1. Lecture 20 Electromagnetic Waves Nature of Light
Reflection and Refraction

2. Light is bent (refracted) as it passes through water, with different wavelengths bending by different amounts (which is called dispersion). Together with reflection, these physical phenomena lead to the creation of a rainbow when light passes through small, suspended droplets of water.
Fig. 22-CO, p.726

3. Electromagnetic Waves are Transverse Waves
The and fields are perpendicular to each other
Both fields are perpendicular to the direction of motion
Therefore, em waves are transverse waves

4. Properties of EM Waves Electromagnetic waves are transverse waves
Electromagnetic waves travel at the speed of light
Because em waves travel at a speed that is precisely the speed of light, light is an electromagnetic wave

5. Properties of EM Waves, 2
The ratio of the electric field to the magnetic field is equal to the speed of light
Electromagnetic waves carry energy as they travel through space, and this energy can be transferred to objects placed in their path

6. Properties of EM Waves, 3
Energy carried by em waves is shared equally by the electric and magnetic fields

7. Properties of EM Waves, final
Electromagnetic waves transport linear momentum as well as energy
For complete absorption of energy U, p=U/c
For complete reflection of energy U, p=(2U)/c
Radiation pressures can be determined experimentally

8. Determining Radiation Pressure
This is an apparatus for measuring radiation pressure
In practice, the system is contained in a vacuum
The pressure is determined by the angle at which equilibrium occurs

9. The Spectrum of EM Waves
Forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths
c = ƒλ
Wavelengths for visible light range from 400 nm to 700 nm
There is no sharp division between one kind of em wave and the next

10. The EM Spectrum Note the overlap between types of waves
Visible light is a small portion of the spectrum
Types are distinguished by frequency or wavelength

11. Notes on The EM Spectrum
Radio Waves
Used in radio and television communication systems
Microwaves
Wavelengths from about 1 mm to 30 cm
Well suited for radar systems
Microwave ovens are an application

12. Notes on the EM Spectrum, 2
Infrared waves
Incorrectly called “heat waves”
Produced by hot objects and molecules
Readily absorbed by most materials
Visible light
Part of the spectrum detected by the human eye
Most sensitive at about 560 nm (yellow-green)

13. Notes on the EM Spectrum, 3
Ultraviolet light
Covers about 400 nm to 0.6 nm
Sun is an important source of uv light
Most uv light from the sun is absorbed in the stratosphere by ozone
X-rays
Most common source is acceleration of high-energy electrons striking a metal target
Used as a diagnostic tool in medicine

14. Notes on the EM Spectrum, final
Gamma rays
Emitted by radioactive nuclei
Highly penetrating and cause serious damage when absorbed by living tissue
Looking at objects in different portions of the spectrum can produce different information

15. Arecibo, a large radio telescope in Puerto Rico, gathers electromagnetic radiation in the form of radio waves. These long wavelengths pass through obscuring dust clouds, allowing astronomers to create images of the core region of the Milky Way Galaxy, which can't be observed in the visible spectrum.
Fig. 21-CO, p.693

16. Radio
FIGURE Observations in different parts of the electromagnetic spectrum show different features of the Crab Nebula. (d) Radio image.
Fig d, p.717

17. X-ray
FIGURE Observations in different parts of the electromagnetic spectrum show different features of the Crab Nebula. (a) X-ray image.
Fig a, p.717

18. optical
FIGURE Observations in different parts of the electromagnetic spectrum show different features of the Crab Nebula. (b) Optical image.
Fig b, p.717

19. Infra-red
FIGURE Observations in different parts of the electromagnetic spectrum show different features of the Crab Nebula. (c) Infrared image.
Fig c, p.717

20. A Brief History of Light
1000 AD
It was proposed that light consisted of tiny particles
Newton
Used this particle model to explain reflection and refraction
Huygens
1678
Explained many properties of light by proposing light was wave-like

21. A Brief History of Light, cont
Young
1801
Strong support for wave theory by showing interference
Maxwell
1865
Electromagnetic waves travel at the speed of light

22. A Brief History of Light, final
Planck
EM radiation is quantized
Implies particles
Explained light spectrum emitted by hot objects
Einstein
Particle nature of light
Explained the photoelectric effect

23. The Particle Nature of Light
“Particles” of light are called photons
Each photon has a particular energy
E = h ƒ
h is Planck’s constant
h = 6.63 x J s
Encompasses both natures of light
Interacts like a particle
Has a given frequency like a wave

24. Dual Nature of Light
Experiments can be devised that will display either the wave nature or the particle nature of light
In some experiments light acts as a wave and in others it acts as a particle
Nature prevents testing both qualities at the same time