Full form of MOEMS is -

Correct — option (2), Micro-Opto-Electro-Mechanical-Systems.

MOEMS is read letter by letter: M for Micro, O for Opto (light), E for Electro (electrical), M for Mechanical (moving parts) and S for Systems.

Wikipedia's article on the technology (March 2021 revision) describes microoptoelectromechanical systems (MOEMS), also known as optical MEMS, as integrations of mechanical, optical and electrical systems that sense or manipulate optical signals at a very small size.

So the expansion is Micro-Opto-Electro-Mechanical-Systems.

The idea to remember: MOEMS is MEMS with optics — very small machines that handle light.

Why the others are wrong

• (1)
  Micro-Optic Electronic Media Source — The letters fit, but the words do not describe the device. It has no "Mechanical" and no "Systems".

  The defining feature of MOEMS is that it joins mechanical, optical and electrical systems at a very small size, as Wikipedia puts it. "Media Source" names neither the moving parts nor the system.

• (3)
  Mega Operations Electronic Media Software — "Mega" signals something very large, the opposite of the micro scale these devices work at. Wikipedia's MEMS article gives MEMS components as 1 to 100 micrometres in size.

  "Operations", "Media" and "Software" are computing words; MOEMS are physical devices with mechanical, optical and electrical parts, not software.

• (4)
  Micro-Optic-Electro-Mechanical Source — This option gets the first four words nearly right, but it ends in "Source". The final S in both MEMS and MOEMS stands for Systems.

  Wikipedia's definition calls MOEMS integrations of mechanical, optical and electrical systems, not a source of anything.

MEMS, microelectromechanical systems, are microscopic devices, particularly those with moving parts. Wikipedia's MEMS article gives their components as 1 to 100 micrometres in size, with devices generally from 20 micrometres to a millimetre.

MOEMS add optics to MEMS: they combine mechanical, optical and electrical parts to sense or manipulate light at a very small size.

Wikipedia lists the optical switch, optical cross-connect, tunable VCSEL and microbolometers as MOEMS devices. It says they are usually made with micro-optics and standard micromachining, using materials such as silicon, silicon dioxide, silicon nitride and gallium arsenide.

Key facts

• MOEMS stands for micro-opto-electro-mechanical systems, also called optical MEMS (Wikipedia, March 2021 revision).
• MEMS stands for microelectromechanical systems: microscopic devices, particularly those with moving parts.
• Wikipedia's MEMS article gives MEMS components as 1 to 100 micrometres; devices generally range from 20 micrometres to a millimetre.
• Wikipedia lists optical switches, optical cross-connects, tunable VCSELs and microbolometers as MOEMS devices.
• At the nanoscale, MEMS merge into nanoelectromechanical systems (NEMS) and nanotechnology (Wikipedia, MEMS).

Common traps

• Source versus Systems: option (4) has the first four words but ends in 'Source'; the S in MEMS and MOEMS is Systems.
• Letters that fit are not enough: 'Media' and 'Software' match the initials but drop the mechanical part that defines the device.
• Micro, not Mega: the prefix marks the tiny scale of the devices, the opposite of mega.

UnlockIAS will link similar questions from RAS Pre 2018 here once that paper is published on this site.

• practice — not a real PYQ

  MEMS, a technology of microscopic devices with moving parts, stands for –

  • (a)Micro-Electro-Mechanical Systems
  • (b)Micro-Electronic Media Systems
  • (c)Mega-Electro-Mechanical Sensors
  • (d)Micro-Energy Management Software
  Answer(1) — Wikipedia's MEMS article gives microelectromechanical systems, also written micro-electro-mechanical systems. Option (2) drops the mechanical part, option (3) uses the wrong scale prefix and wrong final word, and option (4) names software, not devices.
• practice — not a real PYQ

  The full form of NEMS, the nanoscale counterpart of MEMS, is –

  • (a)Nano-Energy Monitoring Software
  • (b)Network-Enabled Mechanical Sensors
  • (c)Nano-Electro-Mechanical Systems
  • (d)Nano-Electronic Media Systems
  Answer(3) — Wikipedia's MEMS article says MEMS merge at the nanoscale into nanoelectromechanical systems (NEMS). Option (1) names software, option (2) replaces Nano with Network, and option (4) drops the mechanical part.