What is the incidence of tax on non residents?

If all the income of non-resident Indians arises or accrues from outside India then it would not be taxable in India. You can read about the Taxation in India – Direct taxes & Indirect Taxes, Features of Taxation System in the given link.

If all the income accrues or arises from outside India, in the case of resident Indians, then that income is taxable in India.

Further readings:

1. Double Taxation Avoidance Agreements (DTAA) – Countries Involved, Income Tax Provisions
2. UPSC Mains General Studies Paper-III Strategy, Syllabus & Structure