Questions About Prisms - Refractive Index & Parallelograms

[Misr]

1-the faces of the prism should be covered with cryolite because the refractive index of cryolite is more than that of air and less than that of glass,so the loss of incident rays could be avoided on entering or leaving the prism.

2-A parallelogram doesn't disperse light because it works as two symmetric reverse prisms so the dispersion caused by one of them is canceled by the other


I don't understand all of this.Do u have a better explanation?

 

[Drakkith]

When light enters or exits something, the angle of entry or exit determines how far it will deviate from it's original path. A parallelogram has parallel opposite sides, so when the light enters it's path is altered a certain amount and when it exits the the mediums are arranged oppositely than entry (going from glass to air instead of air to glass) and the light is refracted opposite the direction that it entered at and ends up converging on itself again. When entering a triangular prism, the entry and exit sides are not parallel to each other, so when the light exits the far side it is refracted in a way that causes the different wavelengths of light to separate and disperse.

That's not a technical description, but I think it is pretty much correct.

 

[Misr]

That's a very good answer,
I hope to find an answer to my first question
Thanks very much