Physics 24: Wave Nature of Light

1. Physics 24: Wave Nature of Light Christopher Chui Physics 24: Wave Nature of Light - Christopher Chui

2. Huygen’s Principle and Diffraction • Every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets. • The bending of waves behind obstacles into the shadow region is known as diffraction • It derives the Snell’s Law: n1 sin q1 = n2 sin q2 • Wavelength in a medium of index of refraction n = ln = l / n Physics 24: Wave Nature of Light - Christopher Chui

3. Young’s Double-Slit Experiment • Thomas Young (1801) explained light interfered as a wave to produce multiple images in a 2-slit expt. • Constructive interference: d sin q = ml, m=0,1,2,… • Destructive interference: d sin q =(m+1/2) l, m=0,1,2,.. • Wavelength (frequency) determines color • Visible spectrum from UV (400 nm)  IR (750 nm) • The spreading of light into its spectrumdispersion • The unexpected diffraction spot produced by a disk • A diffraction pattern exists around any object illuminated by a point source Physics 24: Wave Nature of Light - Christopher Chui

4. Diffraction Equation, sin q = l / D • Single slit diffraction minima: D sin q = ml, m=1,2,3.. • A large number of equally spaced // slitsdiffraction grating; up to 10,000 lines/cm is possible • A diffraction grating containing slitstransmission grating. Reflection gratings are also possible • Principal maxima: sin q = ml / d, m = 0,1,2,.. • A diffraction grating spreads out light into its component wavelengths, the patterna spectrum • A spectrometer measures wavelengths accurately • When a gas is heated, the gasa line spectrum • The Sun’s spectrum contains several dark linesabsorption spectrum Physics 24: Wave Nature of Light - Christopher Chui

5. Newton’s Rings Brewster’s Law • A curved glass surface is placed in contact with a flat glass surfacea series of concentric rings when illuminated by a monochromatic light • A beam of light reflected by a material whose index of refraction is > than that of the materialphase change of ½ cycle • Coating of glass lens to make it “nonreflecting” • Maxwell’s theory predicts light can be polarized • Tourmaline can turn unpolarized light into polarized • A Polaroid can determine if light is polarized • Brewster’s law: tan qP = n, and qr = 90o - qP Physics 24: Wave Nature of Light - Christopher Chui

6. Problems Solving for Interference • Interference depends on two or more waves at the same point in space • Constructive interference occurs when waves arrive in phase with each other. The amplitudes of the waves add to form a larger amplitude. It also occurs when the phase difference is 1l, 2l, 3l, … • Destructive interference occurs when waves arrive out of phase. The amplitudes add in opposite sign and cancel each other. It occurs when the phase difference is a half-integral number of l, or (m+1/2) l • For thin-film interference, reflected light changes ½ l, n1 < n2 Physics 24: Wave Nature of Light - Christopher Chui