1) Light is an electromagnetic wave that travels in straight lines until it encounters a new medium, where it may be reflected or refracted depending on the medium's properties. 2) Reflection occurs when light changes direction upon striking an interface between two media, obeying the law that the angle of incidence equals the angle of reflection. Refraction is when light changes speed and direction when moving between media of different densities, governed by Snell's Law. 3) Lenses and mirrors can converge or diverge light via refraction or reflection, respectively, with their focusing power determined by their shape and the positioning of focal points and object/image distances following specific rules and equations.

1. Light, Reflection, and Refraction
Chapters 14 and 15
OPTICS

2. Electromagnetic Waves
• Magnetic field wave perpendicular to an electric
field wave
• All objects emit EMWs.
⇑ Temp ⇑EMW
• Electromagnetic spectrum
– Range of all frequencies of light
• Visible light is a very small portion of that entire
spectrum.

4. c
• Speed of Light - 3.00 x 108m/s.
• = (wavelength) x (frequency)
• c = λƒ

5. Example
• AM Radio waves
– 5.4 x 105 Hz
– 1.7 x 106 Hz
λ=?

6. Visible Light
• Part of the EMS humans can see
– Red - 750nm (x10-9m)
– Purple - 380nm
• Bees, Birds – UV
• Snakes – IR

7. Reflection
• Light waves usually travel in straight paths
• Change in substance changes direction
• Opaque - does not permit light
– some light reflected
– some light absorbed as heat

8. Reflection
• Texture affects reflection
• Diffuse reflection (rough)
– reflects light in many different directions,
• Specular reflection (smooth)
– reflects light in only one direction
• Smooth – variations in surface < λ

10. Mirrors
• Light striking a mirror reflects at the same
angle that it struck the mirror

11. Flat Mirrors
• p=q
– p- objects distance to the mirror
– q - distance from the mirror to the image
• Virtual image
– Does not exist
– Made by our eyes

13. Ray Diagrams
• Used to predict the location of the image of an
object

14. Concave Spherical Mirrors
• Reflective surface is on the interior of a
curved surface
–
–
–
–
C – center of curvature
R – Radius (distance to C)
f – Focal Point (1/2 R)
Principal axis
• any line that passes through C
• usually oriented with an object

16. Mirror Equations
• 1/object distance + 1/image distance =
1/focal length
1/p + 1/q = 1/f
• Magnification (M) =
Image height/object height (h′ / h)
- (q / p)
• M = h′ / h = - (q / p)

17. Sign of Magnification
Sign of M
Orientation of Image
Type of Image
+
Upright
Virtual
–
Inverted
Real

18. Concave Spherical Mirror Rules
• A ray traveling through C will reflect back
through C
• A ray traveling through (f) will reflect parallel to
the PA
• A ray traveling to the intersection of the PA and
the mirror will reflect at the same angle below the
PA.
• A ray traveling parallel to PA will reflect through
the focal point

19. Ray Diagrams
• Draw three rays
– The image forms at the point of intersection
• Example
– f = 10.0cm
– p = 30.0cm
– h = 3.00cm

20. Convex Spherical Mirrors
• Reflective surface is on the outside of the
curve.
• The points f and C are located behind the
mirror
– negative

22. Rules
• A ray parallel to the PA will reflect directly away
from f.
• A ray towards f will reflect parallel to the PA
• A ray towards C will reflect directly away from C.
• A ray to the intersection of PA and mirror will
reflect at the same angle below the OA.
• Trace the 3 diverging lines back through the
mirror to reveal the location of the image which is
always virtual

24. Example
• f = -8.00cm
• p= 10.0cm
• h = 3cm

25. Parabolic Mirrors
• Rays that hit spherical mirrors far away
from the OA often reflect though other
points causing fuzzy images, spherical
aberration.
• Telescopes use parabolic mirrors as they
ALWAYS focus the rays to a single point.

27. Refraction
• Substances that are transparent or translucent
allow light to pass though them.
• Changes direction of light
• Due to the differences in speed of light

28. Analogy
• A good analogy for refracting light is a
lawnmower traveling from the sidewalk
onto mud

29. Index of Refraction (n)
• The ratio of the speed of light in a vacuum
to the speed of light in a medium
∀⇑ n - ⇓ c

32. Snell’s Law
• ni(sinθi) = nr(sinθr)
∀ θr = sin-1{(ni/ nr)(sinθi)}
• Example
θi = 30.0⁰
– ni = 1.00
– nr = 1.52

33. θi = 30.0⁰
ni = 1.00
nr = 1.52

35. Total Internal Reflection
• If the angle of incidence of a ray is greater
than a certain critical angle the ray will
reflect rather than reflect
• This principal is responsible for the
properties of fiber optic cables.
• Remember the lawn mower analogy…

37. Critical Angle
• sin Θc = nr / ni
• As long as nr < ni
• What is the critical angle for light traveling
from Diamond to Air?

38. nr = 1.000
ni = 2.419

39. Thin Lenses
• Converging
• Diverging
• f- curve of lens & index of refraction

40. Converging Lens Diagram
1. Ray parallel to PA, refracts through far
focal point
2. Ray through center of lens, continues
straight line
3. Ray through near focal point, refracts
through lens, continues parallel to PA
• Treat lens as though it were a flat plane.

42. Diverging Lens Diagram
• Because the rays that enter a diverging lens do not
intersect a virtual image is formed by tracing back
the refracted rays.
• Ray 1 - parallel to PA, refracts away from near f,
trace back to near f.
• Ray 2 - ray toward far f, refracts parallel to PA,
trace back parallel to PA
• Ray 3 - ray through center, continues straight,
trace back toward object

44. Sign Conventions for Lens
Sign
+
–
p
q
Near side of Far side of
lens
lens
Far side of Near side of
lens
lens
F
Converging
Lens
Diverging
Lens

45. Converging Lens Example
• p = 30.0cm
• f = 10.cm

46. Diverging Lens Example
• p = 12.5cm
• f = -10.0cm