想象我们有平行的原子面，就像是微观世界的镜子层层叠叠。X光以角度θ入射，入射角等于反射角。相邻原子面间距为d，那么从相邻两个原子面反射的光线之间的光程差为：
δ=2dsinθ
当这个光程差等于波长的整数倍时，反射光线相互加强，形成明亮的衍射斑点。这就是布拉格父子发现的黄金法则：
2dsinθ=nλ
其中n是任意整数，称为反射级数；d是(hkl)晶面间距；λ是X射线波长；θ是入射线与反射面的夹角，称为掠射角或布拉格角。
布拉格实验发现了"选择反射"现象：只有当X射线以特定角度入射时，才能记录到反射线，就像是晶体在特定角度才会"回应"X射线的"问候"。这是X射线衍射仪的基本原理，它让我们能够解读晶体结构的"密码"。在实际操作中，记录装置与样品台以2:1的角速度同步转动，确保记录装置始终处于接收反射线的最佳位置。---**Extraction Content:**
**Chart/Diagram Description:**
* **Type:** Ray diagram illustrating the geometry of X-ray diffraction (Bragg's Law).
* **Main Elements:**
* **Plane:** A horizontal blue line labeled 'A' at both ends represents a crystal plane or lattice layer.
* **Scattering Centers:** Blue dots on the blue line represent scattering centers (atoms). Two points are explicitly labeled 'M₁' (red) and 'M' (red).
* **Incoming Rays:** Two rays (lines with arrows) pointing towards the plane.
* A red line labeled 'L₁' incident on point M₁.
* A green line labeled 'L' incident on point M.
* **Reflected Rays:** Two rays (lines with arrows) originating from the scattering centers and going away from the plane.
* A red line labeled 'N₁' originating from M₁.
* A green line labeled 'N' originating from M.
* **Angles:**
* An angle labeled 'θ' between the incoming ray L and the blue plane line. This angle is labeled "布拉格角 (掠射角)" (Bragg Angle (Glancing Angle)).
* An angle labeled '2θ' between the dashed green line (representing the original direction of the incoming ray L) and the reflected ray N. This angle is labeled "衍射角" (Diffraction Angle) in a yellow box.
* **Perpendiculars:** Lines perpendicular to the incoming rays (L₁, L) and reflected rays (N₁, N) are drawn from points M₁ and M, indicating the geometry for calculating path difference. Right angle symbols are shown at the intersections.
* **Direction Lines:**
* A black line labeled "同位向" (Isodirectional/Same direction) is drawn parallel to the incoming rays L₁ and L.
* A black line labeled "同光程" (Same path length) is drawn parallel to the reflected rays N₁ and N.
* **Labels and Annotations:** All text labels mentioned above are present, including "A", "M₁", "M", "L₁", "L", "N₁", "N", "θ", "2θ", "布拉格角 (掠射角)", "衍射角", "同位向", "同光程".
* **Relative Position and Direction:** Incoming rays come from the top-left, hit the points M₁ and M on the horizontal plane 'A', and reflect towards the top-right. The angles θ and 2θ are clearly indicated relative to the plane and the original ray direction.
**Textual Information:**
* **Labels:**
* A
* M₁
* M
* L₁
* L
* θ
* 布拉格角 (掠射角) (Bragg Angle (Glancing Angle))
* N₁
* N
* 2θ
* 衍射角 (Diffraction Angle)
* 同位向 (Isodirectional/Same direction)
* 同光程 (Same path length)
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在晶体中，原子按照规律的方式排列，形成了一层层平行的原子面。这些原子面就像微观世界的镜子，当X射线入射时会发生反射。相邻原子面之间的距离用字母d表示，这个距离对于理解X射线衍射现象至关重要。
当X射线以角度θ入射到晶体的原子面上时，会发生反射现象。根据反射定律，入射角等于反射角。图中显示了两条X射线分别入射到相邻的原子面M1和M上，产生相应的反射光线。这种几何关系是理解布拉格衍射的基础。
当X射线从相邻的原子面反射时，由于几何路径的不同，会产生光程差。通过几何分析可以得出，这个光程差等于2d乘以sin θ，其中d是相邻原子面之间的距离，θ是入射角，也就是布拉格角。黄色虚线显示了额外的光程路径。
布拉格父子发现了X射线衍射的黄金法则：当光程差等于X射线波长的整数倍时，从各个原子面反射的光线会发生相长干涉，形成明亮的衍射斑点。这个条件用数学公式表示就是2d sin θ等于n λ，其中n是反射级数，这就是著名的布拉格定律。
布拉格实验发现了选择反射现象：只有当X射线以特定角度入射时，才能记录到反射线，就像晶体在特定角度才会回应X射线的问候。这是X射线衍射仪的基本原理。在实际操作中，记录装置与样品台以2比1的角速度同步转动，确保记录装置始终处于接收反射线的最佳位置，从而解读晶体结构的密码。