1. In this second part of unit 5:
Light
In this second part of unit 5:
Properties of light
Reflection
Colours
Refraction

2. Light
What is LIGHT? WHERE DOES IT COME FROM?
© 2003 Mike Maloney

3. What is Light? Light is a wave, or rather acts like a wave.
How do we know?
Reflection
Refraction
Dispersion
Diffraction
Interference
Polarization
© 2003 Mike Maloney

4. What is Light
Light is a special type of wave What we know as light or VISIBLE LIGHT is actually a type of something called ELECTROMAGNETIC RADIATION. So, what is electromagnetic radiation and electromagnetic waves?
© 2003 Mike Maloney

5. Electromagnetic Waves
When something creates energy it also emits radiation. Depending on the amount of energy, the object will emit different types of electromagnetic radiation. When we studied mechanical waves, they were all transferred through a medium. What medium is light transferred through? LIGHT DOES NOT NEED ONE!
© 2003 Mike Maloney

6. Electromagnetic Waves
Electromagnetic waves are special in the fact that they do not need a medium to propagate through. But what is creating the disturbance? What is emitting this energy? ELECTRONS
© 2003 Mike Maloney

7. Electromagnetic Waves
Electrons in materials are vibrated and emit energy in the form of photons, which propagate across the universe. Photons have no mass, but are pure energy. Electromagnetic Waves are waves that are made up of these “photons”. When these photons come in contact with boundaries, E-M waves interact like other waves would.
© 2003 Mike Maloney

8. Electromagnetic Waves
Electromagnetic waves are everywhere. Light is only a small part of them
Visible/UV/InfraRed ()

9. LIGHT: What Is It? Light Energy Atoms
© 2000 Microsoft Clip Gallery
Light Energy
Atoms
As atoms absorb energy, electrons jump out to a higher energy level.
Electrons release light when falling down to the lower energy level.
Photons - bundles/packets of energy released when the electrons fall.
Light: Stream of Photons
© 2000 Microsoft Clip Gallery

10. LIGHT: Particles or Waves?
Wave Model of Light
Explains most properties of light
Particle Theory of Light
Photoelectric Effect – Photons of light produce free electrons

11. Electromagnetic Waves
Speed in Vacuum
300,000 km/sec
186,000 mi/sec
Speed in Other Materials
Slower in Air, Water, Glass
© 2000 Microsoft Clip Gallery

12. Transverse Waves Energy is perpendicular to direction of motion
© 2000 Microsoft Clip Gallery
Energy is perpendicular to direction of motion
Moving photon creates electric & magnetic field
Light has BOTH Electric & Magnetic fields at right angles!

13. Electromagnetic Spectrum
© 2000 Microsoft Clip Gallery

14. Electromagnetic Spectrum
© 2003 Mike Maloney

15. Speed of E/M Waves V = f * l
It has been found that the speed of E-M waves and light is ---
3 x 108 or 300,000,000 m/s
671,000,000 mph
186,000 miles per second
We call this value “c”
© 2003 Mike Maloney

16. c = f * l
C is constant throughout the universe, as long as light is in a vacuum. When it is in other materials, c can change, but can never be larger than its value in a vacuum. Since “c” is constant, all of E-M waves will have a corresponding frequency to go along with their wavelength.
© 2003 Mike Maloney

17. c = f * l ~~~ f = c / l
Lets find the corresponding frequency ranges for a few of the groups of E-M waves.
© 2003 Mike Maloney

18. Energy in E-M Waves
Which waves have more energy, Radio waves or gamma waves ? The greater the frequency of an E-M wave, the more crests pass a point in a certain amount of time, therefore the more photons pass that point. This means that more energy moves past that point in a certain amount of time or that the wave contains more energy.
© 2003 Mike Maloney

19. Back to Light
So, why can we only see a small portion of these E-M waves?
© 2003 Mike Maloney

20. Our Eyes
© 2003 Mike Maloney

21. Visible Light
We now know what we see is part of the electromagnetic spectrum. We know that the light waves enter our eye, and stimulate parts of it that cause an electrical impulse to be sent to the brain which creates this visual image. But everything does not emit radiation. How do we see those things? And why cant we see a window?
© 2003 Mike Maloney

22. Seeing things
We know that when waves run into a boundary they are partially transmitted and partially reflected. Light behaves as a wave, so it to is reflected. Therefore, an object does not need to emit photons itself to be seen, it just has to reflect light back to our eyes where we can detect it. Objects that do not allow light to pass through them are called opaque. Objects that allow light to pass through them are considered transparent. Objects in between are called translucent.
© 2003 Mike Maloney

23. Light travels in straight lines:
Properties of Light
Light travels in straight lines:
Laser

24. Light travels VERY FAST – around 300,000 kilometres per second.
At this speed it can go around the world 8 times in one second.

