When light passes through two slits, it exhibits wave characteristics due to a phenomenon known as interference. This phenomenon arises from the wave nature of light and is described by the principles of wave interference.
Light is an electromagnetic wave, and when it encounters an obstacle or a small opening, such as two slits, it diffracts or spreads out. This diffraction causes the light to behave as if it were emanating from two coherent point sources located at the two slits. These two secondary wavefronts then propagate and overlap in the region beyond the slits.
As these two waves overlap, they interfere with each other constructively or destructively. Constructive interference occurs when the peaks of the two waves align, resulting in an amplified wave with higher intensity. Destructive interference, on the other hand, occurs when the peaks of one wave align with the troughs of the other wave, leading to a cancellation of the waves and lower intensity.
The interference pattern that emerges on a screen or a detector placed beyond the slits is a result of this constructive and destructive interference. It consists of alternating bright and dark regions, known as interference fringes or bands. The bright fringes correspond to regions of constructive interference, where the waves reinforce each other, while the dark fringes correspond to regions of destructive interference, where the waves cancel each other out.
This behavior of light passing through two slits is analogous to the behavior of water waves passing through two closely spaced openings, which also produce interference patterns. This similarity between light and other wave phenomena led to the conclusion that light itself behaves as a wave.
The wave characteristics of light are further supported by other experiments and phenomena, such as diffraction, polarization, and the ability of light waves to undergo interference with each other. These observations collectively establish the wave nature of light and form the basis of the wave theory of light.