Proverbs 13:9
The light of the righteous shines brightly, but the lamp of the wicked is snuffed out.

Righteousness - godliness as expressed in real life - is associated with light and with rejoicing. There is something wrong with the person who claims to be righteous rarely have evidence of light and rejoicing.

Righteousness is associated with light, but the wicked with darkness. The darkness conceived of here is one that is imposed by the judgment of a righteous God (will be put out).

Application

Read through Matthew chapter 6 and meditate on the acts of righteousness.