Question

What is meant by point source and external source?

Answer 1

We believe that what you might be asking is 'what are point and extended sources', here's the answer in short...

Point sources (of light) are such that there dimensions (or sizes) are very small compared to their surroundings, like a small LED light bulb, stars etc. Point sources are generally taken to be located very far away.

Extended sources (of light) are such that their dimensions (or sizes) are large or compared to objects in their surroundings, like stadium lights, sun etc. Extended sources are generally taken to be located nearby.