  We perform two-dimensional and three-dimensional simulations of cold, dense
clouds, which are accelerated by radiation pressure on dust relative to a hot,
diffuse background gas. We examine the relative effectiveness of acceleration
by ultraviolet and infrared radiation fields, both independently and acting
simultaneously on the same cloud. We study clouds that are optically thin to
infrared emission but with varying ultraviolet optical depths. Consistent with
previous work, we find relatively efficient acceleration and long cloud
survival times when the infrared band flux dominates over the ultraviolet flux.
However, when ultraviolet is dominant or even a modest percentage ($\sim
5-10$\%) of the infrared irradiating flux, it can act to compress the cloud,
first crushing it and then disrupting the outer layers. This drives mixing of
outer regions of the dusty gas with the hot diffuse background to the point
where most dust is not likely to survive or stay coupled to the gas. Hence, the
cold cloud is unable to survive for a long enough timescale to experience
significant acceleration before disruption even though efficient infrared
cooling keeps the majority of the gas close to radiative equilibrium
temperature ($T \lesssim 100$K). We discuss implications for observed systems,
concluding that radiation pressure driving is most effective when the light
from star-forming regions is efficiently reprocessed into the infrared.

Astrophysics of Galaxies