Chapter 19 Diffraction and Interference of Light

Objectives

• 19.1 Relate the diffraction of light to its wave characteristics

• 19.1 Explain how light falling on two closely spaced slits produces and interference pattern and use measurements to calculate wavelengths of light

• 19.1 Apply geometrical models to explain single slit diffraction and two slit interference patterns

Objectives

• 19.2 Explain how diffraction gratings form interference patterns and how they are used in grating spectrometers

• 19.2 Discuss how diffraction limits the ability of a lens to distinguish two closely spaced objects

Chapter 19: Wave model

Diffraction

• Bending around an object.

• Short Waves diffract less

Single Slit

• Reminder: If the wave is too big, won’t fit through

Vocabulary

• Interference pattern: Produced by overlapping of waves (areas of dark and intense light)

• Monochromatic: One wavelength of light• Coherent Waves: Wavelengths where crests

and troughs line up exactly

Usefulness of Young’s Double Slit Experiment

• How we were able to determine the wavelengths of light

• We’ll avoid the math for now

Diffraction Grating

• Lots of slits, able to produce patterns like the double slit

Diffraction Patters

• Double Slit

• Wavelength of light = X (Distance between interference bands on screen) D (distance between slits) / L (Distance from slits to screen)

lxd

Diffraction Patterns

• Single Slit

• Single Slit Equation: X = (Lamda)(L) / W (Width)

wlx

CD: A very crude diffraction grating

Astronomy Connection

• Light from telescopes enters through a tiny opening which ends up diffracting the light just a bit. If two stars are too close, you may not be able to tell if there is one or two

• The Rayleigh criterion for resolution