The Nature of Light

1. The Nature of Light • When studying geometric optics, we used a ray model to describe the behavior of light. • A wave model of light is necessary to describe phenomena such as: • interference • diffraction • A particle model of light is necessary to describe phenomena observed in modern physics, for example, the interaction between light and atoms. We’ll get back to this later... Light as a Wave

2. Wave Nature of Light • Christian Huygens (1629-1695) • contemporary of Newton • developed wave theory of light • Huygen’s Principle • 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 - tangent to all of them Light as a Wave

3. Huygen’s Principle Light as a Wave

4. Diffraction • Huygen’s Principle is useful for understanding diffraction - the bending of waves behind obstacles into the shadow region Light as a Wave

5. Interference • Thomas Young (1773-1829) • definitively (at least temporarily) demonstrates wave nature of light • Young’s Double-Slit Experiment • coherent light passes through 2 slits, S1 and S2 • light from S1 and S2 then interferes and pattern of dark and light spots is observed on the screen Light as a Wave

6. Interference • Constructive interference occurs when • d sin = m  , m = 0,1,2,... • m = order • Destructive interference occurs when • d sin = (m + 1/2)  , m = 0,1,2,... • Source must be coherent • waves at S1 and S2 are in-phase Light as a Wave

7. what you see on the screen: Light as a Wave

8. Think-Pair-Share • Monochromatic light falling on two slits 0.016 mm apart produces the fifth-order fringe at an 8.8 degree angle. What is the wavelength of the light used? Light as a Wave

9. Conceptual Question • What happens to the interference pattern if the wavelength of light is increased from 500 nm to 700 nm? • What happens instead if the wavelength stays at 500 nm but the slits are moved farther apart? Light as a Wave

10. Pair Problem • Light of wavelength 680 nm falls on two slits and produces an interference pattern in which the fourth-order fringe is 38 mm from the central fringe on a screen 2.0 m away. What is the separation of the two slits?(Hint: tan =  for small angles, and angles must be in radians!) Light as a Wave

11. Diffraction by a Disk • Diffracted light interferes constructively at center of shadow • requires a point source of monochromatic light (e.g. laser) Light as a Wave

12. Diffraction by a Single Slit • D sin  = m  • m = 1, 2, 3, ... position of minima for m=1, theta gives 1/2 width of central maximum Motivation for making large diameter telescopes Light as a Wave

13. Diffraction Grating • a large number of equally spaced parallel slits • same relation as double-slit • d sin  = m  • m = 0, 1, 2, ... • produces sharper and narrower interference patterns that double slit Light as a Wave

14. Diffraction Grating (L) • double slit versus diffraction grating • for multi-wavelength light Light as a Wave

15. Emission Tubes • Look at several emission tubes using diffraction gratings & sketch spectrum • Foundation of spectroscopy, a technique used in numerous scientific applications Element Light as a Wave

16. Interference by Thin Films Produces rings of constructive/destructive interference Light as a Wave

17. Thin-Film Interference: Examples Oil on water Soap bubbles Beetles Butterflies Light as a Wave

18. Group Problems • Q2B.6, Q2S.9 Light as a Wave