Chapter 19-1 Interference of Light

Diffraction HISTORY of the concept of diffraction Begins with the old debate: –Is light a wave –Or is light a particle? GRIMALDI: edges of shadows are not perfectly sharp – he named the slight spreading of light around barriers “diffraction” NEWTON: light is a corpuscle (particle) HUYGENS: light is a wave

Wave Model of Diffraction The crest of each wave can be seen as a series of point sources Each point source creates a circular wavelet All wavelets add together: –In the middle is a wave front –At edges circular waves spread out

Monochromatic, Coherent Light Source: Double Slit Diffraction produces bright and dark bands called “interference fringes” caused by constructive and destructive interference Young’s Two Slit Experiment

Double Slit Diffraction of White Light Bright white central band –constructive interference of all light frequencies Bands of spectrum of color –Produced where that color (frequency) undergoes constructive interference

Measuring Wavelength of a Light Wave All light travels same distance to reach P o At P o all light waves interact constructively Produces bright band at P o, the “central band” NOT TO SCALE! Width is actually MUCH wider

Measuring Wavelength (cont) At P: 1 st bright band on either side of central band Light from both slits interferes constructively Called the “First Order Line” The two path lengths differ by one wavelength NOT TO SCALE! Width is actually MUCH wider

x = distance between bright bands d = distance between slits L = distance between slits and screen

Light passes through a single, small opening Light is diffracted A series of bright and dark bands appears Wide bright central band Dimmer bands on either side Single Slit Diffraction

x = distance between bright bands w = width of slit L = distance between slit and screen Wavelength Using Single Slit Interference:

Single slit practice problem:

End 19-1