Chapter2: Light and EM Spectrum

1. Chapter2: Light and EM Spectrum Professor Mohammad Sajjad Alam University at Albany September 28, 2010 Adapted from Web • 18.1 The Electromagnetic Spectrum • 18.2 Interference, Diffraction, and Polarization • 18.3 Special Relativity

2. 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.

3. Chapter 18 Vocabulary Terms • x-ray • spectrum • microwave • index of refraction • electromagnetic wave • spectrometer • gamma ray • radio wave • transmission axis • diffraction grating • special relativity • polarization • polarizer rest • energy destructive • interference • ultraviolet • time dilation • infrared • speed of light • constructive interference • visible light • wavelength

4. Key Question: What is the electromagnetic spectrum? 18.1 The Electromagnetic Spectrum *Students read Section 18.1 BEFORE Investigation 18.1

5. 18.1 The Electromagnetic Spectrum • The energy field created by electricity and magnetism can oscillateand it supports waves that move. • These waves are called electromagnetic waves.

6. 18.1 The Electromagnetic Spectrum • Electromagnetic waves have both an electric part and a magnetic part and the two parts exchange energy back and forth. • A 3-D view of an electromagnetic wave shows the electric and magnetic portions. • The wavelength and amplitude of the waves are labeled λ and A, respectively.

7. 18.1 The Electromagnetic Spectrum • The higher the frequency of the light, the higher the energy of the wave. • Since color is related to energy, there is also a direct relation between color, frequency, and wavelength.

8. 18.1 Speed of Light Wavelength (m) c = f l Speed of light 3 x 108 m/sec Frequency (Hz)

9. 18.1 Calculate wavelength • Calculate the wavelength in air of blue-green light that has a frequency of 600 × 1012 Hz.

10. 18.1 Waves of the electromagnetic spectrum • Visible light is a small part of the energy range of electromagnetic waves. • The whole range is called the electromagnetic spectrum and visible light is in the middle of it.

11. 18.1 Waves of the electromagnetic spectrum • Radio waves are on the low-frequency end of the spectrum. • Microwaves range in length from approximately 30 cm (about 12 inches) to about 1 mm. • The infrared (or IR) region of the electromagnetic spectrum lies between microwaves and visible light.

13. 18.1 Waves of the electromagnetic spectrum • Ultraviolet radiation has a range of wavelengths from 400 down to about 10 nm. • X-rays are high-frequency waves that have great penetrating power and are used extensively in medical and manufacturing applications. • Gamma rays are generated in nuclear reactions.

15. Key Question: What are some ways light behaves like a wave? 18.2 Interference, Diffraction, and Polarization *Students read Section 18.2 AFTER Investigation 18.2

16. 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, Diffraction, and Polarization

18. 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.

19. 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 How a Diffraction Grating Works

20. 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. 18.2 Spectrometer

21. 18.2 Grating Formula distance between grating lines (m) distance between 2 first order bright spots wavelength of light (nm) l = d sinq ~ dw L distance between screen and glasses d= 13,500 lines/inch = ? lines/m

22. Polarization is another wave property of light. The fact that light shows polarization tells us that light is a transverse wave. 18.2 Polarization

24. Polarization is a vector. A wave with polarization at 45 degrees can be represented as the sum of two waves. Each of the component waves has smaller amplitude. 18.2 Polarization

26. 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. 18.2 Polarization

27. 18.2 Applications of Polarizers • 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.