What is the method of obtaining various colors by mixing primary colors in appropriate proportions called?

$(a)$ What is dispersion of white light? What is the cause of such dispersion? Draw a diagram to show the dispersion of white light by a glass prism.
$(b)$ $A$ glass prism is able to produce a spectrum when white light passes through it,but a glass slab does not produce any spectrum. Explain why this is so.

In the spectrum obtained by the dispersion of white light through a prism,which color band is located in the middle?

Which part of the eye helps in changing the focal length of the eye lens to focus the image of objects lying at varying distances from the eye?

$(a)$ What is meant by the least distance of distinct vision?
$(b)$ How does the thickness of the eye lens change when we shift our focus from a distant tree to reading a book?

Why does it take some time to see objects in a dim room when we enter the room from bright sunlight outside?