Chapter 19 diffraction and interference of light

1. Chapter 19diffraction and interference of light Physics Mrs. Martin

2. Diffraction • The bending of light waves around a boundary • Young’s Double Slit Experiment • Allowed for the measurement of the wavelength of light • Confirmed the wave theory of light • Created an interference pattern • The interference lines are equally spaced

3. Young’s double slit • Must use a monochromatic light source • One that emits only one wavelength of light • Example: Laser light • This allows for coherent waves to strike the surface • The waves’ crests reach the same point at the same time • Resulted in banding pattern • Called interference fringes

4. Diffraction of white light • Diffraction of white light causes the appearance of colored spectra instead of light bands and dark bands • This is due to the interference of different wavelengths of light

5. Measuring the wavelength of light • The central band of light is called the central band • Each bright band on either side of the first central band is called the first order line • Where the bright bands are located, constructive interference occurs • Where there are dark bands, destructive interference occurs

6. Measuring the wavelength of light • Formula • λ = xd/L • λ = wavelength (m) • x = distance between ordered bands (m) • d = distance between slits (m) • L = distance between the slit device and the screen (m) • Example • A two slit experiment is performed to measure the wavelength of red light. The slits are 0.0190 mm apart. A screen is placed 0.600 m away and the separation between the central bright line and the first-order bright line is found to be 21.1 mm. What is the wavelength of the red light?

7. Homework Problems • Violet light falls on two slits separated by 1.90 x 10-5 m. A first order line appears 13.2 mm from the central bright line on a screen 0.600 m from the slits. What is the wavelength of the violet light? • Yellow-orange light from a sodium lamp of wavelength 596 nm is aimed at two slits separated by 1.90 x 10-5 m. What is the distance from the central line to the first order yellow line if the screen is 0.600 m form the slits?

8. Single slit diffraction • Produces one wide central band and dimmer, smaller bands on the side • Because of the bending around the slit, destructive and constructive interference occurs

9. Single Slit diffraction • These problems are solved exactly the same way, except the distance between slits now becomes the width of the slit and x is equal to the distance from the central band to the first dark band. • λ = xw/L (where w is the width of the slit) • Example • Monochromatic green light of wavelength 546 nm falls on a single slit with a width 0.095mm. The slit is located 75 cm from the screen. How far from the center of the central band is the first dark band?

10. Homework problems • Yellow light falls on a single slit 0.0295 mm wide. On a screen 60.0 cm away, there is a dark band 12.0 mm from the center of the bright central band. What is the wavelength of light? • Light from a He-Ne laser (λ = 632.8 nm) falls on a slit of unknown width. A pattern is formed on a screen 1.15 m away on which the first dark band is 7.5 mm from the center of the central bright band. How wide is the slit?

11. Diffraction gratings • A device that transmits light in the same way a double slit does. • Transmission gratings are made by scratching fine lines on a piece of glass. • Can have as many as 10,000 lines per centimeter • Reflective gratings are made by making scratches on a reflective surface. As in a CD

12. Diffraction gratings • The interference pattern is the same as if using a double slit but the bright bands are narrower and the dark bands are broader and therefore more easily distinguishable. • Some instruments measure the distance between the first band, while others measure the angle. • Formula • λ = xd/L = d sin Θ • d is the distance between lines on the grating

13. Diffraction grating • Example: • A spectrometer uses a grating of 12000 lines/cm. Find the angles at which red light, 632 nm, and blue light, 4321 nm have first order fringes.

14. Telescope • Can have interference patter because the hole of the lens acts as a slit and spreads the light coming from a star out.