Shiny outer surfaces are not needed in a vacuum flask. In fact, they are often silvered to reduce heat transfer by radiation.

Here's why:

* Vacuum: The primary way a vacuum flask insulates is through the vacuum between its inner and outer walls. This vacuum eliminates heat transfer through conduction and convection.

* Radiation: Even with a vacuum, heat can still transfer through radiation. Shiny surfaces are excellent reflectors of radiation, minimizing the amount of heat that can enter or leave the flask.

* Silvering: Silver is a highly reflective metal, making it an ideal material for reducing radiative heat transfer.

In summary, shiny outer surfaces in a vacuum flask are not needed for insulation, but rather to further reduce heat transfer by radiation.