Light is one of the most obvious elements of interior design. Either natural or man-made, without light other elements namely color, texture and pattern have no significance at all. Light sets in the mood and ambience into a living space and highlights the every other element including space, line and forms. 

Designers cheat sheet:

While smart placement of doors and windows should take care of the natural light, man-made or artificial lighting is broadly divided into three major types namely – 

Task Lighting, 

Accent Lighting and 

Mood Lighting. 

Task light as the name implies, includes light sources like table and bed lamps which have a defined purpose, dedicated for a specific task. Accent lights are meant for highlighting a particular piece or show item like artworks, structures, sculptures and so on. Mood or ambient lighting basically set the mood of the living space and illuminate the overall space.

Textures and dark colours absorb light.

Shine, mirrors, metallics  and light colours reflect it.

Light also guides the way. If you want to emphasize signage or a path to be followed, light it up. A feeling of safety is created with lots of light, but too much or the wrong quality can be oppressive.   There are great light meters out there, but some apps available for phones can do a great job of measuring the amount of light reflecting off a given surface.

Also, where there is directional light, there is shadow.