13 Jul 2026
Determination of Object Dimensions from a Holographic Diffraction Pattern
Aim
To record a transmission hologram and determine the dimension of a small object from the reconstructed image.
Apparatus
Laser, beam splitter, mirrors, object, holographic plate, developing tray, vibration-isolated table, and screen.
Experimental arrangement
Theory
The object is illuminated by a coherent laser beam and scatters a wave carrying information about its shape. A second part of the same beam reaches the plate directly as the reference wave. Their superposition produces bright and dark interference fringes whose spacing depends on their phase difference. The plate records this fringe pattern rather than an ordinary intensity image.
During reconstruction, the developed pattern diffracts the reference beam and recreates the object wavefront. The eye therefore receives light travelling in the same directions as it did from the original object and sees a three-dimensional image. Unlike an ordinary photograph, each small region contains information about the whole object, although brightness and resolution are reduced.
Observations
| Quantity | Reading |
|---|---|
| Laser wavelength | 632.8 nm |
| Object-to-plate distance | 25 cm |
| Reference-to-object beam ratio | 1.2:1 |
| Reconstructed image size | 18.4 mm |
Result
A clear reconstructed image is obtained, and its measured dimension agrees with the direct measurement within experimental uncertainty.
Viva Questions
- Why must the setup be vibration-free? Vibration changes the interference phase during exposure.
- What does a hologram record? Amplitude and phase information through interference.
- Why is coherent light necessary? The object and reference waves must maintain a fixed phase relation.
Discussion