If wavelength is just the distance between 2 consecutive waves, then shouldn’t waveheight be the determining factor on if it can reflect or pass through an object.

Question

For eg, in the wikipedia for radar, it says that they were only able to use radar with radio waves with a wavelength smaller than a plane’s length. But won’t wavelength be perpendicular to it’s direction of travel? But waveheigth is parallel, so won’t that be the deciding factor?

In: Physics

Answer 1

The ability of a material to absorb light is based mostly on the exact arrangement of that material’s electrons. When the light hits the material, the photons hit the electrons and are absorbed^(1), pushing the electron temporarily into a higher energy state. Electrons are able to ‘recover’ from this higher energy state more or less quickly based on their arrangement inside their atom – different atoms absorb photons at different rates, in other words. If a photon hits while an electron isn’t in position to absorb it, it’ll just pass that atom by without being absorbed^(2). The frequency of a light wave (one over the wavelength) determines the time interval between different photons hitting the material. If this frequency lines up well with the ‘recovery’ period of the electrons, the electrons will do a really good job of absorbing all of the photons and basically none of them will get through. If it doesn’t like up as well, only the occasional photon will get reflected/absorbed and almost all of the light will pass through, making the object transparent to that wavelength. This is also why (for example) glass is opaque to UV rays and infrared rays, but transparent to the visible rays between them – it happens to sync up for UV and infrared, but it’s just slightly off for the visible spectrum.

1. Or reflected, but it has to get absorbed before it gets reflected, so I’m just going to say absorbed. Reflection is also a bit more complicated, but in terms of this particular question that extra complexity mostly doesn’t matter.

2. Well, kinda. Photons aren’t quite distinct particles like that, because of quantum physics. And electrons don’t really jump between fixed positions based on energy level either. But you can treat it as working basically like that for the purposes of this explanation.