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Theory
Theory
Interference
Interference
Interference is a process in which multiple waves combine to produce a resultant wave, which is the sum of the amplitudes of the various waves. If the waves are in phase (the same frequency and the maxima happen at exactly the same time), at some points an interference may be observed (see the figure below). In case (a), crests and troughs add creating a wave of a double amplitude. It is called a constructive interference. In case (b), crests and troughs cancel each other creating spots/patterns of no amplitude. It is called a destructive interference.
Figure 1. Constructive (a) and destructive (b) interference
Diffraction
Diffraction
Diffraction is the process by which a beam of light spreads out or the edge rays deflect away from the center of a beam. That can be explained with the Huygens's principle, which states that every point on a wave may be considered a source of spherical wave. Specifically, if the edge point of a wave passing through an opening becomes a source of a new wave, it can propagate in any direction, not only straight ahead. Hence, we observe deflection of light at the edge.
Fig 2. Diffraction
That is the reason that, if you observe a shadow, it does not have sharp edges - the rays on the edge of the object deflect slightly. It seems like the rays of light go around corners (see Figure 3).
Figure 3. The shadow during Solar eclipse (source)
Diffraction & Interference
Diffraction & Interference
Many physicists confuse diffraction with interference, and here is how: Because all the points of the same wave are in phase, the new spherical waves created according to the Huygens's principle interfere creating higher amplitude at some points or no amplitude at some points (Figure 4a, see the two points of purple waves). If the interference patterns of light can be observed on a screen, they form a set of bright and dark fringes.
Figure 4a. Two waves generated by points "in phase" interfere
Figure 4b. Bright and dark fringes observed on a screen.
For the first time, both phenomena were observed by Thomas Young in 1803 (1:30, Video 1 below).
This short video is a good introduction to the Interference and diffraction experiment whose effects can be explained only by assuming the wave nature of light. Notice people's reactions to what they see. Indeed, the results are counterintuitive for those who have not studied physics.
In Video 2, an interesting phenomenon of a light spot in the center of a shadow is explained. This is one of the lesser-known examples of phenomena that demonstrate the wave nature of light.
Video 1
Video 2
Summary
Summary
The key information is the set of light and dark spots created by constructive and destructive interference, respectively.
Experimental Procedure
Experimental Procedure
Use the PhET simulation below.
Qualitative Analysis
Qualitative Analysis
Experimental Settings
Experimental Settings
Select Diffraction in the PhET simulation (the last window in the simulation above).
Click the red button to put the laser on (Figure 5a).
Select the square shape (Figure 5b).
Adjust the slit's width and height (Figure 5c-d).
Figure 5. Diffraction dashboard
Experiemental Procedure
Experiemental Procedure
Part 1.
Use the slide button (Figure 5f) to slowly change the wavelength from 400 nm to 700 nm. Observe the pattern on the screen (Figure 5e). Write down your observations.
Part 2.
Select a wavelength of your choice. Use the slide button to slowly change the slit width from 0.04 mm to 0.4 mm (Figure 5c). Observe the fringe pattern. Write down your observations.
Quantitative Analysis
Quantitative Analysis
Experimental Settings
Experimental Settings
Select "Slit" in the PhET simulation (the third window in the simulation).
Click the green button to initiate the waves (Figure 6a).
Select the laser (the third button, Figure 6b).
Check "Screen" and "Intensity" (Figure 6c).
Select "Two Slides" (Figure 6d).
Figure 6. Slit Dashboard
Experimental Procedure
Experimental Procedure
Part A.
Measure the distance between two light fringes (two maxima) in a double-slit experiment.
In the PhET simulation, select a wavelength and the slit spacing.
Use the measuring tape to measure y (Figure 7).
Write down the results.
Figure 7
Part B.
Calculate the distance between two light fringes (two maxima) in a double-slit experiment for the same setting as above.
Use the measuring tape to measure L (Figure 8).
Use the measuring tape to measure lambda (Figure 9a). You can confirm lambda using the visible light spectrum (Figure 9b)
Calculate y according to the formula derived below (the formula in the red box).
Collect your data in the Data Collection Table (Figure 10).
Repeat Part A and Part B of the Quantitative Experiment for at least five different wavelengths and slit spacing.
Figure 8
Figure 9a
Figure 9b (source: ThoughtCo)
Double Slit Experiment Data Collection Table
Double Slit Experiment Data Collection Table
Figure 10
Formula for the distance between two light fringes in a double slit experiment
Formula for the distance between two light fringes in a double slit experiment
Diagram 1
Diagram 2
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