Deepavali Celebrations and their significance

In this video, Pujya Bhaishri gives us the following sandesh for Deepavali:

1. Why do we light lamps in Deepavali?
2. What is the significance of Deepavali and why so much joy?
3. Description of Deepavali spiritually and scientifically;
4. One thing we must have this Deepavali;
5. How can we make Deepavali fun?

Let us understand this significance in a spiritual and scientific way. Thus, let us enjoy these celebrations with complete joy!