电磁波是一种能穿过真空外层空间的波。机械波与电磁波不同，它需要物质介质的存在，以便将能量从一个地方传输到另一个地方。声波是机械波的例子，而光波是电磁波的例子。电磁波是由电荷的振动产生的。这种振动产生了一种既有电成分又有磁成分的波。电磁波以3.00 x 108 m/s的速度在真空中传输能量(速度值通常用符号c表示)。电磁波在物质介质中传播的净速度小于3.00 x 108 m/s。如下面的动画所示。能量通过介质传输的机理涉及到材料原子对波能量的吸收和再发射。当电磁波撞击物质的原子时，该波的能量被吸收。能量的吸收使原子内的电子发生振动。经过短时间的振动运动，振动的电子产生了与第一个电磁波频率相同的新电磁波。虽然这些振动只发生很短的时间，但它们延迟了波在介质中的运动。电磁波的能量一旦被原子重新发射，它就会在原子间的一小块空间中传播。一旦到达下一个原子，电磁波就被吸收，转化为电子振动，然后以电磁波的形式重新发射。当电磁波以c(3×108 m/s)的速度穿过原子间空间的真空时，吸收和再发射的过程使得电磁波的净速度小于c，如下面的动画所示。
Electromagnetic waves are waves which can travel through the vacuum of outer space. Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 m/s (a speed value commonly represented by the symbol c). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 m/s. This is depicted in the animation below.The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorption of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 108 m/s) through the vacuum of interatomic space, the absorption and reemission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below.