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Why does a hologram create both a real image and a virtual image?
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holography
Figure 1: Gabor's original method for creating holograms.
A hologram creates both a real and a virtual image because the hologram acts as a complex diffraction grating or zone lens when illuminated.12
During the recording process, the interference pattern of the object's light and a reference beam is stored on the plate. When this plate is later illuminated with a coherent laser beam, the fine-detailed fringes diffract the light to reconstruct the original optical field. Because the hologram functions like a zone lens, it splits the light into two distinct components:12
- Virtual Image: Formed on the light source side of the hologram, this primary image is visible to the eye and retains the three-dimensional characteristics and parallax of the original object.12
- Real Image: Formed on the opposite side of the hologram, this conjugate image can be projected onto a screen or captured by a camera.12
In essence, the diffraction pattern recorded on the hologram directs light into two paths: one that converges to form a real focus and another that diverges as if coming from a virtual focus.2