Refractive index of water experiment?

Use a large rectangular piece of cardboard & make a small slit (nt more thn 1-1.5cm for best results) in it using a blade. Place it in front of torch. U don't need to cover the whole torch, just make sure it is large enough to not allow excess light to pass (maybe 15cmx15cm ?). Do it in a dark room at night, turn off the lights & curtains.