Light, by loosest definition, is a form of energy. This energy is termed an electromagnetic wave, which is a wave that consists of both electric and magnetic parts. They do not require a medium to travel, meaning they do not need a substance in order to move. These waves travel at the speed of light. (For information on the speed of light, click here.)
Light travels in straight lines; it cannot bend around objects or change direction. This is why shadows are formed. Shadows are the absence of light behind an object. For example, the shadow behind a tree means that the Sun’s light could not reach the area behind it.
Electromagnetic waves have different energy levels and different lengths. The human eye can only detect certain wavelengths. This is what we call visible light, and it contains the colours of the rainbow: Red Orange Yellow Green Blue Indigo Violet. There are other wavelengths that the human eye cannot detect, such as radio waves, microwaves, X-rays, and gamma rays.