微波爐~光學

That is the diffraction limited of the wave.
If the size of the hole is almost same to the wavelength of the wave, the wave can hardly pass thought the hole. This can be applied to all waves.

Taking X-ray as an example, it can pass throught many things but not metals, this is due to the distance between the metal ions are too small compare with the wavelenght of X-ray. While gamma ray can penetrate thought metals due to its short wavelenght. Wavelenght of gamma ray is about 0.01nm, while the lattice spacing of metal is at order 0.1 to 1nm. X-ray is about 0.1nm.

Similar to your case, the size of the hole is much smaller than the wavelenght (~100 times). In fact, the hole can have a size up to the half of the wavelenght while it can still stopping the wave come out.

即係點解金屬濾網可完全可以阻擋微波的穿透?

This effect has nothing to do with diffraction. It is the phenomenon of [skin depth] of microwave on metals.

When electromagnetic waves (EM waves) incident onto a piece of solid metal, the eelctric and magnetic field components of the EM wave will have interactions with free electrons present in the metal. The time varying electric filed component causes electrons to oscillate, whereas the time varying mangetic field causes induced current in the metal (Faradays Law). As a result of the incident EM wave, electrons in the metal are then caused to oscillate rapidly and eventually disspiate their energy as heat. Thus the energy of the EM wave decreases as it passes through a metal.

The [skin depth] is a measure of how deep the EM wave could penetrate into the metal. Copper, for example, has a skin depth of about 1 micrometer (10^-6 m) at the frequency of microwave. That is to say, microwave cannot penetrate deep into metals.

It has been found that this effect could also happen if the solid metal is replaced by a metallic wire-gauze. As the EM wave could not penetrate deep into each wire of the gauze, the induced current by the changing magnetic field on the wire gauze would produce a magnetic field surrounding each of the wires. This mangetic field could oppose the incident magnetic field component of the EM wave (Lenz Law), thus suppressing its intensity. This leads to the effect of stopping microwave from passing through the holes in the wire gauze.

因為個小孔太小了...微波過唔到..and冇繞射..
即係點解金屬濾網可完全可以阻擋微波的穿透?
ce lv :係