How the thermos flask prevents heat loss by radiation?

A thermos bottle prevents heat transfer to the outside by using a vacuum between the walls of the bottle.

The innerwall is usually made of glass while the outside wall is made of a plastic.

Metal walls also exist.

A vacuum doesnt conduct, since you need a substance to conduct heat.

Same goes for convection, warm and cold liquids / gasses can rise and drop all they want, but the heat cant be passed on to the outside wall.

Radiation is prevented by using reflective surfaces, which bounce back into the substance you are carrying in the bottle.