Opacity is a property of blocks with a wide range of effects on the game world. Its primary purpose is to tell the game engine if it needs to draw behind the block; an opaque block completely obscures the view behind it, while a transparent block does not. Thus, transparency applies not only to clear blocks like ice and glass, but also things like cacti, stairs, and single slabs, which do not completely fill their block space.
By default, opacity also affects how light propagates through the world. Opaque blocks completely prevent light from travelling through them, while transparent blocks diminish it by one intensity per block. This value can be overridden however, and there are several anomalies.
 Types of transparent blocks
 List of things affected by opacity
A lot of the effects of opacity are non-obvious. Minecraft does not have a "solid" property on blocks, so opacity is tested when a test for "solidity" would normally occur.
- Opaque blocks suffocate you, while transparent blocks do not.
- Monsters cannot spawn on transparent blocks, nor can they spawn inside opaque blocks. However, they can spawn inside a non-solid transparent block, such as flowers or Redstone Wires.
- You cannot attach torches or other wall fixtures to transparent blocks, nor can you place signs or other items on top of them. Here's the full list of blocks that are restricted this way: beds, buttons, redstone repeaters, ladders, levers, pressure plates, pumpkins, rails, snow (not snow block, which is OK), torches (including redstone torches), and Redstone wire.
- Opacity determines the direction Dispensers and Furnaces face when they are placed. It also dynamically changes the facing of chests. These test a static version of Opacity that is not affected by the leaves graphics toggle.
- Water which is completely surrounded by water or opaque blocks won't have a current. However, if one of the surrounding blocks is transparent the water will flow down like a waterfall, making it much slower to swim up.
- Opaque blocks overhead can prevent redstone wires from connecting to each other. See the article for details.
- Tree leaves can overwrite transparent blocks when they grow, but they will not overwrite opaque blocks.
- Chests cannot be opened if there is an opaque block on top of them, but transparent blocks above Chests do not prevent them from being opened.
- Opacity affects door orientation when placed. Doors open counter-clockwise by default, but if they have an opaque wall along their left side they open clockwise.
- Ghast fireball explosions only start fires on opaque blocks.
- A bed can only successfully reset the player's spawn point if the block at its head is opaque. Beds with transparent blocks at their heads may still be slept in, but dying will cause the player to respawn at the original spawn point with the message "Your home bed was missing or obstructed."
- Particles that are affected by gravity (e.g. slime particles) will fall through transparent blocks, but will stop on opaque blocks.
- Leaves don't have any extra effect on block light, but they do diffuse sky light (light from the sky will start to get dimmer when it goes through leaves).
- Slabs, stairs, and farmland block light completely, even though they are transparent blocks. Also, for the purposes of most game logic, the light level of the block itself is equal to the maximum light level of any of the blocks directly beside or above it. As a result, if these blocks are lit from the top or sides, they are typically one level brighter than a transparent block would be at the same location.
- Water and ice are transparent, but have a -2 modifier to light propagation, on top of the normal decrease of 1 per block.
- Glass does not reduce the light passing through it, i.e. there is no additional modifier. In other words, light passes through Glass as if it were air.
- Lava is transparent as you would expect, but it is set to completely block light propagation. This is usually unnoticeable since lava has an intrinsic luminance of 15.
- Only opaque blocks can be powered by Redstone power. This can be observed by placing a redstone torch underneath glass, and then noting how adjacent wires are not powered. (If you replace the glass with something opaque they will get power.)
- In the Nether, your spawn point will always be on top of an opaque block. This is hard to observe, since you warp back to the normal world when you die in the nether.
- If a non-transparent block is given transparent pixels in an edited terrain.png, then instead of showing the block behind it through the transparent part, you will see beyond into the void and any open areas beyond. This is because, as explained above in the captions for the pictures demonstrating the anomaly of leaves, the world behind a block is only rendered if the block is set as transparent.