Left sun radical - the indexing component. The square frame with a stroke inside originally pictured the sun with a sunspot or marking inside its disk. Here it provides the bright daylight half of the brightness pairing.
Right moon - the second light source. Note this is genuinely the moon graph (not the meat radical ⺼ that looks identical). Sun + moon together make the perfect compound ideograph for 'bright': both light sources combined, day and night light.
