A Level Physics Practice Exam

How would the appearance of a diffraction pattern change if a non-laser source emitting white light replaces the red laser light?

1. Central fringe is red

2. All fringes will appear the same color

3. Central white fringe with spectral colors at subsidiaries

4. No change in the pattern

The correct answer is: Central white fringe with spectral colors at subsidiaries

When a non-laser source emitting white light is used instead of red laser light, the appearance of the diffraction pattern will indeed change significantly. The key aspect to understand is that white light is made up of multiple wavelengths corresponding to different colors. In a diffraction pattern created with a white light source, the central fringe will still appear white because all colors of light constructively interfere at the center due to their overlapping wavelengths. However, as you move outward from the center, the different wavelengths (or colors) will spread out due to their varying diffraction angles. This leads to the formation of a spectrum of colors (like a rainbow) in the subsidiary fringes. Each color will diffract at a different angle, resulting in the characteristic multicolored appearance of the pattern outside the central fringe. Thus, the correct option reflects that the central fringe will be white while the subsidiary fringes will display distinct colors corresponding to the spectrum of white light. This stands in contrast to the uniform red color of the fringes seen with laser light, which consists of a single wavelength. Understanding this distinction between monochromatic light and polychromatic light sources is crucial in grasping why the diffraction pattern changes in this way.