  We present a synthetic galaxy lightcone specially designed for narrow-band
optical photometric surveys. To reduce time-discreteness effects, unlike
previous works, we directly include the lightcone construction in the
\texttt{L-Galaxies} semi-analytic model applied to the subhalo merger trees of
the {\tt Millennium} simulation. Additionally, we add a model for the nebular
emission in star-forming regions, which is crucial for correctly predicting the
narrow/medium-band photometry of galaxies. Explicitly, we consider,
individually for each galaxy, the contribution of 9 different lines: $\rm
Ly{\alpha}$ (1216\AA), \Hb (4861\AA), \Ha (6563\AA), {\oii} (3727\AA, 3729\AA),
{\oiii} (4959\AA, 5007\AA), $\rm [\ion{Ne}{III}]$ (3870\AA), {\oi} (6300\AA),
$\rm [\ion{N}{II}]$ (6548\AA, 6583\AA), and $\rm [\ion{S}{II}]$ (6717\AA,
6731\AA). We validate our lightcone by comparing galaxy number counts, angular
clustering, and \Ha, \Hb, {\oii} and {\oiiiFd} luminosity functions to a
compilation of observations. As an application of our mock lightcones, we
generate catalogues tailored for J-PLUS, a large optical galaxy survey
featuring 5 broad and 7 medium band filters. We study the ability of the survey
to correctly identify, with a simple \textit{three filter method}, a population
of emission-line galaxies at various redshifts. We show that the $4000\AA$
break in the spectral energy distribution of galaxies can be misidentified as
line emission. However, all significant excess (larger than 0.4 magnitudes) can
be correctly and unambiguously attributed to emission line galaxies. Our
catalogues are publicly released to facilitate their use in interpreting
narrow-band surveys and for quantifying the impact of line emission in broad
band photometry.
