# Is it possible to partially reflect microwaves as we can do with light?

1. Nov 6, 2014

### Xtensity

Different types of mirrors and reflective surfaces can be set up to reflect predetermined amounts of light, like 70% of it, or 30%, or 99% of it, while letting the rest through. Can the same be done with microwaves? It seems even very very weak metal cages can completely reflect microwaves. Is there a way to have some kind of surface that reflects only a certain percentage of the MW radiation while letting the rest pass through (unabsorbed)?

2. Nov 6, 2014

### davenn

walls of brick, wood, concrete etc depending on their thickness will have varying attenuation which ill also be frequency dependant

Any solid metal structure will not allow any RF to pass
a metal mesh cage of a given size of holes will act as a high pass filter allowing frequencies above a certain range to pass through the holes ( gaps)

look at your microwave oven, you will see that there is a metalised mesh screen over the glass door ... it allows the really high freq of light to pass through but not the lower freq of 2.45 GHz RF
that the oven produces to cook the food

Dave

3. Nov 6, 2014

### Xtensity

I understand the basic functioning of a microwave, but my question was more focused on the actual mechanics of how an object might could reflect a fraction of microwaves that hit it, while letting the rest past through, while absorbing none. My curiosity stems from my recent research into LASING and the bridge into the realm of MASING. I just can't seem to find information specific to my question, or whether or not such a thing is even possible.

4. Nov 6, 2014

5. Nov 6, 2014

### davenn

I cannot imagine a system / material where none would be absorbed. I would expect there always to be some % absorbed. With the result the final 100% result would be an addition of
1) what is absorbed
2) what is transmitted through
3) what is reflected

6. Nov 6, 2014

### marcusl

This is a standard problem in electromagnetics. At every interface where there is an impedance change (such as vacuum to dielectric), a portion of the incident wave is transmitted and the rest reflected. These are described by transmission and reflection coefficients that depend on properties of the media, on the angle of incidence, and on the polarization.

7. Nov 7, 2014

### sophiecentaur

Masers were around before Lasers. I remember that my Physics course reading list included "Optical Masers" by O.S. Heavens.

The behaviour of a wave at an interface is easiest to consider in the context of transmission lines. It is relatively easy to produce a discontinuity in a transmission like that will reflect / absorb / transmit any fraction of the power you want. To do it in 3D with an free wave and a large screen you can use a grid of holes of various sizes and shapes in a metal plate and you can also introduce lossy material if you wanted to. You basically need to produce an appropriate complex refractive index.
But, if you are talking in terms of 'an object' then the size of the object will be a major factor in how it affects an incident wave. Measurable diffraction will happen if the object is not many wavelengths wide. Can you be more specific about the context?

8. Nov 7, 2014

### Staff: Mentor

This does not happen with light and mirrors. There is always some absorption also. Same with microwaves.

9. Nov 7, 2014

### davenn

which is what I say way back in post #5 :)

Dave

10. Nov 7, 2014

### Staff: Mentor

Well, superconductors will be highly efficient for reflecting/transmitting (with suitable geometry) microwaves.