diffraction: 2 slits) in the menu at the top. You will see dark regions arranged regularly. This phenomenon can be explained by the concept of destructive interference.

Diffraction by a half-plane

1. Instructions
   • Click here to start the 2D Waves app and select at the top left Diffrazione: semipiano (Diffraction: half-plane);
   • to change the wavelength use the Frequenza (frequency) control of source 1; go to extreme conditions: observe what happens at low frequencies (long wavelengths) and at high frequencies (short wavelengths).
   • when changing the frequency, the wave may become less bright. This can be avoided by increasing its Brillantezza (brightness) with the corresponding control. High brightness values will also make small "ripples" visible.
2. Remarks:
   • the experiment leaves no doubt: the ability of the wave to "diffract" and to reach regions forbidden by the corpuscular theory depends on the relative sizes of the wavelength and the obstacle. When the wavelength is much larger than the size of the obstacle (low frequencies), the wave re-forms almost immediately, as if it had not "seen" the obstacle. When the wavelength is small compared with the size of the obstacle (high frequencies), the obstacle makes its effect felt, producing a "shadow region".
3. To find out more: see diffraction, Huygens' principle, superposition principle

Similar conclusions can be reached with the following experiment:

Diffraction by an obstacle

1. Instructions
   • Click here to start the 2D Waves app and select at the top left Diffrazione: ostacolo (Diffraction: obstacle);
   • to change the wavelength use the Frequenza (frequency) control of source 1; go to extreme conditions: observe what happens at low frequencies (long wavelengths) and at high frequencies (short wavelengths). The shadow region at high frequencies is now less sharp because of the small transverse size of the obstacle.
   • If you want to change the shape and size of the obstacle
     • select Stop and Cancella Onde (clear waves). This way only the obstacle and the slit to be modified will be shown;
     • right-click in the display area and select modifica pareti (edit walls); the mouse works in "toggle" mode: clicking where there is an obstacle deletes it, and vice versa; as a first step, try making the obstacle longer.
     • to restart the animation deselect Stop
       • try repeating the procedure to create a two-dimensional obstacle (with non-negligible longitudinal size).
2. Remarks:
   • the experiment leaves no doubt: the ability of the wave to "diffract" and to reach regions forbidden by the corpuscular theory depends on the relative sizes of the wavelength and the obstacle. When the wavelength is much larger than the size of the obstacle (low frequencies), the wave re-forms almost immediately, as if it had not "seen" the obstacle. When the wavelength is small compared with the size of the obstacle (high frequencies), the obstacle makes its effect felt, producing a "shadow region" and at the same time a reflected wave.
3. To find out more: see diffraction, Huygens' principle, superposition principle

A non-negligible longitudinal size of the obstacle favours the formation of the shadow region.