Why don't two 50% opacity layers of the same color make the solid color in Photoshop?

The opacity is a multiplier. Assume:

• Layer 1 opacity = 50%
• Layer 2 opacity = 50%
• Layer 3 opacity = 50%

Net opacity = Layer1 + (Layer2 * Layer1) + (Layer3 * Layer2 * Layer1)

Or, 50% + 25% + 12.5%. You would mathematically never get to 100%.

The way I look at it, each layer lets 50% of the light through... you don't get 100% with it.

It's like the weatherman saying there's a 50% chance of rain on Saturday and on Sunday, and concluding that it must rain sometime during the weekend. Obviously it doesn't mean that. :-)