Light Block

Light blocks are invisible blocks, primarily intended for map makers, that can produce any light level.

Obtaining
Light blocks cannot be mined, or even targeted, similar to air. Light Blocks with light level 0 are functionally identical to air. They do not attach to any block, meaning that breaking an adjacent block does not remove the light block. However, they can be broken by replacing them with another block. The exception is water (including flowing water) which can occupy the same space as a light block. See this page for details.

Light blocks are not in the player's creative inventory. They can only be obtained using, with any data value from 0 to 15. This value determines the block's light level.

Usage
A light block is normally used in adventure maps (while most just apply Night Vision or Conduit Power effects). This is also used by teachers to light up assignments.

Each light block (as a block or as an item) has an associated light level, which can be anything between 0 and 15 inclusive. In the player's inventory, light blocks display their light level in the top-left corner and appear "brighter" at higher light levels.

When placed as blocks, light blocks are non-solid and completely invisible. Unlike barriers and structure void, they do not reappear when the player is holding another light block.

Light blocks can be pushed and pulled by pistons, are detected by observers, and can be waterlogged.

Light Source
Light blocks produce their associated light level.

Block data
Each light block has a data value between 0 and 15 inclusive, which determines its light level.

Trivia

 * Light Blocks are the only item in vanilla Minecraft to have an HD (32x32 instead of 16x16) texture.

光ブロック Świecący blok Блок света 光源方块