Step-by-step explanation:
aluminum foil be useful in a thermos design because Aluminum foil reflects radiating heat.
Aluminum foil acts as insulation serves productively and efficiently and stops the transmission of heat that helps to reduce wastage of heat.
Aluminum foil work as thermal insulation which is utilized for multiple works where it can be useful for preventing unwanted energy gain or energy loss.
Thus this is the reason aluminum foil is useful in a thermos design.