Light
Light is a level from 0 to 15 held by every block space.
Light level
Every space stores two values from 0 to 15:
- Sky light, which comes from the sky.
- Block light, which comes from blocks that give off light.
A space's light level is the higher of its block light and its sky light minus the sky's darkness.
Sky light
A space open to the sky has sky light 15, however far down it is. A space is open to the sky when nothing above it is a full opaque block or one of the blocks listed below. Any other space takes the brightest sky light among its six neighbours, minus the light lost in its own block.
The Nether has no sky light.
Block light
The blocks below give off light. The space a source stands in has that much block light. Any other space takes the brightest block light among its six neighbours, less the light lost in its own block.
| Icon | Name | Light |
|---|---|---|
 | Lava | 15 |
 | Fire | 15 |
 | Glowstone | 15 |
 | Jack 'o' Lantern | 15 |
 | Locked chest | 15 |
 | Torch | 14 |
 | Furnace | 13 |
 | Portal | 11 |
 | Redstone Ore | 9 |
 | Redstone Repeater | 9 |
 | Redstone Torch | 7 |
 | Brown Mushroom | 1 |
A furnace gives off light only while it burns, redstone ore only while it glows, a redstone torch only while it is on and a repeater only while it is powered.
What stops light
Light loses 1 for each block it passes through. A full, opaque block stops it. Every other block loses 1, as air does, and leaves the space below it open to the sky, except these:
| Icon | Name | Light lost per block |
|---|---|---|
 | Leaves | 1 |
 | Cobweb | 1 |
 | Water | 3 |
 | Ice | 3 |
| Lava | All |
 | Double Stone Slab | All |
 | Stone Slab | All |
 | Wooden Stairs | All |
 | Farmland | All |
 | Stone Stairs | All |
Slabs, stairs and farmland stop light although they do not fill their space. The game reads the light at one of them as the brightest of the spaces above and beside it.
Day and night
The sky's darkness is subtracted from sky light when a light level is read. It is 0 by day and 11 at night, so a space open to the sky reads 15 by day and 4 at night:
| Time of day (ticks) | Sky light read |
|---|---|
| 0 to 12,040 | 15 |
| 12,040 to 13,670 | falls from 15 to 4 |
| 13,670 to 22,330 | 4 |
| 22,330 to 23,960 | rises from 4 to 15 |
Rain darkens the daytime sky to 12, and a thunderstorm to 10. Neither darkens the night.
The game counts it as day while the sky reads 12 or brighter.
The stored sky light never changes with the time of day. Some checks read a space's light without subtracting darkness, and see a space open to the sky as 15 at midnight.
A day is 24,000 ticks, kept by the world clock.
What light affects
| What | Needs | Reads |
|---|---|---|
| Hostile mob spawning | two random tests | stored sky light, then light level |
| Animal spawning | above 8 | light level, ignoring night |
| Crops and saplings growing | 9 or more, in the space above | light level |
| Grass spreading | 9 or more above the grass, 4 or more above the dirt | light level |
| Grass dying | below 4, under water, ice, lava, an opaque block, a slab, stairs or farmland | light level |
| Flowers, roses, tall grass, dead bushes, crops and saplings staying put | 8 or more, or open to the sky | light level, ignoring night |
| Mushrooms spreading and staying put | 12 or less | light level, ignoring night |
| Ice melting | above 8 | block light alone |
| Snow layers melting | above 11 | block light alone |
| Zombies, skeletons and pig zombies catching fire | 12 or more, open to the sky, by day: a chance each tick, from 1 in 120 at 12 to 1 in 25 at 15 | light level |
| Spiders seeking a player | 11 or less | light level |
| Hostile mobs except slimes and ghasts ageing towards despawning | 12 or more, at three times the rate | light level |
| Hostile mobs except slimes, ghasts and giants wandering | prefer darker spaces | light level |
| Giants wandering | prefer brighter spaces | light level |
| Animals wandering | prefer grass, then brighter spaces | light level |