fix: clarify lights

Author: jaspwr

competition/lights/prob.md

@@ -13,7 +13,7 @@ difficulty = 1
 
 # 💡 Point Source Lights
 
-Given the 3D positions and RGB hexcodes of several point light sources, find the colour at 0, 0, 0. The light strength falls off at `1/(r^2)` where `r` is the distance to the light source. The colours from each light are then summed. There is no occlusion.
+Given the 3D positions and RGB hexcodes of several point light sources, find the colour at 0, 0, 0. The light strength falls off at `I_attenuated = I_original / (r^2)` where `r` is the distance to the light source, `I_original` is the full intensity of the light and `I_attenuated` is the strength at a distance `r`. The colours from each light are then summed. There is no occlusion.
 
 ## Input Format
 Each line of the input represents a light source. Each line contains a hexcode (with hash) followed by 3 numbers representing the position all separated by spaces. For example:
@@ -25,3 +25,7 @@ Hexcodes are in the format `#RRGGBB` where `RR`, `GG`, and `BB` are two-digit he
 
 ## Output Format
 Your output should be a hexcode (with or without hash) of the colour at the origin. **There is a ±1 margin of error on each component.** If a component of the solution saturates, just use `ff` for that component.
+
+## Clarifications
+* The length of a 3D vector `(x, y, z)` is `sqrt(x^2 + y^2 + z^2)`.
+* To attenuate a colour, divide each of the R, G, and B components.