The double slit experiment demonstrated the wave-like properties of light. When light passes through two slits, it interferes with itself and creates a pattern of bright and dark bands on a screen due to constructive and destructive interference. This would not occur if light solely behaved as particles. The interference pattern depends on the spatial coherence of the light source, with lasers producing clear patterns due to their high coherence compared to other common sources like light bulbs. The phase difference between light waves determines whether interference is constructive or destructive at each point on the screen.