What is Hologram? Explain the construction and reconstruction of Hologram. How is holography different than photography.

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In conventional photography , a negative is made first and using it a positive print is produced later. The positive print is only a 2D record of light intensity received from 3D object. Only intensity of light recorded and information of phase is lost. Denis Gabor introduced a new technique where intensity and phase both are recorded. This technique is called holography. Holography is done in two stages:

1.construction of image(freezing)

A weak but broad beam of laser light is splitted into wo beams by means of beam splitter. One beam directly goes to the photographic film is called as reference beam and second beam illuminates the object called as object beam. The light scattered by the object moves towards the photographic plate and interferes with the reference beam. The photographic plate carrying complex interference pattern of the object is called hologram. The object is stored in the hologram in the form of interference pattern. Whenever required the object can be viewed by illuminating the hologram as shown in the figure below- enter image description here

2.Reconstruction (unfreezing): A laser beam identical to the reference beam is used for reconstruction of the object. This read out bream illuminates the hologram at the same angle as the reference beam. The hologram acts as a diffraction grating and secondary waves from hologram interferes constructively in certain directions and destructively in other directions. They form a real image in front of the hologram and a virtual image behind the hologram at the original site of the object. An observer sees light waves diverging from the virtual image. An image of the object appears where the object once stood and the image is identical to what our eyes would have perceived in all its details of the object.

enter image description here

Application:

Hologram is reliable medium for data storage

Hologram is used in concerts

Hologram are used for authentication

Holograms are used in exhibitions to avoid possible thefts.

Photography	Holography
coherent light not needed	Monochromatic and coherent light is,necessary
Virtual image is not seen	Virtual image is seen
Only intensity is recorded	Intensity and phase both are recorded
2-dimensional image	3-dimensional image
From a torn or shattered photograph, the,image cannot be reconstructed	The image can be reconstructed from even, a small fragment of the hologram
Information of each point of the object, is recorded at a correspondence point on the image. Hence there is a point to,point correlation between the object and the image	Information of the entire object is, coded at each point of the image.

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