# What's inside the slit in double slit experiment?

If double slit experiment is done in a environment with air, then slits could also contain air made up of (approx. 80% nitrogen, 20% oxygen), then there is not empty space inside the slit. How can it affect the experiment? Has it been taken in account? What are specific requirement to define the slit?

If it were not that way. Why it doesn't affect the measurement?

Air is everywhere, not only in slit, but also in the optical path before and after the slit.

However, interaction with transparent media (e.g. air, water, glass) can be easily included: by use of refractive index $n$. Then you know that wavelength in medium is $$\lambda = \frac{\lambda_0}{n},$$ where $\lambda_0$ is the wavelength in the vacuum.

If in the slit there were glass - still it is easy to calculate the outcome. (Just you need to include the phase shift caused by it.)

• +1, Thanks, I am trying to understand the required properties of the wall material, then understanding the difference with the envieroment helps a lot – HDE Mar 4 '11 at 13:22
• @HDE You can google meta materials and then things get very interesting! – PhysicsDave Aug 20 '19 at 12:45

The air will simply introduce a small amount of additional noise into the experiment due to scattering. The majority of the photons will not scatter off the air molecules.

The optical path lengths will also be changed, but not by much. For example, the troughs in the interference pattern will occur when the optical path lengths of the paths to that point are out of phase. Since the optical path lengths in vacuum and air differ by a factor of $n_\mathrm{Air} = 1.00027$ for an arbitrarily chosen visible wavelength of 500 nm, the air has hardly any effect.

The properties of the wall do have a moderate efect on the interference pattern. However for most opaque walls the simple physical optics model sort of ignoring the wall seems to work reasonably well. Precise mathematics based on Maxwell equations for light are complex and details change significantly with say horizontal or vertical polarisation for a metallic grate. See Sommerfeld edge diffraction or JB Keller geometric theory of diffraction which have good math development in terms of series expansionin wavelength but one needs knowledge of wall properties and seems rather at odds with PO in many ways. However PO gives us useful answers even when we dont know wall properties and where GTD would be rather at a loss.