What does "rightly so" mean here?
Context:
Newton gave a scientific and objective explanation of colors. He used a prism to break white light down into the various colors of the spectrum. And he theorized , rightly so, that different colors are essentially different wavelengths of light.
Why it's not "right so"? We say "right now" very often, do we? Why we don't say "rightly now"?

Question

What does "rightly so" mean here?
Context:
Newton gave a scientific and objective explanation of colors. He used a prism  to break white light down into the various colors of the spectrum. And he theorized , rightly so, that different colors are essentially different wavelengths of light.
Why it's not "right so"? We say "right now" very often, do we? Why we don't say "rightly now"?

Answer

Hi Dinghui, the word 'right' in 'right now' and 'rightly so' serves different functions. 
When we say 'rightly so,' we mean something is correct or somebody did something correct. So in this context, Newton was correct in theorising that different colours are essentially different wavelengths of light.

Another example could be: 
A: did you hear she has quit her job? 
B: yes, and rightly so- she was always under appreciated by her boss. (She did the right thing quitting her hob because her boss never appreciated her.)

Answer

In “rightly so”, “rightly” is an adverb meaning “in a right manner”, “correctly”. That is a normal usage of an adverb of manner formed by adding -ly to an adjective.

“Right now” (right then, right on, etc.), on the other hand, is an idiomatic expression. Here, “right” is an adverb meaning “exactly”.

In some cases, it’s the same either way, for example: “You guessed rightly / right”. In spoken English, we usually use “you guessed right”. Such “flat adverbs” were the norm in English until just a couple of hundred years ago (the blink of an eye in linguistic terms). Check out the Wikipedia article, if you’d like:

https://en.wikipedia.org/wiki/Flat_adverb