We can use 'slight' as an adjective to mean 'very small in degree': 'a slight difference', ' a slight damage'... We can also use it to mean 'small and thin in size': 'a slight woman' and to describe something not deserving serious attention: 'This is a very slight novel'. However, we use the adjective 'light' to mean 'not great in amount, degree, etc.' (Opposite of heavy): 'light traffic' 'light winds' and when talking about work/exercise: 'some light housework' 'take a little light exercise' and to describe food that is small in quantity: 'a light supper' (Opposite of heavy) We can also us it to mean 'cheerful': 'cheerful colours' 'cheerful heart' and to mean 'gentle or delicate' (opposite of 'heavy'): 'She felt a light tap on her shoulder.' So we use the adjective 'slight' mainly to talk about something which is 'small in amount or size or importance'. It is the opposite of 'Significant' or 'Considerable'. And we use the adjective 'light' to mean 'not heavy', 'cheery and happy' and gentle. However, we can use the the adjective 'light' to mean 'of little importance or consequence; trivial'. It is synonym for 'slight' here: 'The loss of his job was no light matter.' (It is the opposite of considerable or important here).