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?

• In English sentences starts with capital ('big') letters. Also: 'environment', 'contain', 'approx.'. And it is good to start questions with a new line. Correctly written and well formatted question has higher chance of being answered well. – Piotr Migdal Mar 4 '11 at 13:22

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.
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.