The Light Card's capture was no ordinary capture, (compared to most of the other Clow Cards) but this is a special card because of what happened before it was sealed. Like Wood and Rain, Light usually appears with Dark. They both take on similar forms, only one is dominated by white, and the other is dominated by black.
In Episode 42, Light appears after Dark interrupts the play Sakura's school is performing. Light appears from within Sakura, and states she was in her heart ever since the Clow Books seal was broken. After reuniting with Dark, Sakura sealed the two. When summoned, it creates a powerful light.