Can you get pregnant with a beam of light?

Yes you can. Genghiz Khan was conceived this way. So was the prophet Zoroaster and maybe Mithra, who was popular with the Roman armies. It was common at the time, of course, to claim the uniqueness of a virgin birth - Alexander, the Ptolemies and the Caesars all did it - but the beam of light was a new thing, recalling Zeus impregnating Danaë with a golden shower (which produced the hero Perseus):

This may have inspired the early Christian writers, since the Virgin Mary is said to have become pregnant when a beam of light entered her ear, at the exact moment when she hears the Word of God from the Angel Gabriel. He tells her that she will be the mother of the Son of God and she won't lose her virginity. Or maybe this was a misunderstanding. After all, why does the beam of light strike her ear (or her halo) and not her womb? Was it to remove any sexual context? And why is there a white dove trapped in the "divine essence" entering Mary in paintings of the Annunciation? It does seem like a representation of sperm.

Whatever the reason, it was important to the early Catholic and Syriac Orthodox churches that Mary never had sex with God, or with her husband, even after Jesus was born.

In Renaissance paintings, the Annunciation was a very popular subject, producing some of the finest work of the time, but very few show the beam of light. By the Renaissance in the 15th century, it was somewhat heretical (too Gnostic perhaps) and Leonardo and Botticelli, for example, don't include it, or the white dove. Fra Angelico kept both (above) in the Prado Altarpiece, originally from the Church of San Domenico in Fiesole, near Florence. That's Adam and Eve on the left.

Below is a 14th century Armenian manuscript illumination that has replaced the beam of light with the divine essence (and the white dove).

There were self-serving reasons for promoting weird ideas of a divine essence and female virginity, but there isn't space to get into that here. Believers would say that the mysteries of life do not need to be explained away; they are inherently unexplainable.

Until they're not. I always considered these stories as fanciful myth making, so what a nice surprise to see science enter the room. In recent years, researchers at Northwestern University have recorded sparks of zinc flying at the exact moment of conception when a human egg is pierced by a sperm cell. Not quite a beam of light from God, but close.

Emily Dickinson, on the other hand, had a more pessimistic take on heavenly beams of light. Her lines conjure up intimations of Death...

There’s a certain Slant of light,
Winter Afternoons —
That oppresses, like the Heft
Of Cathedral Tunes —
Heavenly Hurt, it gives us —