# Two hologram halves brought together

1. May 19, 2015

### trendal

A google search will bring up a lot of pages talking about cutting a hologram in halve and ending up with two smaller copies of the original hologram.

What about the reverse? I can't find anything which says what happens when you stick the two pieces together again. Will you end up with two copies of the hologram side by side? Or will they merge into the original hologram?

If they do not merge, why is that?

If they do, at what point does the image become one? What would one see if they did this with both copies illuminated?

2. May 19, 2015

### Khashishi

If you put two pieces together, you will get the original hologram back.
A hologram looks kind of like a little window that you can peer through to see a (false) view behind it. When you cut the hologram in half, it is like cutting the window in half. The view beyond the window stays the same, but you have to tilt the window around to see it because the opening is smaller.

Why don't you try it, if you can get a hold of a hologram you want to destroy?

3. May 19, 2015

### trendal

At what point do you get the whole hologram back? What I mean is do you see the entire hologram as you bring the pieces together (like looking at something through two windows that are very close), or will you see two separate holograms right up until the point of contact?

4. May 19, 2015

### Khashishi

You get your whole hologram back when the pieces are touching. There's nothing special about this. If you cut a painting in half, and bring it together, at what point does it become one painting?

5. May 19, 2015

### trendal

True...but holograms are hardly paintings. They have this neat property where if you cut one in half you get two copies instead of merely two halves of the whole - as you would get if you cut a painting in half.

It isn't possible to put two halves back together in exactly the same way as before they were cut. There is always some damage to the material itself. You probably can't even get the two halves in direct contact with each other...it's hard to go that last few µm.

To take your painting analogy, you would never get exactly the same painting that you started with...only two halves with a fine line in between.

6. May 19, 2015

### Khashishi

Well, for two pieces of hologram side by side, you get something like two windows side by side, with a fine windowpane between them.

7. May 19, 2015

### A.T.

They aren't identical copies, but subsets of the original viewing angle range of the object.

Their physical contact is irrelevant, only their orientation matters. If you hold the halves coplanar to each other, side by side, they behave like the original with just one picture passing from one half to the other. To see two pictures you have to orient them differently.

Last edited: May 19, 2015