can i just connect the battery ,the light sensor and the bulb in a simple circuit?
Yes you can but the components will have to be exactly the right type to get everything balanced. For example if you use a light dependent resistor you need to make sure that it can take the current and that the fully lit resistance is sufficient to turn your light on. I have done this sort of thing in the past and I have also use a relay directly connected to a light dependent resistor. I was able to use the normally closed contacts to light a flash light bulb that dropped the resistance of the light dependent resistor, thus turning it on, breaking the contact, turning the bulb off which robbed the sensor of light, turned the relay off thus turning the bulb on again. This made a simple oscillator.