Q159 / 160 · AP EAMCET 2019 (21 Apr Shift 1) · Physics

For an $L E D$ to emit light in visible region of the electromagnetic spectrum, it can have energy band gap in the range of, (Plank's constant, $h=6.6 \times 10^{-34} \mathrm{Js}$ and speed of light, $e=3 \times 10^8 \mathrm{~ms}^{-1}$ in vacuum)

  • A
    0.1 eV to 0.4 eV
  • B
    0.9 eV to 1.6 eV
  • Correct answer
    C
    1.7 eV to 3.1 eV
  • D
    0.5 eV to 0.8 eV

Solution

Key Idea Frequency of visible rays range from $4 \times 10^{14} \mathrm{~Hz}$ to $7 \times 10^{14} \mathrm{~Hz}$. Maximum and minimum energy of photon in visible light, $$ \begin{aligned} E_{\max } & =h v=\frac{6.6 \times 10^{-34} \times 7 \times 10^{14}}{1.6 \times 10^{-19}} \\ & =2.88 \mathrm{eV} \simeq 3 \mathrm{eV} \\ E_{\min } & =\frac{6.6 \times 10^{-34} \times 4 \times 10^{14}}{1.6 \times 10^{-19}} \\ & =1.65 \mathrm{eV} \simeq 1.7 \mathrm{eV} \end{aligned} $$ So, the correct option matches between the energy $1.7 \mathrm{eV}$ to $3 \mathrm{eV}$ is $(\mathrm{c})$,

Hence, the correct answer is option: 'C'.

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