In a hypothetical experiment light is used to levitate a transparent glass hemisphere with radius R and mass m having refractive index n. In the medium outside the hemisphere, the index of refraction is equal to one. A parallel beam of monochromatic laser light is incident uniformly and normally onto the central portion of its planar surface as shown in figure. The acceleration of gravity g is vertically downwards. The radius $\delta$ of the circular cross section of the laser beam is much smaller than R. Both the glass hemisphere and the laser beam are axially symmetric with respect to z-axis.
The glass hemisphere does not absorb any laser light. Its surface has been coated with a thin layer of transparent material so that reflections are negligible when light enters and leaves the glass hemisphere. The optical path traversed by laser light passing through the non-reflecting surface layer is also negligible.