Last updated on Sunday, October 15th, 2023

Planck’s Constant Experiment Viva Questions:

Planck’s Constant using LED

Determination of Planck’s constant by using light-emitting diodes (LEDs).

APPARATUS: Planck’s constant kit, wires, graph paper, and 3-4 LEDs.

FORMULA USED: Planck’s constant is h = eV /ν, where e is an electronic charge, V is the voltage reading in a voltmeter, and v is the frequency of a particular LED color.

THEORY:   The energy of a photon is given by the equation: E = h ν                          (1)

Where E is the energy of a photon, ν is its frequency, and (h) is Planck’s constant.

CASE 1: In the case of the photoelectric effect:

1. An electron is emitted from the metal only if the energy of the incident photon is greater than the work function of the metal.
2. These electrons can be attracted by the anode in a circuit and as a result of a current, you can observe with respect to the incident radiations (color).
3. If you need to measure the voltage difference, you can use resistance (the choice is open) in the circuit.
4. This voltage will be corresponding to the particular incident radiation energy.
5. If you need to know any other parameters, you can use this information (the current, the voltage, the color of incident radiation means the wavelength or frequency, so you can find the E=hv of the incident photon).

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CASE 2.  In the case of LEDs, the opposite of the above-mentioned working is true.

1. Here, in LED, If an electron of sufficient electrical energy (eV) is passed across a material then a photon emits.
2. But remember, the meaning of passing the electron across a material here is a diode.
3. That has two types of semiconductors (n- and p-type) and a p-n junction. Near the p-n junction, there is a specific region known as the depletion region.
4. These electrons start from the n-region and after crossing the barrier (depletion region) reach the p-region where they recombine and as a result, a photon emits.
5. But to understand it you have to understand the energy band concept, this explanation is given in the semiconductor laser topic.

Planck’s Constant Experiment Supporting Concepts to LED

Please note that all materials don’t show the photoelectric effect and emission of radiation like in LED. For LED we use Ga As the material that shows the optical properties when electron-hole recombination takes place. If you got the flavor of the second case then it will be clear that you need forward biasing for this purpose. When you provide a sufficient voltage to the electron to cross the barrier only then it recombines with holes. And only then you can see the photons means that light. So initially you can not see the light when you provide potential to the LED. But when you reach the threshold value of voltage where the electron is able to cross the junction then only you see the light. This value of potential you know is known as stopping potential. Now the point is clear, also the emitted photon energy (hv) will be the same as the electrical energy of the electron  (eV). Because of this reason you use; eV  =   h ν                                        h  = eV / ν This equation we will use to determine Planck’s constant.

PLANCK’S EXPT. PROCEDURE:

1. Make the connection in the kit.
2. Take the current measurement of each LED by varying the voltage as given in the table.
3. Plot the curve on the graph paper between the frequency of color on the X-axis and electrical energy on Y-axis for all LEDs.
4. The slope of the curve will give a measured value of Planck’s constant.
5. Compare the measured value with the standard value and check the percentage error.

OBSERVATIONS:

As mentioned in the procedure plot a graph between the last two-column of the above table that is the frequency of the particular LED and energy (eV). Take the slope of this graph and this will be your measured value of Planck’s constant. Now compare it with the standard value (6.62607004 × 10^-34 m² kg / s) and explain the percentage error. Check yourself that what can be the reasons for this percentage error.

What you can analyze in Planck’s Constant Experiment?

From your observation, you can also analyze the stopping potential of all the LEDs. How? Just see you have taken readings for each LED, either that one when the LED starts to glow or also after it with some intervals of potential. So you have a set of reading with potential and current for each LED. When you will plot it for every LED you will observe that every LED starts with a specific value of the potential. This value of the potential is known as the stopping potential and this way, you can analyze the stopping potential for each color LED. But to determine Planck’s constant you will need a graph that points explained above.

How to Observe Planck’s Constant from this Experiment?

1. Take current and voltage readings for each LED (Red, Green, and Blue)
2. Plot a graph on the graph paper using proper scaling of the variables (Voltage and Current), as shown in the above figure.
3. Now find the knee voltage for each one, and make one more graph between stopping potential and frequency as shown in 2nd part of the above picture.
4.  Take the slope from here and use it in the formula as you can see right side of the above figure.
5. Calculate the percentage error by using the formula and check the reasons for it, if more than 20%.

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Points of Care when You make the Graphs:

1. When you are plotting the graph between any two physical variables of the experiment, first make a scale along the X-axis and Y-axis. For example, 10 small boxes are equal to 1 volt along the Y-axis or 1 mA along the X-axis.
2. Then mark the points where it lies
3. After it joins all the points. Now at this point keep remembering, it is not necessary that all your points will lie in a straight line. You have to draw a line that passes over to or near each point. Like, I have drawn the line above, which is passing near to that point which was away from it.
4. Now you can take any two points on the line except for them which were marked earlier. Find the slope and put it in the required formula. This will be your observed value.
5. I hope it will be enough to get my point, for any doubt you can ask through the comment box.

