In case of prism two refracting surfaces are not parallel and hence there is deviation of the Ray and dispersion is visible easily. In case of slab two refracting surfaces are parallel to each other and emergent rays are parallel to the incident ray and they are closely spaced which makes it difficult to see the splitting of white light in different colors.

acha my qsn is a bit similar to ur qsn...I asked that dispersion occurs in glass prism but why it doesn't occur in glass slab?? I managed to get an answer "due to the structure of prism" but I don't get this....can anyone help me out..??

Dispersion in Glass Prisms

Introduction
Dispersion is the phenomenon in which white light splits into its constituent colors when passing through a medium. This phenomenon is commonly observed when light passes through a glass prism. However, in the case of a glass prism, dispersion does not occur to a significant extent. This can be explained by considering the properties of glass and the principles of refraction and dispersion.

Refraction of Light
When light passes from one medium to another, it undergoes refraction. Refraction occurs because light travels at different speeds in different mediums. The angle at which light bends when passing from one medium to another is determined by the refractive index of the mediums involved. The refractive index of a material is a measure of how much it slows down the speed of light.

Dispersion and Refractive Index
The refractive index of a material is dependent on the wavelength of light. Different wavelengths of light bend at different angles when passing through a medium, resulting in the separation of colors. This phenomenon is known as dispersion. In the case of glass, the refractive index is slightly different for different wavelengths, but the difference is not significant enough to cause noticeable dispersion.

Material Composition
The composition of glass plays a crucial role in the occurrence of dispersion. Glass is primarily made of silica, which has a relatively low dispersion compared to other materials like water or certain types of crystal. The low dispersion of silica contributes to the minimal separation of colors when light passes through a glass prism.

Angle of Incidence
Another factor that affects the extent of dispersion in a glass prism is the angle of incidence. The angle at which light enters the prism determines the angle at which it emerges. If the angle of incidence is small, the deviation of light is minimal, resulting in less dispersion. However, if the angle of incidence is large, the deviation of light is more significant, leading to a greater separation of colors.

Conclusion
In conclusion, dispersion does not occur to a significant extent in glass prisms due to the properties of glass, including its composition and the low dispersion of silica. Additionally, the angle of incidence plays a role in determining the extent of dispersion. While some dispersion may occur in glass, it is not as pronounced as in other materials, such as water or certain crystals.