In the second mode of working, the crystal fibre uses the regular array of holes to influence light in much the same way as the arrangement of atoms in a crystal can determine whether electrons are able to travel easily through it and hence whether the crystal is electrically conducting or not.

ECONOMIST: A new approach to making optical fibres

With such an option, also called a brokerage window, you can essentially turn your 401(k) plan into a regular brokerage account where you can trade a wide array of mutual funds, in some cases also exchange-traded funds and stocks.

FORBES: Gray Lady Points Readers To a 401(k) Trap