Light Absorption and Reflection by Opaque Objects

The reflection coefficients calculated from Fresnel's laws apply to transparent materials. For opaque materials, the absorption coefficient for visible light can vary greatly. A high absorption coefficient for all visible wavelengths gives a surface a black appearance. A black surface will absorb energy more rapidly than a white object. An example occurs in the demonstration device sometimes called a "radiometer", though it is not immediately evident how it responds to wavelengths other than the visible range.

The device has light metallic vanes that are black on one side and white or highly reflective on the other. The framework of vanes is suspended on a pin so that it can rotate freely. Placed near a directional source of light, the vanes will turn in a direction that shows that the force on the dark surface is larger than that on the white surface. The direction of rotation is attributed to the greater heating of the black surface. 该装置有轻质金属薄片，一面为黑色，另一面为白色或高度反光。薄片的框架通过一个销子悬挂，可以自由旋转。当这些薄片靠近一个方向光源时，薄片会向某一方向旋转，这表明黑色表面受到的力大于白色表面受到的力。旋转方向归因于黑色表面的加热程度更大。