# Determine the Planck’s Constant h by Laser

Item no.: F22

##### Introduction

Rather than the energy continuity considered in classic physics, the energy level of a photon or an electron, viewed from quantum theory, exhibits an integer multiple of hf (here h is planks constant and symbol
of f indicates the frequency of light wave). That means a light is emitted from or absorbed by an electron and the energy, hf, will be quantized. While laser light is irradiated to a light-emitting diode, the induced current will be further retarded due to weaker forward voltage unable to overcome the energy barrier, i.e., most electrons are still constrained in depletion layer. However, a current of fast growth starts being induced if potential supplied V0 is just adequate to break through the barrier. Thus planck’s constant h might be determined from the equivalence of energy emitted from laser diode, hf, and power absorption of photon e ( V-V0).

##### Objectives
1. Measure the Laser’s wavelength by reflective diffraction or grid diffraction.
2. Utilize the V-I characteristic of diode Laser to find the emitted voltage.
3. Determining Planck’s constant.

Experiment—Laser wavelength by Reflective diffraction

Base on the pattern of interference fringes reflected from the multi- gratings on steel straightedge, the wavelength of laser might be determined. Here the bright fringes occurs at δ=dCosθ -dCos(θ +φ n )=nλ    where f= vc/λ

##### Experiment—V-I characteristic of diode laser

Base on V-I characteristic profile accessed from diode laser, an approximated equation following the linear behavior might be given. Here Vo, an interested point of linear equation and horizontal axis ( induced current), indicates the breakdown voltage of diode laser. And then the estimated planck’s constant h, yielded below, is close to the result measured by Millikan (1916).

1. 測量半導體雷射波長
2. 推算半導體雷射截止電壓
3. 推定普朗克常數
##### 實驗器材：
1. 半導體雷射3V及俯角調整器 x1
2. 半導體V-I 特性實驗模組 x1
3. 短鋼尺x1
4. 400孔/2.54 cm不鏽鋼網光柵片x1
5. 三用電錶 x2
6. DC 電池盒 x1
7. 導線 x4