The nano-particles of double perovskite oxide Sm2NiMnO6 (SNMO) are synthesized by the sol-gel method. The synthesized SNMO nano-particles are characterized by the X-ray diffraction, transmission electron microscopy and UV-visible absorption spectroscopy. By combining experiments with the density functional theory (DFT) calculations, we suggest that SNMO can be used as a promising solar cell absorber. The measured direct optical band gap of 1.41 eV is found to be very close to the Shockley-Quiesser band gap value of a single junction solar cell. DFT calculations show that SNMO has a large value of absorption coefficient in the visible region, which is comparable to that of some standard photovoltaic materials such as Si, CH3NH3PbI3, etc. The calculations also show that SNMO possesses a strong birefringence (-0.26), which is also very promising for various nonlinear optical applications. In addition the photocatalytic degradation of Rhodamine B under the visible light and photocurrent density measurements under a simulated sunlight (AM1.5G) are performed to elucidate the photosensitive nature of SNMO.