  Light refracted by the planet's atmosphere is usually ignored in analysis of
planetary transits. Here we show that refraction can add shoulders to the
transit light curve, i.e., an increase in the observed flux, mostly just before
and after transit. During transit, light may be refracted away from the
observer. Therefore, even completely transparent planets will display a very
similar signal to that of a standard transit, i.e., that of an opaque planet.
We provide analytical expression for the amount of additional light deflected
towards the observer before the transit, and show that the effect may be as
large as $10^{-4}$ of the stellar light and therefore measurable by current
instruments. By observing this effect we can directly measure the scale height
of the planet's atmosphere. We also consider the attenuation of starlight in
the planetary atmosphere due to Rayleigh scattering and discuss the conditions
under which the atmospheric lensing effect is most prominent. We show that, for
planets on orbital periods larger than about 70 days, the size of the transit
is determined by refraction effects, and not by absorption within the planet.
