Looking up...
to become bright or illuminated
太陽が出て部屋が明るくなった
The sun came out, and the room became bright.
Often used to describe physical brightness or illumination.
to become cheerful or optimistic
彼の話を聞いて気分が明るくなった
After hearing his story, my mood became cheerful.
Can also describe a shift in emotional state to a more positive one.
The phrase can describe both physical light and emotional cheerfulness.
Determine from context whether it refers to light or mood.
Combination of 明るい (akaru, bright) and なる (naru, to become).
Can refer to both physical brightness and emotional cheerfulness.