Electromagnetic energy interacts with earth surface features through transmission, reflection, and absorption. The proportion of energy transmitted, reflected, or absorbed depends on factors like the material composition of the feature and the wavelength of the energy. Remote sensing is interested in the reflected energy, which can be specular or diffuse depending on the roughness of the surface relative to the wavelength. The spectral reflectance of a feature varies with wavelength and is characteristic of the material, showing absorption features related to constituents like water and chlorophyll in vegetation. Thermal emission or absorption also depends on the temperature of the feature relative to its surroundings.