You can still get interference if the medium is thicker than a wavelength, but it needs to be close. This effect is quite often seen in CCD detectors used in in

You can still get interference if the medium is thicker than a wavelength, but it needs to be close. This effect is quite often seen in CCD detectors used in infrared spectroscopy, as well as certain detectors used for visible spectroscopy. It's called etaloning. But as you suggest, in thicker media there's much less chance of it happening. http://www.andor.com/learning-academy/optical-etaloning-in-charge-coupled-devices-technical-article http://www.andor.com/learning-academy/optical-etaloning-in-c...