# Hologram question

1. Nov 17, 2009

### Ja4Coltrane

Someone I know has asked about a certain property of holograms which he said he was shown while he was in school, but i can't figure out why the property should be true.

He says that he once saw a video of a demonstration where a light (presumably a laser) was shone through a hologram, and the image could be seen on the wall behind the hologram.

This doesn't seem to make sense to me. I thought that a lens would be needed to resolve the image onto a screen.

Any help is appreciated!

2. Nov 17, 2009

### mgb_phys

A hologram can act like a lens.
The pattern on the wall isn't strictly speaking an image, you can produce a hologram which will create pretty much any pattern of outgoing light rays when you shine a laser through it.
That's how the add-on lenses for laser pointers that project a line or star or other shapes work - they are holograms.

3. Nov 17, 2009

### Ja4Coltrane

I'm still confused. Is this just like if I have say, a stars shaped object that happens to send light out from every point, then if I look at a wall next to it, I will see a blurry star?

4. Nov 17, 2009

### mgb_phys

No, the hologram splits the incoming laser beam and sends out copies of the beam at different angles.
If you get the arragement of the angles right they make a pattern on the wall.
It's not an image because it makes the same pattern although at a different scale at different distances.

If you prefer you can think of the hologram as being a grid of little mirrors (or prisms) each bending the incoming light in a particular direction

5. Nov 17, 2009

### Ja4Coltrane

Okay and the pattern you see on the wall looks like the three dimensional thing that you see when you look into the hologram?

6. Nov 17, 2009

### mgb_phys

No it's just a 2d pattern of points.
You generally can't see anything looking into this kind of hologram - it's not the same effect as projecting a transmission hologram.