# Huygen's Principle, Snell's law and velocity of light

Can you deduce Snell's law from Huygen's Principle without knowing in which medium the velocity is going to less? Of all the derivations I have seen of Snell's law using Huygen's Principle (light going from rarer to denser medium with $$v1$$ and $$v2$$ velocities respectively), they assume $$v2 < v1$$ and hence a smaller circle is drawn in denser medium and Snell's law follow immediately. I get that. What I want to know is what if you didn’t know that velocity is less in denser medium can you still derive Snell's law from Huygen's principle? I remember reading somewhere that Huygen's proved that velocity is less in denser medium. I was wondering if someone could show me how.