JEE Main · 2021 · 26 Feb 2021 · Shift 1 · Modern Physics

LED is constructed from

Ga - As - P

semiconducting material. The energy gap of this LED is 1.9 eV . Calculate the wavelength of light emitted and its colour.

[h = 6.63 \times 1 0^{- 34} Js

and

c = 3 \times 1 0^{8} ms^{- 1}

]

Answer: (D)

Solution

A photon emitted when an electron crosses the gap carries the gap energy, so

λ = \frac{h c}{E}

with

E = 1.9 \times 1.6 \times 1 0^{- 19} = 3.04 \times 1 0^{- 19}

J. Substituting,

λ = \frac{6.63 \times 1 0 ^{- 34} \times 3 \times 1 0 ^{8}}{3.04 \times 1 0 ^{- 19}} = 6.54 \times 1 0^{- 7} m

That is

654

nm, which lies at the long-wavelength end of the visible range, so the diode emits light of wavelength

654

nm and red colour.

JEE Main · 2021 · 26 Feb 2021 · Shift 1 · Single correct option