21. GaAs LED’s emit radiation in the :
(a) Ultraviolet region
(b) Visible region
(c) Infrared region
(d) Invisible region
22. LED’s are fabricated from :
(a) Si
(b) Ge
(c) Si or Ge
(d) GaAs
23. Which material is generally used to build LED ?
(a) Compounds of gallium
(b) Compounds of phosphorus
(c) Compounds of germanium
(d) Compounds of silicon
24. A laser diode :
(a) Produces always light of single wavelength
(b) Produces always light of multiple wavelength
(c) Can be made to produce light of single and multiple wavelengths
(d) Produces visible light spectrum
25. The isolation capacitance of an opto coupler is in the order of :
(a) 0.3 – 2.5 μF
(b) 0.3 – 2.5 F
(c) 0.3 – 2.5 pF
(d) 0.3 – 2.5 nF
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