Question

Why do we study the Dispersion phenomenon using a glass prism and not a glass slab ?

• A. Light dosen't undergo any deviation inside a glass slab; so cannot be used to study dispersion.
• B. In glass slab all of dispersion that takes place at the first interface is exactly reversed at the second parallel surface; undoing the effect of the first interface.
• C. We can use glass slab to study dispersion.
• D. None of the above.

Answer 1

The correct option is B In glass slab all of dispersion that takes place at the first interface is exactly reversed at the second parallel surface; undoing the effect of the first interface.

We study the Dispersion phenomenon using a glass prism and not a glass slab because when light pass through glass slab, it encounters TWO interfaces--one entering and the other leaving. All of the bending (and dispersion) that takes place at the first interface is exactly reversed at the second, 'undoing' the effect of the first interface; so although the emerging ray of light is displaced slightly from the entering ray, it travels in the same direction as the incoming ray and all wavelengths that separated at the first interface are re-combined.

In case of prism the second interface is NOT parallel to the first, the effects of the first interface are NOT reversed and the colors separated at that interface continue along different paths upon leaving the prism.

Hence, option $\left(b\right)$ is the correct answer.