  Optical $UBVRI$ photometry and low resolution spectroscopy of the type IIb
supernova SN 2011dh in M51 are presented, covering the first year after the
explosion. The light curve and spectral evolution are discussed. The early
phase light curve evolution of SN 2011dh is very similar to SN 1993J and SN
2008ax. In the late phase, however, SN 2011dh declines faster than SN 1993J.
The late phase decline in the $B$-band is steeper than in the $R$ and $I$
bands, indicating the possibility of dust formation. With a peak $V$-band
absolute magnitude of $M_V = -17.123\pm0.18$ mag, SN 2011dh is a marginally
faint type IIb event. The reddening corrected colour curves of SN 2011dh are
found to be redder than other well studied type IIb supernovae. The bolometric
light curve indicates $\sim$ 0.09 M$_\odot$ of $^{56}$Ni is synthesized during
the explosion. The HeI lines were detected in the spectra during the rise to
maximum. The nebular spectra of SN 2011dh show a box shaped emission in the red
wing of the [OI] 6300-6363 \AA\ feature, that is attributed to H$\alpha$
emission from a shock excited circumstellar material. The analysis of nebular
spectra indicates that $\sim 0.2$ M$_\odot$ of oxygen was ejected during the
explosion. Further, the [CaII]/[OI] line ratio in the nebular phase is $\sim$
0.7, indicating a progenitor with a main sequence mass of 10-15 M$_\odot$.
