From Ephesians by Peter S. Williamson, commenting on Ephesians 5:11-14:
Paul expects that when Christians confront immorality, whether by shining the light of Christian witness before the world, by gently reproving a brother or sister, or by openly censuring public wrongdoing, the situation will change: but everything exposed by the light becomes visible, for everything that becomes visible is light. In other words, whatever light shines on becomes illuminated, enlightened. Paul hopes for the conversion of people who are in darkness as a result of the light of Christ shining through Christians.
Of course, things do not always work out this way. Jesus, the perfect light, let his light shine by word and example. Some people received the light and became light; others rejected the light (John 1:9–11; 3:19–21). As followers of Jesus, we can expect the same mixed response to our testimony. However, we can be sure that if we want to overcome darkness, we need to turn on a light.