韦尔代常数（英语：Verdet constant）是一个光学常数，用以描述某特定物质中的法拉第效应强度。

韦尔代常数对大部分物质来说是极小且与波长相关的，它在含有顺磁性离子（如铽离子）的物质中强度最强。韦尔代常数的最大实验值是在密火石玻璃或铽镓石榴红晶体（TGG）中被找到，这个材质有极佳的透光度，并且能够在相当的程度上抵抗雷射光的破坏。然而，原子蒸气的韦尔代常数在某些情况下可以比TGG还要大上几个数量级，但仅限在非常窄的波长区间内。杜伦大学的原子与分子研究团队表示，碱金属蒸气因此可作为光学隔离器（也就是光的二极管）。

The Verdet constant is an optical "constant" that describes the strength of the Faraday effect for a particular material.

The Verdet constant for most materials is extremely small and is wavelength dependent. It is strongest in substances containing paramagnetic ions such as terbium. The highest Verdet constants in bulk media are found in terbium doped dense flint glasses or in crystals of terbium gallium garnet (TGG). These materials have excellent transparency properties and high damage thresholds for laser radiation. Atomic vapours, however, can have Verdet constants which are orders of magnitude larger than TGG, but only over a very narrow wavelength range. Alkali vapours can therefore be used as an optical isolator (in essence, a diode for light), as demonstrated in Durham University's Atomic and Molecular Physics research group.