Diffraction: Mechanical Waves and Light Waves4.3.5 – 4.3.74.4.9 – 4.4.13A.S. Due Monday, May 18

Diffraction

• Observations (first officially recorded in the 1600’s!)• Frequency, wavelength and speed of waves remain same after diffraction• Direction of waves will change—the waves will bend around the edge of barriers• Effect of diffraction is most obvious when aperture (width of the slit) is approximately equal to the size of the wavelength• Amplitude of diffracted wave is less than incident wave; energy is distributed over a larger area.

Huygens’ Principle

On a mechanical wavefront, imagine each point to be an individual point source (that creates a circular wavefront)

• An infinite number of point sources will add together (constructive interference) and form a straight wavefront

• As the wavefront passes through an opening (aperture), the point sources at the edges no longer have a neighboring wave with which to interfere, so the circular wavefront is visible, making the wave “bend” around the barrier.

Principle of Superposition--reminder

• When 2 or more waves are in the same place at the same time, the amplitudes at that hoff position will add together to form a single wave

• Constructive interference: The result of two positive amplitudes adding together (or two negative amplitudes) to form a single wave that has a larger amplitude than either of the two individual waves.

• Destructive interference: The result of a positive amplitude and a negative amplitude adding together to form a single wave with a smaller overall amplitude.• Total Destructive Interference: with the two amplitudes are equal, but opposite—the overall amplitude will be zero, and the medium will appear to not be oscillating

Mechanical Waves through 2 slits (or 2 point sources)

Diffraction of Light• Even through a single slit, Light shows patterns

that mimic mechanical waves through 2 slits:

• Central maximum: The brightest section of the light’s interference pattern, directly across from the opening (slit)

• Bright fringes (maxima): areas where constructive interference occurs

• Dark fringes (minima): areas where total destructive interference occur

How do these patterns come about?• Basic Trigonometry, and the fact that light

diffracts around both edges of the slit

• Path difference:the difference in thedistance from one edgeof the slit to a point onthe screen and the distance from the otheredge of the slit to thatsame point

Diffraction patterns

• In summary:• If the path difference is equal to an integral multiple of the wavelength of light, a maxima will occur (nl) (imagine two crests meeting)• If the path difference is equal to ((n+1/2)l), then a minima will occur. (imagine a crest meeting a trough)

• Fringe order: (Order of the fringe)—is the “n” value