Many modern electronic devices are designed using quantum mechanics. Examples include lasers, electron microscopes, magnetic resonance imaging (MRI) devices and the components used in computing hardware. The study of semiconductors led to the invention of the diode and the transistor, which are indispensable parts of modern electronics systems, computer and telecommunication devices. Another application is for making laser diodes and light-emitting diodes, which are a high-efficiency source of light. The global positioning system (GPS) makes use of atomic clocks to measure precise time differences and therefore determine a user's location.