What is spectrum? What is the name of glass shape used to produce a spectrum?

What will happen if another similar glass prism is placed upside down behind the first prism?

When a beam of white light is passed through a prism, it splits to form lights of seven colours. Is it possible to recombine the lights of seven colours to obtain the white light again? Explain your answer.

How many colours are there in a full spectrum of white light? Write the various colours of spectrum in the order, starting with red.

What is meant by dispersion of white light? Describe the formation of rainbow in the sky with the help of a diagram.