25. Light travels much faster than sound. For example:
Thunder and lightning start at the same time, but we will see the lightning first.
2) When a starting pistol is fired we see the smoke first and then hear the bang.

26. We see things because they reflect light into our eyes:
Homework

27. Luminous and non-luminous objects
A luminous object is one that produces light.
A non-luminous object is one that reflects light.
Luminous objects
Reflectors

28. Shadows are places where light is “blocked”:
Rays of light

29. Properties of Light summary
Light travels in straight lines
Light travels much faster than sound
We see things because they reflect light into our eyes
Shadows are formed when light is blocked by an object

30. Reflection Reflection from a mirror: Mirror Normal Angle of incidence
Incident ray
Reflected ray
Angle of incidence
Angle of reflection
Mirror

31. Angle of incidence = Angle of reflection
The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at ____ _____ angle it hits it.
The same !!!

32. Regular vs. Diffuse Reflection
Smooth, shiny surfaces have a regular reflection:
Rough, dull surfaces have a diffuse reflection.
Diffuse reflection is when light is scattered in different directions

33. Using mirrors
Two examples:
2) A car headlight
1) A periscope

34. Colour
White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow.
We can demonstrate this by splitting white light with a prism:
This is how rainbows are formed: sunlight is “split up” by raindrops.

35. The colours of the rainbow:
Red Orange Yellow Green Blue Indigo Violet

36. Adding colours
White light can be split up to make separate colours. These colours can be added together again. The primary colours of light are red, blue and green:
Adding blue and red makes magenta (purple)
Adding blue and green makes cyan (light blue)
Adding red and green makes yellow
Adding all three makes white again

37. Only red light is reflected
Seeing colour
The colour an object appears depends on the colours of light it reflects.
For example, a red book only reflects red light:
Homework
White
light
Only red light is reflected

38. A white hat would reflect all seven colours:
A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):
Purple light
A white hat would reflect all seven colours:
White
light

39. Using filters Red Filter
Filters can be used to “block” out different colours of light:
Red Filter
Magenta Filter

40. Refraction
Refraction is when waves are bent, or slow down due to travelling in a different medium. A medium is something that waves will travel through. When a pen is placed in water it looks like this:
In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______.
Words – water, air, bent

41. LIGHT: Refraction of Light
Refraction – Bending of light due to a change in speed.
Index of Refraction – Amount by which a material refracts light.
Prisms – Glass that bends light. Different frequencies are bent different amounts & light is broken out into different colors.

42. Refraction (Cont.)

45. Polarization
Polarization is a phenomenon of light that is used in sun-glasses and 3-D movies. (Play with the polarizing glasses for a few minutes and note what is happening and see if you can think of any reasons for it.)
© 2003 Mike Maloney

46. Polarization Hint
Light vibrates in all directions. A polarizing filter acts like a picket fence. It only lets certain direction vibrations pass through it. Therefore, if you pass light through two of them you can completely block the light from passing through. HOW?
© 2003 Mike Maloney

47. Polarization
© 2003 Mike Maloney

48. Flux
We now know how light behaves, but we must measure how strong it is. The rate at which a source emits light is called the LUMINOUS FLUX (P). What do you think this is measured in? What are light bulbs measured in. LUMINOUS FLUX (P) is actually measured in something called a lumen (lm). A typical 100-W bulb emits 1750 lm.
© 2003 Mike Maloney

49. Illuminance
Flux is the total of all the light that is emitted from a source. This is not very useful, often we would like to know how much of that light is hitting a surface at some point. The illumination of a surface is called illuminance, E. It is measured in lumens per square meter, lm/m2
© 2003 Mike Maloney

50. References
updated 2/1/97

51. References http://www.isc.tamu.edu/~astro/color.html
updated 11/22/97

52. WORKS CITED
updated 11/22/97  
updated 2/1/97