18.2 Interference, Diffraction, and Polarization

Chapter 18 Objectives Calculate the frequency or wavelength of light when given one of the two. Describe the relationship between frequency, energy, color, and wavelength. Identify at least three different waves of the electromagnetic spectrum and an application of each. Interpret the interference pattern from a diffraction grating. Use the concept of polarization to explain what happens as light passes through two polarizers. Describe at least two implications of special relativity with regards to energy, time, mass, or distance. 2

Chapter 18 Vocabulary Terms diffraction grating electromagnetic spectrum electromagnetic wave gamma ray inference pattern microwave polarization polarizer radio wave rest energy special relativity spectrometer spectrum time dilation transmission axis visible light x-ray

Inv 18.2 Interference, Diffraction, and Polarization Investigation Key Question: What are some ways light behaves like a wave? 4

18.2 Interference, Diffraction, and Polarization In 1807, Thomas Young (1773-1829) did the most convincing experiment demonstrating that light is a wave. A beam of light fell on a pair of parallel, very thin slits in a piece of metal. After passing through the slits, the light fell on a screen. A pattern of alternating bright and dark bands formed is called an interference pattern.

18.2 Interference An interference pattern is created by the addition of two waves.

18.2 Diffraction gratings A diffraction grating is a precise array of tiny engraved lines, each of which allows light through. The spectrum produced is a mixture of many different wavelengths of light.

18.2 How a Diffraction Grating Works When you look at a diffracted light you see: the light straight ahead as if the grating were transparent. a "central bright spot". the interference of all other light waves from many different grooves produces a scattered pattern called a spectrum.

18.2 Spectrometer A spectrometer is a device that measures the wavelength of light. A diffraction grating can be used to make a spectrometer because the wavelength of the light at the first-order bright spot can be expressed in a mathematical relationship.

Distance between grating lines (m) 18.2 Grating Formula Distance between grating lines (m) Wavelength of light (nm) l = d sinq Angle 

18.2 Polarization Polarization is another wave property of light. The fact that light shows polarization tells us that light is a transverse wave. Most of the light that you see is unpolarized. That does not mean the light has no polarization. Unpolarized light is actually an equal mixture of all polarizations. We call ordinary light unpolarized because no single polarization dominates the mixture. 11

18.2 Polarization The direction of polarization is a vector and can be resolved into components in two directions. A wave that has 45-degree polarization is the addition of two smaller-amplitude component waves with horizontal and vertical polarizations. Most of the light that you see is unpolarized. That does not mean the light has no polarization. Unpolarized light is actually an equal mixture of all polarizations. We call ordinary light unpolarized because no single polarization dominates the mixture. 12

18.2 Polarization A wave with polarization at 45 degrees can be represented as the sum of two waves. Each of the component waves has smaller amplitude. Most of the light that you see is unpolarized. That does not mean the light has no polarization. Unpolarized light is actually an equal mixture of all polarizations. We call ordinary light unpolarized because no single polarization dominates the mixture. 13

18.2 Polarization A polarizer is a material that selectively absorbs light depending on polarization. A polarizer re-emits a fraction of incident light polarized at an angle to the transmission axis. Most of the light that you see is unpolarized. That does not mean the light has no polarization. Unpolarized light is actually an equal mixture of all polarizations. We call ordinary light unpolarized because no single polarization dominates the mixture. 14

18.2 Applications of polarization Polarizing sunglasses are used to reduce the glare of reflected light The LCD (liquid crystal diode) screen on a laptop computer uses polarized light to make pictures.