1. Light as a Wave Part 2
SNC2D

2. The Speed(s) of Light
Light will travel slower in denser media (e.g. glass), but the speed in air is still effectively 3.0 x 108 m/s and

3. The Spectrum
The wavelength (and therefore the frequency) of a light wave will depend of the scale of the disturbance in the EM fields.

4. The Spectrum
Visible light is actually a very small part of the EM spectrum:

5. The Spectrum
Very short (i.e. gamma ray) wavelengths would be generated by very small-scale transitions (i.e. in the nucleus).

6. Wavelength and Frequency
Short wavelengths are associated with high frequencies:

7. Wavelength and Frequency
Short wavelengths are associated with high frequencies:

8. X-rays
Still-short wavelength x-rays are useful for diagnostic imaging.

9. Ultraviolet
Ultraviolet light rays, originally called deoxidizing or chemical rays for their chemical reactivity, can cause skin cancer.

10. Infrared
Infrared light is felt as heat. You are emitting infrared radiation.

11. Microwaves and Radio Waves
We use longer-wavelength microwaves to cook food and still-longer-wavelength radio waves to communicate.

12. Visible wavelengths
In the visible spectrum, wavelength translates to colour.

13. Visible wavelengths Red light has the longest l, ~700 nm.
Violet light has the shortest l, ~400 nm.

14. Visible wavelengths
Red light has the longest l, ~700 nm (700 x 10-9 m)
Violet light has the shortest l, ~400 nm (400 x 10-9 m)

15. Visible wavelengths
Red light has the longest l, ~700 nm (700 x 10-9 m)
Violet light has the shortest l, ~400 nm (400 x 10-9 m)
Practice Question: What is the frequency of red light?

16. Visible wavelengths
Practice Question: What is the frequency of red light?

17. Visible wavelengths
Practice Question: What is the frequency of red light?

18. Visible wavelengths
Practice Question: What is the frequency of red light?

19. Visible wavelengths
Practice Question: What is the frequency of red light?

20. Visible wavelengths
Practice Question: What is the frequency of red light?

21. Resonance
Different atoms and molecules have different natural frequencies of vibration and will emit and absorb different frequencies of light.
(When light is absorbed, the wave energy is converted to the thermal energy of the particles.)

22. Resonance
e.g. hydrogen atoms, when excited, will emit the frequencies:

23. Resonance
e.g. hydrogen atoms, when excited, will emit the frequencies:
so when light is reflected from or transmitted through hydrogen, it will absorb those same frequencies:

24. These lines are created by electrons jumping between the second and higher orbital shells.
(An electron falling from the higher to the second emits light; an electron jumps up when light is absorbed.)

25. Resonance
Gas discharge tubes produce these characteristic frequencies by exciting the gas/vapour with an electric current.
The frequencies can
be viewed using a
spectroscope.

26. White Light
“White” light is the sum of all frequencies (ROYGBIV), which can be seen by viewing white light through a spectroscope or, similarly, by refracting light through a prism.

27. Colour
The perceived colour of an object illuminated with white light therefore depends on which frequencies are reflected.
Example:

28. Colour
The perceived colour of an object illuminated with white light therefore depends on which frequencies are reflected.
Example:
Reflects GREEN
and
Appears GREEN

29. Black = Hot
A shirt that appears black is absorbing all colours (and therefore all that energy of the light waves).
A shirt that appears white is reflecting all colours.

30. Black = Hot
A shirt that appears black is absorbing all colours (and therefore all that energy of the light waves).
A shirt that appears white is reflecting all colours.
So wear white to stay cool.

31. Primary Colours
White light can also be produced by combining only three colours. These colours are called primary colours, and in physics are

32. Primary Colours
White light can also be produced by combining only three colours. These colours are called primary colours, and in physics are Red, Blue, and Green.

33. Secondary Colours
Secondary colours are formed by the addition of 2 primary colours:

34. Secondary Colours
Secondary colours are formed by the addition of 2 primary colours:
Blue + Red =

35. Secondary Colours
Secondary colours are formed by the addition of 2 primary colours:
Blue + Red = Magenta

36. Secondary Colours
Secondary colours are formed by the addition of 2 primary colours:
Blue + Red = Magenta
Blue + Green =

37. Secondary Colours
Secondary colours are formed by the addition of 2 primary colours:
Blue + Red = Magenta
Blue + Green = Cyan

38. Secondary Colours
Secondary colours are formed by the addition of 2 primary colours:
Blue + Red = Magenta
Blue + Green = Cyan
Red + Green =

39. Secondary Colours
Secondary colours are formed by the addition of 2 primary colours:
Blue + Red = Magenta
Blue + Green = Cyan
Red + Green = Yellow

40. More Practice
p. 391 #3 – 5, 7 – 13
Tomorrow: Reflection