Using Snell's law, which expression gives the refractive index when light moves from air into glass?

• A. sin i / sin r
• B. sin r / sin i
• C. cos i / cos r
• D. tan i / tan r

Answer: (A)

Snell's law shows how light bends when it passes into a new medium: n1 sin i = n2 sin r. When light moves from air into glass, the ratio n2/n1 equals sin i / sin r. This ratio is the refractive index of glass relative to air. Since n in air is about 1, the refractive index of glass is approximately sin i / sin r. So the expression sin i / sin r is the correct one. The other options aren’t derived from Snell's law for this situation—they involve the wrong ratio or use cos/tan, which aren’t part of the law’s relation.