Posted on 31 March 2011 at 17:44h

After doing about the diffraction grating with my HL class I decided to give them a treat and explain how a hologram is made then show them the puppy hologram I keep hidden under the sink (sounds dubious).  I know it’s no longer on the syllabus but I do it anyway because the way it works is so neat. My final year project at university was doing holographic interferometry on loudspeaker cones, first  had to make a hologram. The first 20 didn't work at all because the box of photo plates had been exposed.

I found this really good explanation in an old journal some years ago when holography was still on the syllabus, the way the photo of the fringes recreates the original wave front is almost magical. To record all the information that is contained in a wave front we need to record the intensity, wavelength and phase. A photograph only records intensity and phase which is why unlike the original scene; it looks the same if viewed from different angles. To record the relative phase of different parts of the wave front we can interfere it with a reference wave.

Consider the blue wave front below.

The relative phase of the wave front can be recorded by superposing it with the red reference beam. The two beams coincide on a photo plate which darkens where the waves add to give a series of dark bands. If the photo plate is the used as a diffraction grating for the red wave it diffracts the light in exactly the same direction as the original blue beam. The blue beam has been reconstructed.

If the blue plane wave front is now replaced with a circular one the diffraction grating produced has uneven lines. If this is now used to diffract the reference beam a circular wave front is recreated. As shown below in my second diagram which shows scant regard for the consistent use of colours.

If it works for one point it will work for the millions of points that make up any 3D object. And by the way, the beams aren't really blue and red they are both the same colour since the waves won't interfere to give fringes unless they are COHERENT. (a class joke)

I’m just waiting for the first holographic computer screens and TV’s. I guess watching 3D TV will take a bit of getting used to; maybe we’ll have to have special empty rooms to view them in as it might be quite strange watching all the action with your furniture in the background.