IB DP Physics: HL复习笔记9.3.2 Two-Slit Interference Patterns

For two-source interference fringes to be observed, the sources of the wave must be:
- Coherent (constant phase difference)
- Monochromatic (single wavelength)

When two waves interfere, the resultant wave depends on the phase difference between the two waves
- This is proportional to the path difference between the waves which can be written in terms of the wavelength λ of the wave
As seen from the diagram, the wave from slit S2 has to travel slightly further than that from S1 to reach the same point on the screen
- The difference in this distance is the path difference

8.2.2.3-Path-difference-equations

Interference is caused by the variation in path length between the two slits

For constructive interference (or maxima), the difference in wavelengths will be an integer number of whole wavelengths
The condition for constructive interference can be written as:
n is the order of the maxima (bright fringes) / minima (dark fringes)
- n = 0 is taken from the middle, n = 1 is the next peak and so on
When waves undergo destructive interference their amplitudes subtract to form a resultant combined wave at the point where they meet
The equations above are the same versions as found in your data booklet
- Don't let other versions of these equations confuse you
- Constructive and destructive interference can be calculated with alternative equations for specific values of n

Interference pattern of light waves shown with orders of maxima and minima

An interference pattern shows the intensity of light at different distances away from the central maxima
- For a double-slit interference pattern the intensity of the light is the same for all maxima
This is different to the double-slit diffraction pattern that is observed on a screen (These are explained in 9.3.3 Diffraction Grating Patterns)

The diffraction pattern made by waves passing through two slits of separation d can be observed on a screen placed a large distance, D, away

9-3-2-double-slit-geometry