# Intuitive, conceptual understanding of holography

• B
• BiGyElLoWhAt
In summary, optical holography involves reconstructing a wavefront by illuminating an interference pattern created by the object wave and reference wave. The hologram film captures phase differences between the two waves, resulting in constructive and destructive interference. The reconstructed wavefront appears to have been generated by the original object, creating a 3D effect. The negative film is used to adjust for the 180 degree phase shift, and our eyes do not distinguish phase.
BiGyElLoWhAt
Gold Member
I'm trying to understand how optical holography works. I made a hologram using LitiHolo C_RT20 instant develop hologram film and am now trying to understand how this actually works. It almost makes sense to me. What I'm having trouble understanding, however, is this:
Reference wave + object wave = speckle on the film. Speckle + reference wave = object wave?
This feels very counter intuitive to me. I looked through the wikipedia page here, and everything follows, but the end result is very lack-luster. It's not intuitively obvious to me that what you get at the end resembles the object wave.

From the article:
##
\mathbf{t} A_\text{R} \exp {i \varphi_\text{R}} =
\left[ \mathbf{t}_0 + \beta \left[ A_\text{R}^2 + A_\text{O}^2 + A_\text{R}A_\text{O} \exp {i (\varphi_\text{R} - \varphi_\text{O})} + A_\text{R}A_\text{O} \exp{-i (\varphi_\text{R} - \varphi_\text{O})}\right]\right] A_\text{R} \exp {i \varphi_\text{R}}
##
This can be split into three terms:
##
\begin{align}
\mathbf{U}_1 &= \left[\mathbf{t}_0 + \beta A_\text{R}^2 + \beta A_\text{O}^2\right] A_\text{R} \exp {i \varphi_\text{R}} \\
\mathbf{U}_2 &= \beta A_\text{R}^2 A_\text{O} \exp i\varphi _\text{O} \\
\mathbf{U}_3 &= \beta A_\text{R}^2 A_\text{O} \exp{i (2 \varphi_\text{R}- \varphi_\text{O})}
\end{align}
##

##\mathbf{U}_1## is a modified version of the reference wave. The first term is a reduced amplitude version, the second is also a reduced amplitude version if the reference wave amplitude is uniform. The third term produces a halo round the transmitted reference wave which is negligible when the amplitude of the object wave is much less than that of the reference wave

##\mathbf{U}_2## is the reconstructed object wave which is identical to the original wave except that its amplitude is reduced. When the object wave is generated by light scattered from an object or objects, a virtual image of the object(s) is formed when a lens is placed in the reconstructed wave.

##\mathbf{U}_3## is known as the conjugate wave. It is similar to the object wave but has the opposite curvature. When the object wave is generated by light scattered from an object or a series of objects, a real image is formed on the opposite side of the hologram plate to where the object was located and is deflected from the normal axis by twice the angle between the reference wave and the normal direction.

Andy Resnick
Decades ago, in college, I had fun in the laser lab doing holograms.
At the intuitive level, I always looked at it as wave front reconstruction.
Of course, the film is a negative, so technically, you are reconstructing a wavefront that is 180 degrees out of phase - but that doesn't change the amplitudes or the way the wavefront progresses.

What the film is capturing is the objective wave and reference wave phase differences. Where they are the same, there is constructive interference and the film is darkened. Where they are the directly out of phase, there is destructive interference and the film ends up transparent. So when the reference wave illuminates the developed film, the selective transparency results in a reconstructed wavefront - all be it 180 degrees out of phase from the original.

I hope you are enjoying the experiments. I stayed up into the wee hours creating holograms that really showed the 3-D effects. In one case, after spending a few frustrating hours trying to project a real image of a vacuum tube, I slouched back in my chair to see the mirror image of the printing on the tube directly in front of my face. For an instant, I wondered how I managed to get myself inside the tube. So I packed up and went back to my dorm room for some needed sleep.

BiGyElLoWhAt and berkeman
BiGyElLoWhAt said:
I'm trying to understand how optical holography works. I made a hologram using LitiHolo C_RT20 instant develop hologram film and am now trying to understand how this actually works. It almost makes sense to me. What I'm having trouble understanding, however, is this:
Reference wave + object wave = speckle on the film. Speckle + reference wave = object wave?
This feels very counter intuitive to me. I looked through the wikipedia page here, and everything follows, but the end result is very lack-luster. It's not intuitively obvious to me that what you get at the end resembles the object wave.
I've used the LitiHolo system as well, it's great!

Holography is also called 'wavefront reconstruction', because that's what happens when you illuminate the hologram properly. The pattern on the film is indeed an interference pattern (what you call 'speckle' is a spatial interference pattern), and so phase information is retained in the hologram pattern. By illuminating the interference pattern with the reference beam, the 'original' object wavefront is reconstructed, generating a wavefront that appears to have been generated by the object (when in fact is it is generated by the interference between the hologram pattern and a new reference beam).

There are some subtleties here, so I encourage you to play around!

BiGyElLoWhAt
Ahhh. @.Scott
I think the piece I was missing was the negative. So basically I was looking at wave 1 + wave 2 = interference. Interference + wave 1 = wave 2, but that doesn't make sense. It should be Interference - wave 1 = wave 2. However I'm more so looking at wave 1 + wave 2 = -interference -> -wave 1 -wave 2 = interference. interference + wave 1 = - wave 2, but we don't care about the negative sign because our eyes don't distinguish phase.
Pseudo qualitative math of course.
Thanks a lot.

## 1. What is holography?

Holography is a technique that uses light to create a three-dimensional image of an object. It involves splitting a beam of light and directing one part of it onto the object and the other part onto a photographic plate. The interference pattern between the two beams is recorded on the plate, creating a hologram that can be viewed under specific lighting conditions to see a three-dimensional image of the object.

## 2. How does holography work?

Holography works by capturing the interference pattern between two beams of light. One beam is directed onto the object, while the other is directed onto a photographic plate. The two beams then combine and create an interference pattern, which is recorded on the plate. When the plate is illuminated with a laser, it recreates the original beams of light, resulting in a three-dimensional image of the object.

## 3. What is the purpose of holography?

The purpose of holography is to create realistic three-dimensional images of objects. It has many practical applications, such as in security features on credit cards and passports, as well as in art and entertainment industries. It is also used in scientific research to study objects that are difficult to observe directly.

## 4. How is holography different from traditional photography?

Holography differs from traditional photography in that it captures the full three-dimensional information of an object, while traditional photography only captures a two-dimensional representation. Holograms also require specific lighting conditions to be viewed, while traditional photographs can be viewed under normal lighting.

## 5. What is the concept of intuitive, conceptual understanding of holography?

The concept of intuitive, conceptual understanding of holography refers to the ability to understand and visualize how holography works without needing to know the complex mathematical and physical principles behind it. It involves developing an intuitive understanding of the interference patterns and how they relate to the three-dimensional image of the object.

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