diff --git a/doc/2.physics.md b/doc/2.physics.md
index 3f63012..121f5d0 100644
--- a/doc/2.physics.md
+++ b/doc/2.physics.md
@@ -231,7 +231,7 @@ $$
     ≈ 4 \frac{ω}{c} {{\text {Im}}(N_{\perp w})}
         \frac {N_{\perp}} {|{∂ Λ}/{∂ {\bf N}|}}  \biggr|_{Λ=0}
     = 2{{\text{Im}}(k_{\perp w})} \frac{v_{g\perp}} v_{g}.
-$$ {eq:alpha}
+$$ {#eq:alpha}
 being $N_{\perp w}$ (and $k_{\perp w}$) the perpendicular refractive index (and
 wave vector) solution of the relativistic dispersion relation for EC waves
 $$