LED's have following advantages over incandescent low power lamps.
A. Low operational voltage and less power.
B. No warm up time required.
C. LED must at least have a band gap of 1.8 eV.
D. \( \mathrm{GaAs_{1-x}P_{x}} \) is used for making LED's of different colours
E. The band width of emitted light in LED's is 100 Å to 500 Å
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → All correct
A light emitting diode is a heavily doped p-n junction which, under forward bias, gives out light when electrons and holes recombine. The energy of the photon is about the band gap \( E_g \) of the semiconductor.
(A) Low operational voltage and less power. — Correct. An LED works at a few volts and uses far less power than a filament lamp.
(B) No warm up time required. — Correct. The light comes from recombination at the junction, so it starts and stops at once; there is no filament to heat up.
(C) LED must at least have a band gap of 1.8 eV. — Correct. For visible light the photon energy must be at least that of red light (about 700 nm):
\( \displaystyle E = \frac{hc}{\lambda} \approx \frac{1240\,\mathrm{eV\,nm}}{700\,\mathrm{nm}} \approx 1.8\,\mathrm{eV} \)
(D) \( \mathrm{GaAs_{1-x}P_{x}} \) is used for making LED's of different colours — Correct. Changing the fraction \( x \) changes the band gap, and so the colour of the emitted light.
(E) The band width of emitted light in LED's is 100 Å to 500 Å — Correct. The emitted light is nearly monochromatic, with a spectral width of only 100 Å to 500 Å, unlike the very wide spectrum of a filament lamp.
Statements C, D and E are properties rather than advantages, but all five are true.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Light Emitting Diode – “… The semiconductor used for fabrication of visible LEDs must at least have a band gap of 1.8 eV (spectral range of visible light is from about 0.4 μm to 0.7 μm, i.e., from about 3 eV to 1.8 eV). The compound semiconductor Gallium Arsenide – Phosphide (GaAs1–xPx) is used for making LEDs of different colours. … LEDs have the following advantages over conventional incandescent low power lamps: (i) Low operational voltage and less power. (ii) Fast action and no warm-up time required. (iii) The bandwidth of emitted light is 100 Å to 500 Å or in other words it is nearly (but not exactly) monochromatic.”