Hint: Recall the concept of dispersion and refraction of light because of which the white light produces a spectrum when passed through a prism. Check the difference between the construction of a prism and a glass slab. From this difference, conclude why the spectrum is not produced when the white light passes through the glass slab.
When a ray of light passes from one medium to the other medium, the refraction of light causes.In a prism the faces are inclined with each other and not parallel to each other. Hence, when a ray of white light passes through the one face of a prism, the white light undergoes dispersion and separates into its seven constituent colors.
These seven colors are distinct from each other because the constituent colors of the white light also undergo refraction. Due to this refraction, the seven colors in the white light deviates with some angle and appears distinct.When these seven colors of white light pass through the second face of the prism, they again undergo refraction and deviation causes them to look distinct.
But in a rectangular glass slab, the opposite faces of the slab are parallel to each other.When a ray of white light passes through the rectangular glass slab, it undergoes dispersion and separates into seven constituent colors. But when these dispersed colors of light emerge from the opposite face of the glass slab, the emerging rays of light again combine with each other to form white light as the refraction does not deviate these seven colors.
Hence, the light emerging from the second face of the glass slab does not produce any spectrum.
Note:The students should keep in mind that when the white light passes through the first face of the glass slab, it undergoes refraction and dispersion. But when this dispersed light emerges from the opposite face of the glass slab, it undergoes refraction but does not undergo dispersion because of the parallel face. Hence, the students should not think that the light from the second face of the glass slab does not undergo refraction.