Question

Question: An LED is connected in series with a resistor and a power supply of variable voltage. The supply voltage starts from —5 V and increases linearly with time to a maximum of +5 V. The figure shows the supply voltage the voltage across the diode, and the current in the Circuit as functions of time. The LED does not glow for the whole time shown. (a) Explain the plots, and identify the period when the LED glows. (b) Approximately what color is the light produced by the LED?

Short answer

Answer

(a) The band gap of the LED is 2.5e V .

(b) The light produced by the LED is cyan color light.

Step-by-step solution

Given data

The given diagram is shown in figure 1.

Figure 1

Concept of the energy of a photon

The expression to determine the wavelength of the light is, $\mathit{\lambda }\mathbf{=}\frac{\mathbf{h}\mathbf{c}}{\mathbf{V}}$ .

Determine the explanation for the plots and identify the period when the LED glows

(a)

The LED only glows when the current flows in the circuit and the time period is $15\text{s}<t<20\text{s}$.

From the above graph the voltage required by the LED to turn on is, V= 2.5 e V .

Therefore, the band gap of the LED is V = 2.5 e V .

Determine the color of the light produced by the LED

(b)

Calculate the wavelength of the light.

$$\begin{gathered} \lambda =\frac{hc}{V} \\ =\frac{1240 \text{eV.nm}}{2.5 \text{eV}} \\ =496 \text{nm} \end{gathered}$$

The above value shows that the light is of cyan color.

Therefore, the light produced by the LED is cyan color light.