In a double slit experiment, the key difference between a laser and a flashlight lies in the characteristics of the light they produce.
A laser (which stands for "Light Amplification by Stimulated Emission of Radiation") emits coherent light. Coherence refers to the property of light waves having a fixed phase relationship with each other. Laser light consists of waves that are in sync, meaning they have a constant phase difference between them. This coherence is maintained over long distances, resulting in a well-defined and concentrated beam of light. In a double slit experiment, when a laser beam is used, the coherent light passing through the two slits produces an interference pattern on the screen placed behind them. This interference pattern arises due to the constructive and destructive interference of the coherent waves.
On the other hand, a flashlight emits incoherent light, which means the light waves are not in sync with each other. The emitted light consists of a range of wavelengths and phases, and the waves are not correlated. When an incoherent light source, such as a flashlight, is used in a double slit experiment, each individual wave passing through the slits does not maintain a consistent phase relationship with other waves. As a result, the interference pattern on the screen is not well-defined or distinct. Instead, the pattern appears blurred and is generally not as pronounced as with a laser beam.
In summary, the main difference between a laser and a flashlight in a double slit experiment is that a laser produces coherent light with a well-defined phase relationship, resulting in a clear interference pattern, while a flashlight emits incoherent light, leading to a less distinct or observable interference pattern.