半波带法是处理次波相干叠加的一种简化方法。菲涅尔的衍射公式要求对波前作无限次分割,半波带法则用较粗糙的分割代替,虽然不够精细,但可以很方便地得出衍射图样的某些特征。下面是采用半波带法解释泊松亮斑的产生。
图1
如图1,取波前∑为以点源为中心的球面,即等相面,设其半径为R,其顶点O与场点\(P_0\)的距离为b,以\(P_0\)为中心,分别以\(b+\frac{\lambda}{2}\),\(b+\lambda\),\(b+\frac{3\lambda}{2}\),\(b+2\lambda\),……为半径做球面,将波前∑分割为一系列环带。由于这些环带的边缘点\(O\),\(M_1\),\(M_2\),\(M_3\),\(M_4\),…,\(M_n\)到\(P_0\)的光程逐个相差半个波长\(\frac{\lambda}{2}\),故称为半波带。用\(\mathrm{\Delta}{\widetilde{U}}_1\left(P_0\right) \),\(\mathrm{\Delta}{\widetilde{U}}_2\left(P_0\right) \),…,\(\mathrm{\Delta}{\widetilde{U}}_n\left(P_0\right) \)
代表各半波带发出的次波在O点产生的复振幅,则
\(\mathrm{\Delta}{\widetilde{U}}_1\left(P_0\right)=A_1\left(P_0\right)e^{i\phi_1}\)
\(\mathrm{\Delta}{\widetilde{U}}_2\left(P_0\right)=A_2\left(P_0\right)e^{i\left(\phi_1+\pi\right)}\)
\(\mathrm{\Delta}{\widetilde{U}}_3\left(P_0\right)=A_3\left(P_0\right)e^{i\left(\phi_1+2\pi\right)} \)
则在O点的合振幅为
\(A\left(P_0\right)=\left|\widetilde{U}\left(P_0\right)\right|=\left|\sum_{i=1}^{n}{\Delta{\widetilde{U}}_i\left(P_0\right)}\right|=A_1\left(P_0\right)-A_2\left(P_0\right)+A_3\left(P_0\right)-\cdots+\left(-1\right)^{n+1}A_n\left(P_0\right) \).
考查\(A_n\):
由惠更斯-菲涅尔原理知
\(A_n\propto f\left(\theta_n\right)\frac{\mathrm{\Delta}\ \mathrm{\Sigma}_\mathrm{n}}{r_n}\).
\(\mathrm{\Delta}\mathrm{\Sigma}_\mathrm{n}\):第n个半波带的面积;
\(r_n:第n个半波带到场点[latex]P_0\)的距离;
\(f\left(\theta_n\right) \):倾斜因子.
图2
如图2所示的球振幅,其面积
\(\Sigma_n=2\pi R^2\left(1-\cos{\alpha_n}\right) \);
\(cos\alpha_n=\frac{R^2+\left(R+b\right)^2-r_n^2}{2R\left(R+b\right)} \).
两式分别取微分
\(d\Sigma_n=2\pi R^2sin\alpha_nd\alpha_n\);
\(sin\alpha_nd\alpha_n=\frac{r_ndr_n}{R\left(R+b\right)} \).
联立得
\(\frac{d\mathrm{\Sigma}_\mathrm{n}}{r_n}=\frac{2\pi R\ d\ r_n}{R+b}\).
因为\(\lambda\ll\ r_n\),故可以把式中得\(dr_n\)看作相邻半波带间\(r\)的差值\(\frac{\lambda}{2}\),\(d\mathrm{\Sigma}_\mathrm{n}\)看作半波带面积\(\mathrm{\Delta}\mathrm{\Sigma}_\mathrm{n}\).
则\(\frac{\mathrm{\Delta}\mathrm{\Sigma}_\mathrm{n}}{r_n}=\frac{\pi R\lambda}{R+b}\)作为\(\lambda的最低级近似,[latex]\frac{\mathrm{\Delta}\mathrm{\Sigma}_\mathrm{n}}{r_n}\)与n无关,即\(A_n\)的大小由倾斜因子\(f\left(\theta_n\right) \)决定。
相邻半波带之间的\(\theta_n\)值变化极小,从而\(f\left(\theta_n\right) \)和\(A_n\)随n值的增加而缓慢地减小,当\(\theta_n\rightarrow\frac{\pi}{2}\)(菲涅尔最初假设)或当\(\theta_n\rightarrow0\)(基尔霍夫理论)时,\(f\left(\theta_n\right)\rightarrow0\).
将
\(A\left(P_0\right)=A_1\left(P_0\right)-A_2\left(P_0\right)+A_3\left(P_0\right)-\cdots+\left(-1\right)^{n+1}A_n\left(P_0\right) \).
中各加减交替项用上下交替矢量表示,如图3.
由图可见,合成振幅
\(A\left(P_0\right)=\frac{1}{2}\left[A_1+\left(-1\right)^{n+1}A_n\right] \).
在自由传播情况下,最后一个半波带\(f\left(\theta_n\right)\rightarrow0\),则\(A_n\rightarrow0\),故
\(A\left(P_0\right)=\frac{1}{2}A_1\left(P_0\right) \).
圆屏衍射中,设圆屏遮挡住前k个半波带,则
\(A\left(P_0\right)=A_{k+1}\left(P_0\right)-A_{k+2}\left(P_0\right)+\cdots+\left(-1\right)^{n+1}A_n\left(P_0\right)=\frac{1}{2}A_{k+1}\left(P_0\right) \).
则无论k是奇是偶,中心总是亮的.