My first idea:
A light bulb (or similar contraption) which will be connected to a bunched-up optical fibre cables whose other end will be connected to a dish-antenna-kind-of mirror (this can be made to track the sun, a device invented by Pranav Mistry) which is installed on the terrace of the building. The sun-light falling on the mirror will travel through the OFC bunch and will illuminate the bulb on the other end.
This way we can stop the usage of electricity to illuminate the interiors during day time.
I appeal to scientists to come up with such a thing.