Light comes from natural sources like the sun or fire, and artificial sources like light bulbs. Light travels in a straight line and can pass through some materials, but is blocked by others causing shadows. Objects can either reflect light by bouncing it off their surface, or absorb light. Reflection occurs when light hits a surface and changes direction, either in the same direction for smooth surfaces, or many directions for rough surfaces. Light can pass through transparent materials but changes speed and direction when moving between different materials like air and water, which is called refraction. White light can be refracted and separated into the colors of the visible spectrum.