Question

A ray of white light passes through a rectangular glass slab, entering and emerging at parallel faces. The angle of incidence, measured from the normal to the glass surface, is large.

• A. White light will emerge from the slab.
• B. The light emerging from the slab will have a number of parallel, coloured rays.
• C. The emergent rays will not form a spectrum on a screen.
• D. Colours will be seen if the emergent rays enter the eye directly.

Answer 1

Answer: (A), (C)

The correct options are
A White light will emerge from the slab.
C The emergent rays will not form a spectrum on a screen.

When light passes through a glass slab it slows down at the first interface and speeds back up at the second. The white light which enters is bent and the different colors of light are bent depending on their wavelengths. All of the bending that takes place at the first interfaces is exactly reversed at the second, so although the emerging ray of light is displaced slightly from the entering ray, it travels in the same direction as the incoming ray and all wavelengths that separated at the first interface are re-combined and white light thus emerges from the second interface.