An opaque object is something that allows no light through it. Concrete, wood, and metal are some examples of opaque materials. Some materials can be opaque to light, but not to other types of electromagnetic waves. For example, wood does not allow visible light to pass through it, but will allow other types of electromagnetic waves, such as microwaves and radio waves to pass. The physical characteristics of the material determine what type of electromagnetic waves will and will not pass through it.