Chapter 18 – The Electromagnetic Spectrum and Light

1. Chapter 18 – The Electromagnetic Spectrum and Light Jennie L. Borders

2. Section 18.1 – Electromagnetic Waves • Electromagnetic Waves are transverse waves consisting of changing electric fields and changing magnetic fields. • An electric field in a region of space exerts electric forces on charged particles. • A magnetic field in a region of space produces magnetic forces.

3. Electromagnetic Waves • Electromagnetic waves are produced when an electric charge vibrates or accelerates. • This is a transverse wave because the particles are vibrating perpendicular to the direction of the wave.

4. Electromagnetic Waves • Electromagnetic waves can travel through a vacuum, or empty space, as well as through matter. • The transfer of energy by electromagnetic waves traveling through matter or across space is called electromagnetic radiation.

5. Speed of Electromagnetic Waves • The speed of light in a vacuum is 3.0 x 108 m/s. • In a vacuum, all electromagnetic waves travel at the same speed. • Electromagnetic waves vary in wavelength and frequency.

6. Electromagnetic Wave Speed • Formula for speed of a wave – v = l x u For electromagnetic waves, v is always the speed of light which is represented by a c. c = l x u c = speed of light (3.0 x 108 m/s) l = wavelength (m) u = frequency (Hz)

7. Sample Problem • A radio station broadcasts a radio wave with a wavelength of 3.0m. What is the frequency of the wave? c = l x uu = c/l c = 3.0 x 108 m/s u = 3.0 x 108 m/s / 3.0m l = 3.0m u = 1.0 x 108 Hz u = ?

8. Practice Problems • The radio waves of a particular AM radio station vibrate 680,000 times per second. What is the wavelength of the wave? • A global positioning satellite transmits a radio wave with a wavelength of 19cm. What is the frequency of the radio wave? c = l x ul = c/ul = 3.0 x 108 m/s / 680,000 1/s = 440m 19cm = 0.19m c = l x uu = c/l u = 3.0 x 108 m/s / 0.19m = 1.6 x 109 Hz

9. Wave or Particle? • Electromagnetic radiation behaves sometimes like a wave and sometimes like a particle. • Evidence for a wave includes the fact that light can produce constructive and destructive interference.

10. Wave or Particle? • Evidence for a particle includes the fact that light causes the photoelectric effect. • The emission of electrons from a metal caused by light striking the metal is called the photoelectric effect.

11. Photons • In 1905, Albert Einstein proposed that light, and all electromagnetic radiation, consists of packets of energy called photons. • Each photon’s energy is proportional to the frequency of the light.

12. Intensity • Photons travel outward from a light source in all directions. • The intensity of light decreases as photons travel farther from the source.

13. Section 18.1 Assessment • How fast does light travel in a vacuum? • What makes electromagnetic waves different from one another? • What happens to the intensity of light as photons move away from the light source? • What is the wavelength of an AM radio wave in a vacuum if its frequency is 810 kHz? 810 kHz = 810,000Hz c = l x ul = c/ul = 3.0 x 108 m/s / 810,000 1/s = 370m

14. Section 18.2 – The Electromagnetic Spectrum • A scientist observed that when light is passed through a prism the temperature of the light is lower at the blue end and higher toward the red end. • He also concluded that there was invisible radiation beyond the visible spectrum.

15. Electromagnetic Spectrum • The full range of frequencies of electromagnetic radiation is called the electromagnetic spectrum. • The electromagnetic spectrum includes radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays.

16. Radio Waves • Radio waves are used in radio and television technologies, as well as in microwave ovens and radar. • In AM radio, the stations have signals with varying amplitudes. • In FM radio, the stations have signals with varying frequencies.

17. Radio Waves • The shortest radio waves are called microwaves. • Microwaves also carry cell phone conversations.

18. Infrared Rays • Infrared rays are used as a source of heat and to discover areas of heat differences. • Thermograms are color-coded pictures that show variations in temperature.

19. Visible Light

20. Ultraviolet Rays • Ultraviolet radiation has higher frequencies than violet light. • Ultraviolet rays have applications in health and medicine, and in agriculture.

21. X-Rays • X-rays have high energy and can penetrate matter that light cannot. • X-rays are used in medicine, industry, and transportation to make pictures of the inside of solid objects.

22. Gamma Rays • Gamma rays have the shortest wavelengths in the electromagnetic spectrum, about 0.005nm or less. • Gamma rays are used in the medical field to kill cancer cells and make pictures of the brain, and in industrial situations as an inspection tool.

23. Section 18.2 Assessment • List the kinds of waves included in the electromagnetic spectrum, from longest to shortest wavelength. • How are AM radio waves different from FM radio waves? • What type of electromagnetic waves are microwaves?

24. Section 18.3 – Behavior of Light • Materials can be transparent, translucent, or opaque. • A transparent material transmits light, which means it allows most of the light that strikes to pass through it. • A translucent material scatters light. • An opaque material either absorbs or reflects all of the light that strikes it.

25. Interactions of Light • When light strikes a new medium, the light can be reflected, absorbed, or transmitted. • When light is transmitted, it can be refracted, polarized, or scattered.

26. Reflection • An image is a copy of an object formed by reflected (or refracted) waves of light. • Regular reflection occurs when parallel light waves strike a surface and reflect all in the same direction. • Diffuse reflection occurs when parallel light waves strike a rough surface, and reflect in many different directions.

27. Refraction • A mirage is a false or distorted image.

28. Polarization • Light with waves that vibrate in only one plane is polarized light.

29. Scattering • Scattering means that light is redirected as it passes through a medium.

30. Section 18.3 Assessment • Explain the differences among opaque, transparent, and translucent materials. • List and explain three things that can happen to a light wave when it enters a new medium. • What is the difference between diffuse reflection and regular reflection?