Look at the picture of a thermos flask showing five of its most important features. What is the function of the vacuum in the thermos flask?

In thermos flask a vacuum is created in the gap between the glass walls by pumping out air in the space between the walls and sealing it near the neck and foot. The vacuum between the glass walls prevents heat transfer by conduction. This also reduces the heat loss as there is no medium present to carry the heat away.

Look at the picture of a thermos flask showing five of its most important features. Why are the walls of the flask made of glass?

A thermos flask maintains the temperature of the substance present in it. This is achieved by minimising the exchange of heat by conduction, convection and radiation. The inner flask is generally made of some type of glass (for example, borosilicate glass) as glass is a poor conductor of heat. When heat radiation tries to enter or leave the flask, the silvered surfaces of glass reflect it straight back, and thus, reduce heat exchange due to radiation.