**Difference in Optical Properties**

**Glass Prism:**

A glass prism is a three-dimensional object with two triangular ends and rectangular sides. It is specifically designed to refract (bend) light and separate it into its constituent colors, creating a spectrum. This phenomenon is known as dispersion.

When white light passes through a glass prism, it enters at one face and undergoes refraction. The angle of incidence and the refractive index of the prism determine the extent and direction of this refraction. Since different colors of light have different wavelengths and hence different speeds in a medium, they refract at different angles upon entering and leaving the prism.

As a result, when white light passes through a prism, the light is dispersed into its component colors, forming a spectrum. This occurs due to the different degrees of bending or refraction experienced by different colors.

**Glass Slab:**

A glass slab, on the other hand, is a two-dimensional object with parallel faces. Unlike a prism, it is not specifically designed to separate light into its constituent colors. When white light passes through a glass slab, it undergoes refraction at both the entry and exit faces of the slab. However, since the faces are parallel, the light rays are not separated or dispersed into different colors.

The refraction that occurs in a glass slab is governed by Snell's law, which relates the angle of incidence, the angle of refraction, and the refractive indices of the media involved. In the case of a glass slab, the angle of incidence and the refractive index of the slab determine the angle of refraction.

Since the faces of the glass slab are parallel, the angle of refraction at both the entry and exit faces is the same. Consequently, the light rays passing through the slab remain parallel to each other and do not separate or spread out into a spectrum.

**Conclusion:**

In summary, the ability of a glass prism to produce a spectrum while a glass slab does not is due to their different geometric shapes and the resulting refraction patterns. The triangular shape of a prism allows for dispersion, while the parallel faces of a slab prevent the separation of light into its constituent colors.

In the case of a glass slab, the two interfaces the light ray has to pass are parallel to each other. In this case the emergent ray is also a white light because the constituents of whitelight which are refracted at different angles at the first interface recombine at the second interface and emerge as a single white light. In the case of a prism,  the two interfaces the light ray has to pass are not parallel to each each other and are inclined at an angle. In this case, the effects of the first interface are not reversed and the colors separated at that first interface continue along different paths upon leaving the glass at the second interface. Hence we observe a spectrum on the other side.