1. HOW DO WE SEE? WHAT IS LIGHT? HOW
DOES IT CAUSE US TO SEE THINGS?

2. FACTS ABOUT LIGHT
• It is an electromagnetic wave that transmits energy
• It is a part of the Electromagnetic Spectrum and the only part we can really see

4. FACTS ABOUT LIGHT
The speed of light, c, is constant in a vacuum.
Light can be:
•REFLECTED
•ABSORBED
•REFRACTED
Light is an electromagnetic wave in that it has wave like properties which can be influenced by
electric and magnetic fields.

5. CHRISTIAAN HUYGENS
(1629-1695)
• Used a wave model to explain diffraction
• Huygens’ principle--light waves spreading out from a point source are made up of tinier
wave fronts.

6. CHARACTERISTICS OF LIGHT
• What we recognize as “white” light is actually light that can be separated into elementary
colors of the visible spectrum:
ROY G. BIV
Red, orange, yellow, green, blue, indigo, violet

7. LIGHT HAS BOTH PARTICLE AND WAVE
CHARACTERISTICS
• A particle:
• Definition: a small discrete quantity of matter that has an interface with the
surrounding environment
• Travels in a straight line, like a bullet
• Bounces off objects, like a ball off of a wall
• Its speed varies in different types of matter, like a ball bearing in molasses
• A wave:
• It travels in a straight line
• It bounces off of object (we can hear our own echo)
• Its speed varies in different types of medium

8. WAVE-PARTICLE DUALITY THEORY
• Light is a more complex phenomenon than just a simple wave or a simple beam of
particles!
Phenomenon Explained in terms of Explained in terms of
waves? particles?
Reflection ✔ ✔
Refraction ✔ ✔
Interference ✔ ✗
Diffraction ✔ ✗
Polarization ✔ ✗
Photoelectric Effect ✗ ✔

9. LIGHT
vocabulary activities
• Absorption 1. Laser lights
• Reflection 2. Light around an obstacle
• Refraction 3. Pig mirage
• Dispersion 4. Polarizing filters and mica
• Diffraction 5. Write and see squares
• Polarization 6. Concave mirror
• Concave mirrors 7. Triple slits & prisms
8. Observing a pin through water

10. DOUBLE-SLIT EXPERIMENT
Two slits made in a screen. Light was shined through to see what image would display on the
other side.
• If light was a particle, the experiment would show similar results to the ones below:

11. THOMAS YOUNG--FAMOUS EXPERIMENT IN 1801
• This experiment was a MAJOR reason for the discrediting of the particle theory of light
• Monochromatic light was interfering constructively and destructively when passed
through narrow slits onto a screen
• Bright and dark
fringes formed
on the screen.

12. THE PHOTOELECTRIC EFFECT
• Showed that if photons are shined at an object, an electric current could be produced in
the object.
• Found that by increasing intensity (amplitude), more electrons will be emitted
• supports WAVE THEORY: more intensity, more electrons knocked out of orbit!
• BUT, it also found that if you increase the frequency (shift toward blue end of
spectrum), the energy of each electron does increase
• Supports PARTICLE THEORY: according to wave theory, frequency should not
influence the amount of energy – only intensity should
• Einstein: light comes in discrete units called photons that have a specific energy
associated with in depending on its frequency

13. SEEING COLOUR
• The colour an object appears depends on the colours of light it
reflects.
For example, a red book only reflects red light:
White Only red light is
light reflected

14. 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

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

16. PART 2 - REFLECTION
• Reflection from a mirror:
Normal
Incident ray Reflected ray
Angle of incidence Angle of
reflection
Mirror

17. • 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 !!!

18. CLEAR VS. DIFFUSE REFLECTION
• Smooth, shiny surfaces have a clear reflection:
Rough, dull surfaces have a diffuse reflection.
Diffuse reflection is when light is scattered in
different directions

19. USING MIRRORS
• Two examples:
2) A car headlight
1) A periscope

20. REFLECTION
• Sun rays are hit by spherical water droplets and are reflected off the back surface.
• Red is bent the least (since it has the lowest wavelength) so it reaches the observer’s
eye from higher in the sky!

21. REFRACTION
Refraction is based on the idea that LIGHT is passing through one MEDIUM into
another. The question is, WHAT HAPPENS?
Suppose you are running on the beach with
a certain velocity when you suddenly need
to run into the water. What happens to your
velocity?
IT CHANGES!
Refraction Fact #1: As light goes from one
medium to another, the velocity CHANGES!

22. REFRACTION
Suppose light comes from air, which in this case will be considered to be a vacuum,
strikes a boundary at some angle of incidence measured from a normal line ,and
goes into water.
The ratio of the two speeds can be compared.
The denominator in this case will ALWAYS be smaller
and produce a unitless value greater or equal to 1.
This value is called the new medium’s INDEX OF
REFRACTION, n.
All substances have an index of refraction and can be used to identify the material.

