General concepts do not require the plural suffix. In this case, it says apples in general, not specific ones, are light. Also, the suffix is usually dropped when the noun is inanimate even if it refers to a specific one.
가볍다-> Is used for light relative to weight.
빛-> A noun that indicates light relative to ilumination (조명) and shine.
조명하다-> To illuminate something.
And 피울 is indeed 피우다 (original form) that means burn, kindle, ignite something.