1. Interference and Diffraction

2. Combining Light Waves
Interference – the combining of waves via the superposition principle
Add the amplitudes (displacements)
Combined wave is called the resultant wave
Interference only takes place when waves have the same wavelength
Constructive interference – resultant amplitude is greater than component amplitudes
Destructive interference – resultant amplitude is less than the greatest component amplitude
Component amplitudes at least partially cancel each other out

3. Combining Light Waves
Waves must have a constant phase difference to be observed
Phase difference – how the waves overlap
Crest-to-crest is 0° – in phase
Crest-to-trough is 180° – out of phase
Coherence – the correlation between the phases of two or more waves
The waves do not shift relative to one another
The source of such waves is called coherent
Sources that do not produce observable interference due to the waves not having a constant phase difference are called incoherent

4. Combining Light Waves Equation for Constructive Interference
The path difference between two waves = an integer multiple of the wavelength
d * sinθ = ±m * λ m = 0, 1, 2, 3…
Equation for Destructive Interference
The path difference between two waves = an odd number of half wavelengths
d * sinθ = ±(m + ½) * λ m = 0, 1, 2, 3…

5. Demonstrating Interference
You can produce interference patterns in light waves by passing the light through a single slit and then two parallel slits
Monochromatic (single wavelength or color) light produces a pattern of alternating bright and dark bands
Bright bands are formed from constructive interference
Dark bands are formed from destructive interference
White light or other polychromatic light produces bands of color
The pattern will be indistinct because the different colors have constructive and destructive interference at different positions

6. Demonstrating Interference
Path difference – the difference in the distance traveled by two beams when they are scattered un the same direction from different points
Difference in the distance traveled by two interfering waves
Distance between slits on a filter (for light waves)
If the path difference is 0 or a whole number multiple of the wavelength, the waves are in phase and constructive interference results
If the path difference is an odd multiple of ½ the wavelength, the waves are 180° out of phase and destructive interference results