PHYSICS EXPERIMENT 2

Experiment No. 2

Object: -     To determine Planck’s constant using light emitting diodes [LED’s] by observing the ‘reverse photo-electric effect’.

Apparatus Required:- Planck’s Constant kit and LED’s

Theory:   The energy of a photon is given by the equation:

                                                       E = h                            ……..(1)

Where E is the energy of photon   is its frequency, and h is a constant. In the case of the photoelectric effect, an electron is emitted from a metal if the energy of the photon is greater than the work function of the metal. If the energy of said photon is greater than the work function of a given material then the electron emitted possesses a voltage, which equals the difference in these energies. In the case of an LED’s the opposite is true. If an electron of sufficient voltage is passed across a material then a photon is emitted whose energy is equivalent to the work function of that material. The voltage at which this effect observed is the ‘turn on voltage’. This effect is not normally observed in metals and other typical substances because the photons emitted are usually outside the range of visible light, usually somewhere in the infrared.         The energy of the photons emitted should then be the same as the energy of a given electron. Since:

                                                P = IV                                      ……. (2)

Where P is power, I is current and V is the voltage of a system. The energy of one electron is the charge of an electron (i.e. the current flow of one electron per second in amps) times the voltage. Using this knowledge we then from the equation:

                                                E = eV                                      ……… (3)

Where,   e = 1.6 × 10^-19 C (electron charge)

We then solve equation (1) for h and replace the E term with the equivalent of E       in equation (3), as well as replace  with:

                                                                       …… (4)

     Where c = 3×10⁸ m/sec (speed of light)

We then get:

                                                                     …… (5)

          This equation we will use to determine Planck’s constant.

Procedure:

1.    Make the connection in the kit as shown in the figure 1.

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 Voltage on X axis and current on Y axis.

4.    The linear portion of the I/V curve is extra plotted back to the X axis .

5.    The intercept is the point at which the voltage equals to the barrier potential. Put this value in table 2. 

DIAGRAM:- 

Observation Table:

S.No.	Voltage (Volts)	Current     IRed (μA)	Current          Iyellow  (μA)	Current          IGreen  (μA)	Current     IBlue (μA)
1
2
3
4
5
6
7
8
9
10

Calculation:

                   Planck’s Constant is                                            

Where c = 3×10⁸ m/sec (speed of light)

                               e = 1.6 × 10^-19 C (electron charge)

S. No.	L.E.D. Colour	Voltge V [Volts]	Wavelength λ [nm]	Frequency [Hz]	Energy [J]
1	Blue		430
2	Green		565
3	Yellow		590
4	Red		627

Result:

Observed value of Planck’s constant is h=…………………Js.

Percentage error:

                                                                                                                                                        (Standard value~ Observed Value) X 100

                  % ERROR =

                                                                    Standard Value

Source of Error:- 

1.          Some noise may be created in the system during Experiment which can be minimized by immersing the diode in the Ice during Experiment [if available].

2.          Connect the LED properly to Jack provided on front panel.

VIVA-VOCE

Q.1. Define Photoelectric effect?

Ans: When light falls on metal surface, an electron is emitted from a metal if the energy of the photon is greater than the work function of the metal.

Q.2 What is Reverse Photoelectric effect? 

Ans:  If an electron of sufficient voltage is passed across a material then a photon is emitted whose energy is equivalent to the work function of that material. The voltage at which this effect observed is the ‘turn on voltage’. In case of LED reverse photoelectric effect works.

Q.3 Can we observe reverse photoelectric with Metal surface?

Ans : This effect is not normally observed in metals and other typical substances because the photons emitted are usually outside the range of visible light, usually somewhere in the infrared Range.

Q.4  What is the full form of LED?

Ans: Full form of LED is Light Emitting Diode.

Q.5 What is the standard value of h.

Ans:   h= 6.6x10^-34 Js.

Q.6: What is LED?

Ans: A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it.

Q.7 What is the learning outcome of this practical?

Ans: Using this practical we can find out the value of Planck constant using LED.