1. Light as a Wave
                                          
SPH4U
Young Star Cluster NGC 7129

2. What is Light?
                                               
All charged particles have an electric field. When they move, they change the electric field (and create a magnetic field). These field distortions propagate through space as an electromagnetic wave, aka light. Light is a charged particle saying, “Hey, I just moved.”

3. Electromagnetic Waves
The electric and magnetic field distortions are perpendicular to each other and to the direction of propagation:

4. Wavelength
The wavelength is determined by the scale of the charged particle’s displacement:
A displacement within an atomic nucleus (10-12 m) will create light with a wavelength of m.
A displacement within a radio antenna (1 m) will create light with a wavelength of 1 m.

5. Wavelength
The wavelength is determined by the scale of the charged particle’s displacement:
A displacement within an atomic nucleus (10-12 m) will create light with a wavelength of m.
A displacement within a radio antenna (1 m) will create light with a wavelength of 1 m.
gamma rays
radio waves

6. Light Production by Wavelength
Big Metal Antennae: Radio
Vibrating Molecules: Infrared
Excited Electrons: Visible Light
Chemical Bonds: UV
Ionized Electrons: X-rays
Atomic Nuclei: Gamma-Rays
(Note that there is more energy involved as you go to shorter wavelengths.)

7. The Electromagnetic Spectrum

8. The Electromagnetic Spectrum
Visible light is a very small part of the spectrum: between 700 nm (red) and 400 nm (violet).

9. Human Perception of Light
The colour detectors on our retinas (cones) are sensitive to red, green, and blue.

10. Light Speed
                                                    
Electromagnetic waves travel at: in a vacuum.

11. Light and Radiation
                                                    
Astronomical distances are measured in light travel time:
Planets/Sun: Light minutes away
Nearest Stars: Light years away
Nearest Galaxies: Millions of Light years
The Edge of the Observable Universe: Billion Light years
(light from further away hasn’t had time to reach us yet)

12. Frequency
Frequency is how many crests arrive each second:
                                                                                  

13. The Wave Equation

14. Example
What is the frequency of a light wave with a wavelength of 420 nm?

15. Example
What is the frequency of a light wave with a wavelength of 420 nm?

16. Example
What is the frequency of a light wave with a wavelength of 420 nm?

17. Blackbody Radiation
Objects emit radiation (lose energy from particle motion) at wavelengths related to their temperatures:
Blackbody curve

18. Blackbody Radiation
Wien’s Law

19. What is your peak wavelength?

20. What is your peak wavelength?
Body temperature: 310 K

21. What is your peak wavelength?
Body temperature: 310 K
too long to be visible: peak is in the infrared

22. The light seen here is reflected visible light.

23. This is the emitted infrared.
T (Fahrenheit)

24. This is the emitted infrared.
Cold-blooded

25. Emitted Energy
This light is emitted in all directions: L (Luminosity): total light energy/time (Watts) F (Flux): light energy/time/unit area (Watts per m2)

26. Emitted Energy
This light is emitted in all directions: L (Luminosity): total light energy/time (Watts) F (Flux): light energy/time/unit area (Watts per m2)

27. Doppler Shift
Light also exhibits other wave behaviours, e.g., the Doppler Effect

28. Doppler Shift
For an unmoving source of light, the waves arrive with the same spacing in all directions.
The wavelength is simply the space between each wave.

29. Doppler Shift
For a moving source of light the waves in front bunch up – the wavelength gets shorter!
Blue light is shorter wavelength: we call this a blue shift.

30. Doppler Shift
For a moving source of light the waves behind spread out – the wavelength gets longer!
Red light is longer wavelength: we call this a red shift.

31. Doppler Shift
Red-shifted absorption lines in the spectrum of a galaxy moving away from ours.

32. (Non-relativistic) Doppler Shift
l wavelength of signal
f frequency of signal
v velocity of recession (away)
c speed of signal

33. Example
A source’s blue hydrogen line is shifted from nm to nm. What is the speed of the source relative to us?

34. Example
A source’s blue hydrogen line is shifted from nm to nm. What is the speed of the source relative to us?

35. Example
A source’s blue hydrogen line is shifted from nm to nm. What is the speed of the source relative to us?

36. Example
A source’s blue hydrogen line is shifted from nm to nm. What is the speed of the source relative to us?
[away from us]

37. More Practice
“Light as a Wave Homework Questions”