Looking at Light

In the same way that Mary gave birth to Jesus, the Light of the World, and yet remained a virgin, light can pass through glass without altering the glass. When the glass is coloured, the symbolism deepens: the light takes on the same colour as the glass, just as God had “passed through” Mary, and took on her nature, humanity, in the form of Jesus.

Richard Stemp, The Secret Language of
Churches & Cathedrals