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 u u = 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. c = l x u u = c/l u = 3.0 x 108 m/s / 0.19m = 1.6 x 109 Hz
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 u l = c/u l = 3.0 x 108 m/s / 680,000 1/s = 440m
19cm = 0.19m
c = l x u u = 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 u l = c/u l = 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?