Light crossing a boundary changes speed, and a wave that changes speed at a slant changes direction. Snell's law puts the number on it: $n_1 \sin\theta_1 = n_2 \sin\theta_2$ — read aloud n-one sine theta-one equals n-two sine theta-two. Theta is the Greek letter, said “THAY-ta”.

The subscripts are not a ranking; they are a map, and the convention is fixed. Subscript 1 is the side the light STARTS on: $n_1$ is that medium's index, a naked number, and $\theta_1$ is the angle of incidence, in degrees. Subscript 2 is the side the light ENDS on: $n_2$ its index, $\theta_2$ the angle of refraction. Four quantities, and whichever one the question withholds is the one you solve for. Note what that means for a ray leaving a glass block: the glass is then medium 1, because that is where the ray began.

Now the wound this lesson exists to prevent. Every angle in optics is measured from the NORMAL — the imaginary line perpendicular to the surface — never from the surface itself. A ray quoted at 30° sits 30° from that perpendicular and 60° from the glass. Read a diagram the other way and every number after it is 90 minus the truth. Marks do not come back from that.

Two habits will carry you. First, the behaviour without arithmetic: into a denser medium (bigger n) a ray bends toward the normal; out into a rarer one it bends away. Check your answer against that before you write it down. Second: Snell's law is about sines, not angles. $\sin 30° = 0.500$ but $\sin 60° = 0.866$, not double — so never divide 30° by 1.5 and call it refraction.