Which type of glass is used in the manufacturing of lenses and prisms?

Which of the following statements is correct?

$SiCl_4$ on hydrolysis forms '$X$' and $HCl$. Compound '$X$' loses water at $1000^{\circ} C$ and gives '$Y$'. Compounds '$X$' and '$Y$' respectively are:

What is the increasing order of stability of dihalides of $Si$,$Ge$,$Sn$,and $Pb$?

What would be the major product of the following reaction: $SiCl_4 \xrightarrow{\text{Excess of } H_2O}$?

On controlled hydrolysis and condensation,$R_3SiCl$ yields