Dispersion in Prism:
When white light passes through a prism, it gets dispersed into its constituent colors. This phenomenon is known as dispersion. The reason why a prism shows dispersion while a rectangular glass slab does not is due to the shape and refractive index distribution within the prism.

Shape of Prism:
A prism has a triangular shape, which causes light to undergo refraction at two different surfaces. The angle of incidence and the angle of refraction are not the same at these surfaces. This variation in the angle of refraction leads to the separation of different colors of light, resulting in dispersion.

Refraction and Change in Velocity:
When white light enters a denser medium, such as a prism, it undergoes refraction. Refraction occurs because the speed of light changes as it moves from one medium to another, due to the change in refractive index. The refractive index of a medium determines how much the light gets bent when passing through it.

Velocity of Light:
The velocity of light in a medium is inversely proportional to the refractive index of that medium. As white light enters a denser medium like a prism, its velocity decreases due to the higher refractive index. The different colors of light have different wavelengths, and each wavelength experiences a different change in velocity.

Dispersion and Wavelength:
When white light enters a prism, its constituent colors (different wavelengths) experience different amounts of refraction due to their different velocities. This causes the colors to bend at different angles and separate from each other. The shorter wavelengths (violet and blue light) bend more than the longer wavelengths (red and orange light). As a result, a spectrum of colors is observed, creating the dispersion of white light into its constituent colors.

Conclusion:
In summary, a prism shows dispersion because of its triangular shape, which causes the light to refract at different angles at its surfaces. When white light enters a denser medium like a prism, its velocity decreases due to the change in refractive index. This change in velocity leads to the separation of different colors of light, resulting in dispersion. Conversely, a rectangular glass slab does not show dispersion because it does not have the necessary shape and refractive index distribution to cause the separation of colors.

I think glass slab has two parallel sides so it can't show dispersion.. according to Snell's law sin r2/ sin i2= g mew a where at last we find the result sin r1= sin r2 thats mean obviously it would be i1= i2..