23. REFRACTION
Suppose you decide to go spear fishing, but unfortunately you aren’t having
much luck catching any fish.
The cause of this is due to the fact that light
BENDS when it reaches a new medium. The
object is NOT directly in a straight line path,
but rather it’s image appears that way. The
actual object is on either side of the image
you are viewing.
Refraction Fact #2: As light goes from one
medium to another, the path CHANGES!

24. REFRACTION
What EXACTLY is light doing when it reaches a new medium?
Not ALL of the light refracts.
Some of the light REFLECTS off the boundary
and some of the light REFRACTS through the
boundary.
Angle of incidence = Angle of Reflection
Angle of Incidence > or < the Angle of refraction
depending on the direction of the light

25. REFRACTION – GOING FROM AIR TO WATER
The index of refraction, n, for air (vacumm)
is equal to 1. The index of refraction for
water is 1.33.
If you are going from a LOW “n” to a HIGH
“n”, your speed DECREASES and the angle
BENDS TOWARDS the normal

26. REFRACTION – GOING FROM WATER
INTO AIR The index of refraction, n, for air (vacumm)
is equal to 1. The index of refraction for
water is 1.33.
If you are going from a HIGH “n” to a LOW
“n”, your speed INCREASES and the angle
BENDS AWAY the normal
Note: If the angles are EQUAL, then
the “n” must be equal for each. The
ray will pass straight through.

27. REFRACTION – SNELL’S LAW
A scientist by the name of Snell discovered that the ratios of the index ’s
and the ratio of the sine of the angles are the same value!
n1 sin  2

n2 sin  1
Snell' s Law
n1 sin 1  n2 Sin 2

28. EXAMPLE
The refractive index of the gemstone, Aquamarine, is 1.577. Suppose a ray
of light strikes a horizontal boundary of the gemstone with an angle of
incidence of 23 degrees from air.
Calculate the SPEED of light in Aquamarine
8
c c 3x10
n  vm  
vm n 1.577
vm  1.90 x 108 m/s
Calculate the angle of refraction within Aquamarine
n1 sin 1  n2 sin  2
(1)(sin 23)  1.577 sin  2
sin 23
  sin ( 1
) 14.34 degrees
1.577

29. COLOUR
• White light is not a single colour; it is made up of a mixture of the seven colours of the
rainbow.
• The act of splitting light into its colors using a prism is called dispersion
• Each color has a different wavelength, so they bend differently when going
into a new medium since index of refraction depends on the wavelength for
that media.
• It is greater for shorter wavelength
We can demonstrate this by splitting
white light with a prism:

30. LIGHT & USES: DIFFRACTION
• Diffraction – Bending of waves around the edge of a barrier. New waves are formed from
the original. breaks images into bands of light & dark and colors.
• Refraction – Bending of waves due to a change in speed through an object.

31. LIGHT & USES: DIFFRACTION
© 2000 Microsoft Encarta
• A diffraction grating. Each space between the ruled grooves acts as a
slit. The light bends around the edges and gets refracted.

32. CONVERGING (CONCAVE) MIRROR
Since the mirror is spherical
it technically has a CENTER
OF CURVATURE, C. The
focal point happens to be
HALF this distance.
C
f 
2
C 2f
We also draw a line through the center of
the mirror and call it the PRINCIPAL AXIS.

33. RAY DIAGRAM
A ray diagram is a pictorial representation of how the light travels
to form an image and can tell you the characteristics of the
image.
object C f
Principal axis
Rule One: Draw a ray, starting from the top of the object, parallel to the principal axis and
then through “f” after reflection.

34. RAY DIAGRAMS
object C f
Principal axis
Rule Two: Draw a ray, starting from the top of the object, through the focal point, then parallel
to the principal axis after reflection.

35. RAY DIAGRAMS
object C f
Principal axis
Rule Three: Draw a ray, starting from the top of the object, through C, then back upon itself.
What do you notice about the three lines? THEY INTERSECT
The intersection is the location of the image.

36. RAY DIAGRAM – IMAGE CHARACTERISTICS
object C f
Principal axis
After getting the intersection, draw an arrow down from the principal axis to
the point of intersection. Then ask yourself these questions:
1) Is the image on the SAME or OPPOSITE side of the mirror as the object?
Same, therefore it is a REAL IMAGE.
2) Is the image ENLARGED or REDUCED?
3) Is the image INVERTED or RIGHT SIDE UP?

37. POLARIZED WAVES
• Definition: a wave that is polarized so
that oscillations only occur in a certain
plane.