This is because the electrons in the absorbing medium are driven to oscillations of much larger amplitudes than occurs for non-resonant frequencies. These large amplitudes result in energy transfer to neighboring atoms and an increase in internal energy rather than a reemission of light. The greater number of interactions per distance tends to slow the light and result is a smaller average speed. Transparency or opaqueness is determined by the match between incident light frequencies and the resonant frequency of the material. A substance that is transparent to a range of light frequencies will be opaque to those frequencies that match its own resonant frequency.
Clouds are transparent to ultraviolet light, which is why clouds offer no protection from sunburn.
Glass, however, is opaque to ultraviolet light, and will therefore shield you from sunburn. The sunglasses will be warmer in sunlight than regular reading glasses. This is because the reading glasses transmit most of the light energy that is incident upon them, whereas the sunglasses absorb more light energy, which increases their internal energy. Any shadow cast by a faraway object such as a high-flying plane is filled in mainly by light tapering in from the sun, which is not a point source. If the plane is low to the ground, however, the tapering of light around the airplane may be insufficient to fill in the shadow, part of which can be seen.