An interference pattern is formed by sending red light through a. pair of narrow slits. If blue light is then used, the spacing of the bright areas (where constructive interference takes place) won't be the same. How will it be different? Why?

Imagine throwing two pebbles into a calm pond and watching the ripples overlap. Where the waves crest at the same point, they add up to create a larger wave. This visual is akin to constructive interference, a fundamental concept in wave physics, which occurs when two waves meet in phase - that is, their crests and troughs align to enhance each other.

In the context of light, when two beams of photons traveling in the same direction overlap, their electric fields can interact. If they match up such that the crest of one wave aligns with the crest of another, and the trough aligns with a trough, their amplitudes combine, resulting in a brighter, or 'constructive' spot. This principle underlies the bright bands seen in a double-slit interference pattern, where light waves pass through two narrow slits and overlap on a screen, generating regions of higher intensity where constructive interference occurs.

Every color of light we can see in the spectrum carries a distinct wavelength, which is the distance between successive peaks (or crests) of a wave. The human eye perceives different wavelengths as different colors, with red light having longer wavelengths (about 650-700 nanometers) and blue light having shorter (around 450-495 nanometers).

The wavelength not only determines the color but also has a profound effect on how light behaves when encountering obstacles or slits, like in a double-slit experiment. A fundamental property to remember is that different wavelengths bend or 'diffract' to varying degrees. Shorter wavelengths diffract less than longer ones, implying that blue light with its shorter wavelength will form closer spaced interference patterns compared to red light when passed through the same slits.

When a wave encounters an obstacle or a slit, it doesn't just stop; it bends around it, spreading out. This is diffraction. In the double-slit interference experiment, light waves diffract upon passing through two slits and then interfere with each other, creating a series of light and dark bands known as an interference pattern on a screen.

The interference pattern is a hallmark of wave behavior and it's the result of the constant combining and canceling out of waves (constructive and destructive interference). The position and spacing of these patterns are closely tied to the wavelength of light used. Hence, when different colors of light are used, the interference patterns visibly change, directly illustrating the wave nature of light and the diversity within the spectrum of colors visible to our eyes.