We are going to determine the value of Planck's constant h.
We connect the LED to a power supply V.
We put one two meter stick perpendicular to the other one meter stick, and we one diffraction grating with 300 lines/mm at the end of two meter stick. We put the LED at the other end of the two meter stick, which is the same location of the intersection of two sticks.
We have the LED glow. We have the LED oriented right. We solder the resistor. We also screw the wires down in alligator clips. The battery "fresh" at least 9V.
We have the energy of one electron in a completed circuit E = qV, and we have the energy of one photon is E=hc/(wavelength)
We have the E=qV and E=hc/(wavelength)
we thus have h=qV(wavelength)/c and V=hc/q(wavelength)
We measure the voltage V across the LEDS with different color.
One student observe the fringe of different color of the LEDs through diffraction grating, and the other point the position of the fringes and mark the reading of that position.
We have the data below:
| LED | Voltage (V) | Wavelength (m) | h(Js) | % Error (%) |
| RED | 1.85 | 8.25083E-08 | 8.14081E-35 | 87.72124573 |
| GREEN | 2.57 | 1.415E-07 | 1.93949E-34 | 70.74672778 |
| BLUE | 2.67 | 1.20299E-07 | 1.71306E-34 | 74.16193823 |
| YELLOW | 1.9 | 9.33804E-08 | 9.46255E-35 | 85.72768391 |
We see there are mixture of color of LED through the spectroscope. All of the LED are mixture of color. It is because the LED is not made prefect, the electrons in the LED may jump to different levels that it is designed. The red LED gives the largest of error as the red LED is not made perfect. We can see the wavelength of the LEDs is proportional to potential. We have the voltage across the sky and earth is lower in the day time, as the voltage is smaller, we have the wavelength of the sky is shorter, the sky is blue. We have the voltage across the sky and earth is higher in the setting of sun, as the voltage is larger, we have the wavelength of the sky is longer, the sky is red.
| LED | Voltage (V) | 1/Wavelength (m^-1) |
| RED | 1.85 | 12120000 |
| GREEN | 2.57 | 7067142.857 |
| BLUE | 2.67 | 8312593.22 |
| YELLOW | 1.9 | 10708887.22 |
The equation y = -2^-7 x + 3.93. The slope is -2 * -7, and it represents the value of hc/q. So we have the Planck's constant 3.616*10^-34 J/Hz.
We successfully measure the voltage across each led. We have the Planck's constant calculated that is same digital but half of the theoretical value 6.63*10^-34. There exists errors, for example, there is light come from background, which influence the pattern of light observed through the grating slit. We thus measure the wavelength inaccurate.
