Yes, the act of observing or measuring a double-slit interference pattern can indeed have an effect on its appearance. This phenomenon is known as the observer effect or the measurement problem in quantum mechanics.
In the double-slit experiment, a beam of particles or waves (such as electrons or photons) is directed at a barrier with two slits. When the particles or waves pass through the slits and interact with each other, an interference pattern is formed on a screen placed behind the barrier. This pattern consists of alternating bright and dark bands, indicating constructive and destructive interference of the waves.
However, when the experimenter tries to determine which slit a particle passes through, by placing detectors or any other means of measurement, something interesting happens. The interference pattern disappears, and the particles behave more like individual particles rather than waves. Instead of producing an interference pattern, they create a simple pattern of two distinct bands corresponding to the two slits.
This change in behavior occurs because the act of measurement or observation affects the quantum system. When we try to determine the path of a particle, we introduce interactions that disturb the delicate wave-like nature of the system. The act of measurement collapses the particle's wave function, forcing it to take on a definite position or path, and thereby destroying the interference pattern.
So, the presence of an observer or measurement apparatus does indeed affect the appearance of the double-slit interference pattern. Without observation or measurement, the pattern exhibits wave-like interference behavior, while with observation or measurement, it behaves more like a classical particle. This effect highlights the peculiar nature of quantum mechanics and the fundamental role of observation in shaping the behavior of quantum systems.