{"id":652,"date":"2026-07-25T06:11:02","date_gmt":"2026-07-25T06:11:02","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=652"},"modified":"2026-08-29T21:49:21","modified_gmt":"2026-08-29T21:49:21","slug":"refractive-index","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/waves\/refractive-index\/","title":{"rendered":"Refractive Index (n = c\/v): Formula & Table"},"content":{"rendered":"\n
Definition<\/div>

\n\nRefractive index (n) measures how much a material slows light: it is the ratio of the speed of light in a vacuum, c, to its speed in the material, v, so n = c\/v. Water has n = 1.33, typical glass about 1.5 and diamond 2.42 \u2014 the larger the value, the slower the light and the more strongly it bends.\n\n<\/p><\/div>\n\n

Hold a cheap cubic zirconia next to a real diamond and, at arm’s length, they look identical. Tilt them under a lamp, though, and the diamond erupts with far more flash. Almost the entire difference comes down to one number: 2.42 versus 2.16.<\/p>\n\n

That number is the refractive index, and this page is built around it. The full lookup table comes first, then the ratio behind it, how to measure it yourself, and where those decimal places quietly run the modern world \u2014 from spectacle lenses to fibre broadband.<\/p>\n\n

Refractive Index Table: n Values for 15 Common Materials<\/h2>\n\n

The refractive index of common materials runs from exactly 1 for a vacuum, through 1.33 for water and about 1.5 for glass, up to 2.42 for diamond. Unless stated otherwise, the values below are measured with yellow sodium light (589 nm) at roughly 20 °C \u2014 the standard reference conditions used in data tables.<\/p>\n\n

\n\n\n\n\n
Material<\/th>\nRefractive index (n)<\/th>\nSpeed of light inside (v = c\/n)<\/th>\n<\/tr>\n<\/thead>\n
Vacuum<\/td>1 (exact)<\/td>3.00 × 108<\/sup> m\/s (c itself)<\/td><\/tr>\n
Air (0 °C, 1 atm)<\/td>1.000293<\/td>≈ c (0.03% slower)<\/td><\/tr>\n
Ice (0 °C)<\/td>1.31<\/td>2.29 × 108<\/sup> m\/s<\/td><\/tr>\n
Water (20 °C)<\/td>1.333<\/td>2.25 × 108<\/sup> m\/s<\/td><\/tr>\n
Ethanol<\/td>1.361<\/td>2.20 × 108<\/sup> m\/s<\/td><\/tr>\n
Fused quartz (silica)<\/td>1.458<\/td>2.06 × 108<\/sup> m\/s<\/td><\/tr>\n
Olive oil<\/td>1.47<\/td>2.04 × 108<\/sup> m\/s<\/td><\/tr>\n
Glycerol<\/td>1.473<\/td>2.04 × 108<\/sup> m\/s<\/td><\/tr>\n
Crown glass<\/td>1.52<\/td>1.97 × 108<\/sup> m\/s<\/td><\/tr>\n
Table salt (NaCl)<\/td>1.544<\/td>1.94 × 108<\/sup> m\/s<\/td><\/tr>\n
Polycarbonate<\/td>1.586<\/td>1.89 × 108<\/sup> m\/s<\/td><\/tr>\n
Flint glass (dense)<\/td>1.62<\/td>1.85 × 108<\/sup> m\/s<\/td><\/tr>\n
Sapphire<\/td>1.77<\/td>1.69 × 108<\/sup> m\/s<\/td><\/tr>\n
Cubic zirconia<\/td>2.16<\/td>1.39 × 108<\/sup> m\/s<\/td><\/tr>\n
Diamond<\/td>2.417<\/td>1.24 × 108<\/sup> m\/s<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n\n

Notice the pattern as you read down. Gases barely rise above 1, everyday liquids cluster between 1.3 and 1.5, glasses and clear plastics sit around 1.5 to 1.6, and gemstones own the top of the chart.<\/p>\n\n

Even air’s tiny 1.000293 matters more than it looks. Laser interferometers measure lengths in wavelengths of light, so precision labs must correct for air’s index \u2014 NIST maintains dedicated calculators<\/a> (the Ciddor and Edlén equations) for exactly that correction.<\/p>\n\n

One caveat before you quote a value in an exam: n shifts in the third decimal place with the colour of the light and with temperature. More on that below.<\/p>\n\n

\"Refractive<\/figure>\n

The refractive index scale: every everyday transparent material sits between vacuum (1) and diamond (2.42).<\/p>\n\n

What Is the Refractive Index?<\/h2>\n\n

The refractive index is a dimensionless number that tells you how many times slower light travels in a material than in a vacuum. Glass with n = 1.5 slows light to two-thirds of its vacuum speed; diamond, at 2.42, drags it to well under half.<\/p>\n\n

You will also see the same quantity called the index of refraction<\/strong> \u2014 the usual term in American textbooks \u2014 or described through “optical density”: the optically denser medium is simply the one with the higher n. The reference speed, c, is the fastest anything in the universe can travel, a story we unpack in our guide to the speed of light<\/a>.<\/p>\n\n

Why does light slow down at all, and why does slowing make a ray bend? That mechanism \u2014 waves, wavefronts and the marching-band turn \u2014 is the territory of our reflection and refraction guide<\/a>. Here, we stay with the number itself: what it equals, how to look it up, and how to use it.<\/p>\n\n

The Refractive Index Formula: n = c\/v<\/h2>\n\n

The refractive index formula is n = c\/v: divide the speed of light in a vacuum by the speed of light in the material.<\/p>\n\n

n = c \/ v<\/div>\n\n