Question

A prism disperses a ray of white light while a rectangular glass slab does not. This is because
A. In a glass slab a ray of white light cannot split into its constituent colors
B. In a glass slab a ray of white light simply passes straight
C. A glass slab can be considered as an achromatic combination of two prisms, the dispersion produced by one prism is canceled by the dispersion produced by the other
D. None of these

Answer 1

Hint: Recall the concept of dispersion of a white light. Recall the basics of dispersion of white light when it passes through a prism. Also, check why the dispersion of light does not occur when the white light passes through a rectangular glass slab.

Complete step by step answer:
-The phenomenon of splitting of light in its constituent colors is known as the dispersion. In a glass slab, when a ray of white light enters from the first interface, the dispersion of the white light into its constituent colors takes place. But when the light exits from the opposite interface, the dispersion effect of light gets cancelled as the dispersed colors travel parallel to each other.Therefore, in the glass slab light can be dispersed.Hence, the option A is incorrect.
-When the ray of light enters from the air medium into the glass medium of the glass slab, the refraction (bending) of the light must occurs due to the change in medium.Therefore, the light cannot pass straight through the glass slab.Hence, the option B is incorrect.
The rectangular glass slab is not the combination of the two prisms.Hence, the option C is incorrect.
-When a ray of white light passes through one interface of a prism, the light gets dispersed into seven colors with different angles of deviation due to different velocities of these 7 colors in any medium. The second interface from which the dispersed light emerges out of prism is not parallel to the first interface. Hence, the different colors of light again get dispersed and emerge out of prism.For a rectangular slab, after the dispersion of light from the first interface, the light emerges out from the opposite and parallel interface of the glass slab. As the second interface is parallel to the first interface, the seven colors of light emerge out and combine again to form white light. Hence, the dispersion does not occur in glass slab.These reasons are not given in any options.

Hence, the correct option is D.

Note:The option C may appear correct to the students as the same kind of process occurs in a rectangular glass slab. But the glass slab is not a combination of two prisms. Also, the dispersion from the first interface is cancelled by the second interface and not the dispersion from the first prism is cancelled by the second prism.