Final answer:
No interference is observed with two side-by-side lightbulbs like sodium lamps because they are not coherent light sources, lacking a consistent phase relationship. The same is true for independent laser pointers, which also do not maintain a fixed phase relationship to produce an interference pattern.
Step-by-step explanation:
If two lightbulbs, such as small sodium lamps, are placed side by side, no interference is observed because the light waves they emit do not have a consistent phase relationship with each other. For interference patterns to occur, the light sources need to be coherent, meaning the waves emitted by each light source have to maintain a constant phase relationship over time. Sodium lamps, operating independently, are not coherent light sources and thus their light waves are emitted with random phases relative to one another.
Similarly, if the sodium lamps were replaced by two independent laser pointers held close together, one would still not observe an interference pattern for the same reason. While laser light is normally coherent, when the lasers are operating independently, they do not necessarily maintain a fixed phase relationship; thus, they cannot produce an interference pattern when placed side by side. To create an interference pattern, both lasers would need to be derived from the same initial laser beam or be precisely synchronized to ensure coherence.