A Level Physics Practice Exam

In a double slit system, which action would decrease the separation of the fringes seen on the screen?

1. Moving the screen closer to the double slits

2. Increasing the width of each slit while keeping separation constant

3. Increasing the separation of the slits

4. Exchanging the light source for one with a shorter wavelength

The correct answer is: Moving the screen closer to the double slits

In a double slit experiment, the separation of the interference fringes on the screen is influenced by the geometry of the setup and the properties of the light being used. The fringe separation, often denoted as \( \Delta y \), can be described by the formula: \[ \Delta y = \frac{\lambda L}{d} \] where \( \lambda \) is the wavelength of the light, \( L \) is the distance from the slits to the screen, and \( d \) is the separation between the slits. When the screen is moved closer to the double slits, the distance \( L \) decreases. Since the fringe separation is directly proportional to \( L \), reducing this distance results in a smaller \( \Delta y \), meaning the fringes become closer together on the screen. This principle explains why moving the screen closer to the double slits leads to a decrease in the fringe separation. The other actions listed would not serve to reduce the fringe spacing in the same manner. For example, increasing the width of each slit does not affect the separation of fringes directly, while increasing the separation of the slits would result in a larger fringe separation. Changing to a light source with a shorter