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You have to collect armour with increasing levels of light. For instance, if i had a rare chest piece that has say...19 light. I have to get different armour pieces with larger amounts of light to continue past level 20. Try getting some exotics from xur or legendary armour. Even using some rare stuff is fine to level up for a while... once you hit level 20 you earn xp only for motes of light...
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Level 20 is the experience level cap. To ascend higher, you begin to acquire and level up armor with 'light'. The more you play, the more gear you'll get. Eventually you will need to start wearing legendary and exotic gear to continue to rank up. If you aren't lucky enough to get good drops, be sure to rank up your vanguard past 3 and purchase your gear with Vanguard marks! Same principle applies with Crucible!
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Do the strike playlists and wear the armor with light