For radio emission at the frequency corresponding to the second harmon
ic of the local plasma frequency, the optical thickness in the solar a
tmosphere is calculated. Three types of models are assumed: the model
with radio emission from the narrow transition region,and models with
radio emission from a cool and dense plasma filament embedded in hotte
r plasma at the transition region and in the corona. The optical thick
ness is computed by integration of the collisional (Free - free) absor
ption along a radio-ray path radial in the solar atmosphere. In all mo
dels considered the optical thickness can be sufficiently low for appr
opriate parameters. For example, in the narrow (< 100 km) transition r
egion where the density scale height is much less than that of the pre
ssure one, the optical thickness can be lower than 1. Furthermore, the
optical thickness can be decreased if the radio emission is generated
in the cool and dense plasma filament surrounded by hotter and thinne
r plasma. But the models differ in density scale heights and thus in d
istances between plasma emission levels. This difference is essential
for the interpretation of high frequency type III radio bursts.