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Light-Type:
>Offensive
x2 against Ghost (Shadows), Fairy , Electric, Water
x1/2 against Psychic, Ice and Rock (Diffraction)
x0 against Grass (Photosynthesis)
>Defensive
Weak to Poison, Grass, Psychic
Resists Ghost, Fairy, Water, maybe a 4th thing
>Retyped moves:
Laser Focus, Lock On, Morning Sun, Tail Glow, Flash
Signal Beam, Solar Beam, Aurora Beam, basically any move that can be boiled down to "Laser Beam"
>Retyped Pokemon:
Starmie (Water/Light), Ampharos (Electric/Light), Chinchou (Electric/Light), Volbeat (Bug/Light), Manaphy (Water/Light), Luxray (Light). Maybe a few other bio-luminescent ones I'm forgetting.
>Why/How is this different from Psychic, Fire, Fairy, etc?
Psychic moves focus on telepathy and shit. There are a few moves like Psycho Cut that are "hardlight". Those can stay. Most Psychic-types aren't known for producing light, they're known for being really smart and shit, so only Starmie would need to be retyped.
Moonlight and Dazzling Gleam could go either way, yeah, but so could plenty of moves. Only Fairy know for producing light is Shiinotic, which is a Gliscor situation and could be Grass, Fairy or Light.
Fire moves attack with fire, not light. Same deal with Electric moves even though there is a visible light component.
For types that would lose moves to this new type, new moves can be created to replace them, so there is no problem there, either. Light hits Water SE anyway, so Zard/heatran wouldn't miss it.