1. Physics 102 Wave Optics Moza M. Al-Rabban Professor of Physics mmr@qu.edu.qa Lecture 8

2. 2 Models of Light Is light a particle or a wave? Physics in the 20 th century has shown that this simple question does not have a simple answer. The behavior of light, depending on the circumstances, can be described by three distinct (and seemingly contradictory) models. We will introduce all of them and learn the conditions and circumstances under which each is valid. The Wave Model: This model works in many circumstances. When it is applicable, light shows the same interference behavior as water waves and sound waves. Lasers and electro-optical devices, critical technologies of the 21 st century, are best understood with the wave model of light, which we will call wave optics. The Photon Model: This model works in circumstances where energy detection is important, as describes light as a stream of photons, particles that carry packets of energy called quanta. The Ray Model: Light travels in straight lines, modified by reflection and refraction. This model works best for optical instruments and lenses, and is the basis for ray optics.

3. 3 The Interference of Light

4. 4 Young’s Double-Slit Experiment There are an alternating pattern of bright and dark “fringes” that arise from the interference of light passing through the two slits.

5. 5 Double-slit interference Experiment There are an alternating pattern of bright and dark “ interference fringes” that arise from the interference of light passing through the two slits. The fringes are numbered m= 0,1,2,3.. Going outward the center. The brightest fringe, at the midpoint of the viewing screen, with m=0, is called the central maximum.

6. 6 Analyzing Double-Slit Interference

7. 7 Bright fringes Dark fringes Constructive interference The fringe spacing

8. 8 Example: Double Slit Interference of a Laser Beam Light from a helium-neon laser ( = 633 nm) illuminates two slits placed 0.40 mm apart. A viewing screen is placed 2.0 m behind the slits. What are the distances  y 2 between the two m=2 bright fringes and  y’ 2 between the two m=2 dark fringes?

9. 9 Example: Measuring the Wavelength of Light A screen is placed 1.0 m behind a pair of slits, which are spaced 0.30 mm apart. When this system is illuminated by a certain frequency of monochromatic light, ten bright fringes are found to span a distance of 1.65 cm on the screen. What is the wavelength of the light?

10. 10 Intensity of the Double-slit Interference Pattern

11. End of Lecture 8