Why do secondary maxima decrease in intensity?

Secondary maxima decrease in intensity due to the destructive interference of light waves. This phenomenon is a key aspect of studying types of waves, which is foundational in understanding wave behaviour.

In more detail, the phenomenon of secondary maxima decreasing in intensity is a result of the wave nature of light. When light passes through a double slit, it diffracts and interferes, creating a pattern of bright and dark fringes on a screen, a concept further explored in the study of wavefronts and rays. This is known as the double-slit interference pattern. The bright fringes, or maxima, are areas where the light waves interfere constructively, meaning their peaks and troughs align. The dark fringes, or minima, are areas where the light waves interfere destructively, meaning the peak of one wave aligns with the trough of another, cancelling each other out.

The brightest fringe, or the primary maximum, is found directly opposite the double slit, where the path difference between the two slits is zero or a whole number of wavelengths. This is where the constructive interference is strongest. The secondary maxima are found on either side of the primary maximum, where the path difference is a half or a whole odd number of wavelengths. Here, the constructive interference is not as strong, because the waves are not perfectly in phase. This results in the secondary maxima being less bright, or having a lower intensity, than the primary maximum.

Furthermore, the intensity of the secondary maxima decreases progressively as you move away from the primary maximum. This is because the path difference, and therefore the phase difference, increases. As the phase difference increases, the waves become more out of phase, leading to more destructive interference and less constructive interference. This results in a decrease in the intensity of the secondary maxima. To visualise this concept, examining diffraction patterns can provide a clearer understanding of how light behaves when it encounters obstacles.

IB Physics Tutor Summary: In simpler terms, when light waves go through two slits, they spread out and mix together, creating bright and dark lines on a screen. The secondary maxima are the less bright lines on either side of the brightest line. They're dimmer because the light waves aren't perfectly lined up, causing some of the light to cancel out, making these areas less bright.