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 \pm 0.18 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 Å 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 } .