实验背景

前提

1. 光可以被偏振这一事实说明了光是横波，而不是纵波(因为纵波无法被偏振)

2. 光的能量是通过光子束传播的

3. 光在传播过程中，一般而言，电向量、磁向量与传播方向三者互相垂直

偏振介绍

定义

光在传播过程中，电磁波振动方向不变，而电场振动方向在特定平面内振动的现象

偏振光的类型(type)

1. $Partially \ polarized$

• 光照到透明介质上，部分反射，部分折射,其中反射光被偏振，折射角被称为Brewster’s angle，在下图中有以下关系

$$ \pmb{折射率 n = tan i} \\\ \\ \pmb{反射光与折射光垂直} $$

2. $Totally \ polarized$

偏振光的状态(States)

1. $linear$

• 电场矢量在特定平面内振动，而垂直方向上不发生振动，这个垂直方向被称为偏振方向，一旦确定就不能改变

• 线偏振光可以通过偏振片实现

2. $circular$

• 电场矢量大小不变，但始终绕光的传播方向旋转

3. $elliptical$

• 被视为线偏振光与圆偏振光的混合

Polarizer

1. 举例

• Polaroid sunglasses

  • 它可以偏振阳光反射产生的眩光(glare from reflected sunlight)

对光偏振的方式

1. 使用偏振片

2. 让光照射透明物体

3. 让光穿过各向异性的材料

实验

实验目的

1. 使用起偏器获得线性偏振光

2. 使用起偏器判断偏振的类型和状态

• 偏振器可以使自然光变为线偏振光

• 检偏器可以鉴别线偏振光、自然光和部分偏振光

• 了解half-wave plate和quarter-wave plate的功能

• 起偏器和检偏器在本质上是一样的

3. 了解波板的功能

实验器材

1. Diode laser: 二极管激光器

2. Detector (to measure the intensity of light):侦测器

3. Two polaroid sheets:两块偏振片

4. microammeter (to display the intensity of light): 微安计

5. Quarter-wave and half-wave plates: 四分之一波板与半波板

• 沿着fast axis的光直接穿过波板，而垂直方向上的光会与它产生$\frac{\lambda}{2}$或$\frac{\lambda}{4}$的相位差

• 这就是双折射现象

实验图示

• Law of Malus

$$ I = I_0 cos^2\theta \\\ \\ \theta表示图中transmission \ axis的夹角 $$

实验过程

实验1: 证明Malus定律

实验2：证明半波板的功能

实验3：证明四分之一波板的功能

实验结论

1.

数据和问题

问题

1. 偏振现象如何证明光是横波

• If we stack two polarizing filters together and make their polarization directions perpendicular to each other, then unpolarized natural light cannot pass through. Only when the polarization direction of the second polarizing filter is the same as that of the first one, can the polarized light pass through.

• This demonstrates that light must be a transverse wave, because only transverse waves can be filtered to have a single direction of polarized light.

2. 区分自然光和偏振光

• 区分自然光和线偏振光

  • Place a polarizer in front of the light source and gradually rotate the direction of the polarizer. If the intensity of light remains unchanged, the light is linearly polarized because only linearly polarized light has polarization in a specific direction.

  • If the intensity of light changes as the polarizer rotates, the light is elliptically polarized or circularly polarized because they can be produced by the superposition of two orthogonally polarized linearly polarized lights.

  • Natural light can be seen as the superposition of linearly polarized light in all possible polarization directions, so the intensity of light passing through the polarizer will change as the polarizer rotates.

• 区分自然光和圆偏振光

  • Passing natural light and circularly polarized light through a polarizer, such as a polarizing mirror or polarizing filter, reveals that the intensity of natural light is reduced by half after passing through the polarizer, while the intensity of circularly polarized light remains unchanged.