Even when there is a propagating electromagnetic wave produced e. A hallmark of an evanescent field is that there is no net energy flow in that region. Since the net flow of electromagnetic energy is given by the average Poynting vector , this means that the Poynting vector in these regions, as averaged over a complete oscillation cycle, is zero. In many cases one cannot simply say that a field is or is not evanescent. For instance, in the above illustration, energy is indeed transmitted in the horizontal direction.
Penetration depth is a measure of how deep light or any electromagnetic radiation can penetrate into a material. When electromagnetic radiation is incident on the surface of a material, it may be partly reflected from that surface and there will be a field containing energy transmitted into the material. This electromagnetic field interacts with the atoms and electrons inside the material. Depending on the nature of the material, the electromagnetic field might travel very far into the material, or may die out very quickly.
TIRF microscopy delivers images with an outstandingly high axial resolution below nm. This allows the observation of membrane-associated processes. This is achieved by employing an evanescent wave for excitation of the fluorophores instead of direct illumination via light delivered by an arc lamp, LEDs or lasers. The evanescent field occurs if incident light is totally reflected at the interface of two transparent media with different refractive indices. In biological applications the incident light is usually laser light and the interface the glass of the coverslip and a film of aqueous solution between coverslip and adherent cells.