VIVA QUESTIONS Planck’s Constant Experiment

1. How Planck’s constant is determined? “Planck’s constant is determined by using the light-emitting diode. The LED works on the principle of semiconductor diode, to flow the current from n to p-type semiconductor a minimum potential is required. When electron reaches the p-type semiconductor they fuse there and emits a photon. So in this way, we can compare the potential energy eV₀ with the energy of the photons h and nu.

Q1. How it is different from Si/Ge diode?

Ans. LED is made of Ga As Gallium Arsenide semiconductor material which shows the optical properties when electron-hole recombination takes place.  While Si/Ge-made diode or semiconductor shows thermal properties, they start to heat up when current flows. On the other hand LED glow.

2Q. How LED works?

2Answer:  When forward bias gives to the light emitting diode (LED), immediately LED doesn’t glow and takes some time. The minimum potential at which LED starts to glow is known as the stopping potential. The light from LED is the result of electron and hole recombination in the depletion region.

3Q. Why Minimum potential is required to glow the LED?

3Ans. There is a potential barrier for the charge carrier to cross the junction, to overcome it they required this amount of potential energy. And then after with small change in potential, they cross the junction, and current flows through the LED.

4Q. In the photoelectric effect, a suitable frequency of photon falls on an electron in an atom and ejects the electron. In LED when electron-hole recombination takes place a photon emits. How do you see these two phenomena?

4Ans: Both phenomena are different, in the case of the “photoelectric effect” to emit the electron, from the surface of a material, a minimum energy of threshold frequency is required. While on the other hand, in a light-emitting diode (LED) photon emits when electron-hole recombination takes place above the threshold value of the voltage; known as stopping potential.

5Q. Why do you put two different energies eV and hv equal, what is the condition that they satisfy in the LED?

Answer: From the question, no. 3 you understood the stopping potential, and a small potential above it shows the deflection in current, simultaneously glowing in the LED. The potential energy eV is responsible to recombine the electron-hole recombination, by which a photon of the energy hv emits. Because of this reason, one can put eV = hv

6Q.Which material do we use in the LED?

Answer: Gallium Arsenide which is of a semiconductor nature.

7Q. How are photons emitted from the LED and from which section of the LED?

Answer: When electron-hole recombine photons emit, these emit from the depletion region.

8Q. How do you explain the working of LED by using the energy band diagram in forward biasing?

Answer: You have to follow a detailed lecture for this purpose. Click Here Working of LED.

7Q. What happens when you provide the forward bias to the LED in terms of the conduction band & valence band in the depletion region?

Answer: Conduction band and valence band drift in the depletion region, for more info watch this video.

8Q. Why does not LED starts to glow immediately when you provide the forwarding bias to that?

Answer: Because of the potential barrier at the junction for the charge carriers.

9Q. Explain the concept of stopping potential in semiconductor diode by V-I Characteristics.

Answer: In the above figure, it is pointed out for the red, green, and blue LEDs.

10Q. Why does Blue color LED stopping potential greater than the Red color LED?

Answer: from the relation eV₀=hν

further  ν = c/λ so  eV₀=hν

eV₀=hc/λ in this relation e, h, and c are constants, so you can see V₀   ∝1/λ

for small wavelength, stopping potential is higher than the larger wavelength. As you can see in Table. 1 the blue color wavelength is 475 nm while for the red it is 650 nm.

11Q. Can we achieve the population inversion process in LED too? if yes what is the condition for that? if not then why?

Answer: Highly doped semiconductor material is required, the first condition, so not possible in LED. and the second reason is a threshold current beyond that we achieve population inversion in the semiconductor LASER.

12 Q. What symbol do we use for the Light Emitting Diode?

Answer: Similar to the normal diode but with arrows, which indicate the emission of light

↑↑↑

—-|>-

13Q. What information do we get from Planck’s Constant, and how one can say that radiation is in a discrete form of energy?

Answer: Energy is in the discrete form that signifies by the photon energy;  hv

Sources/Information Required to Explain/Understand the Experiment :

1. Working of a p-n Junction diode 2. Depletion region Concept/Idea along with potential barrier for Si/Ge 3. Semiconductor Material name that shows the optical property 4. Mainly energy band diagram of the p-n junction diode and how electron transit from n to p side in depletion region in case of forward biasing. 5. The basic idea of electrical and radiation energy so can understand why they keep equal two different energy. 

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