# Controlling a material's opacity

1. Jan 5, 2012

### SuperFrisbie

So I'm a sophmore in college working on a project, and I am stumped. The problem is this:
I need to be able to control the opacity of some normally transparent material. So for example, I would have a transparent object sitting on the table, and when I shine a UV laser or an IR laser through it, the part that the laser is passing through of it becomes translucent, or any part of it becomes translucent. Another possibility would be if I applied a magnetic or electric field through the material, it would become translucent. The color it turns when it loses it's transparency would preferably be white or some light color. It must be able to change to translucent rapidly, to the degree of several nanoseconds, ~(10^-9 - 10^-7)s. And yes, it must be able to change back to transparent just as quickly.
If anyone has any relevant information, including even the name of the property of material to adjust opacity, it would be greatly, greatly appreciated.

2. Jan 5, 2012

### Simon Bridge

Photoreactive materials perhaps?
You can also use magnetic fields to line up polarizing crystals in a material.
You can put a liquid crystal between two crossed polarizers - like in an LCD display - apply a current and the crystal changes the rotation of the light passing through it.

That sort of thing?

3. Jan 5, 2012

### sophiecentaur

They are possible solutions but I think not for someone with limited means. The principles were well known but need high voltages (Kerr Cell) or high tech (LCD) to implement.

You can buy thermochromic paints and dyes which react slowly to heat. You could focus an IR beam on the table to produce a spot, perhaps. But I'm not sure whether such dyes would go transparent at low temperature. Try googling the term.

4. Jan 5, 2012

### Simon Bridge

Well I haven't seen any constraints - perhaps OP is just looking for something that can be looked up and read about - maybe write an essay?

I also thought of just having three stacked polarized sheets - cross the top and bottom one and put the middle one on an armature run off a motor or whatever so it rotates when an electric field is applied.

Fits the description but would probably be cheating.