<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_reflectance(self, projectables, optional_datasets):
 """Calculate 3.x reflectance with pyspectral."""

        _nir, _tb11 = projectables
        LOG.info('Getting reflective part of %s', _nir.attrs['name'])

        sun_zenith = None
        tb13_4 = None

        for dataset in optional_datasets:
            wavelengths = dataset.attrs.get('wavelength', [100., 0, 0])
            if (dataset.attrs.get('units') == 'K' and
                    wavelengths[0] <= 13.4 <= wavelengths[2]):
                tb13_4 = dataset
            elif ("standard_name" in dataset.attrs and
                  dataset.attrs["standard_name"] == "solar_zenith_angle"):
                sun_zenith = dataset

        # Check if the sun-zenith angle was provided:
        if sun_zenith is None:
            if sun_zenith_angle is None:
                raise ImportError("No module named pyorbital.astronomy")
            lons, lats = _nir.attrs["area"].get_lonlats_dask(CHUNK_SIZE)
            sun_zenith = sun_zenith_angle(_nir.attrs['start_time'], lons, lats)

        return self._refl3x.reflectance_from_tbs(sun_zenith, _nir, _tb11, tb_ir_co2=tb13_4)