The consonants on the end of a Korean orthographic syllable is called a "final." (받침)
These "finals" only have seven phonetic values which are ㅇ, ㅁ, ㄴ, ㄹ, ㅂ, ㄷ, ㄱ.
The rule is
ㄱ,ㄲ,ㅋ => ㄱ
ㄴ => ㄴ
ㄷ,ㅅ,ㅆ,ㅈ,ㅊ => ㄷ
ㄹ => ㄹ
ㅁ => ㅁ
ㅂ,ㅍ => ㅂ
o => o
Thus, 빛 is pronounced as 빋
The ㅊsound can be pronounced with 이,을
빛이 => 비치
빛을 => 비츨