1. Light

2. What is Light? Is light a particle or a wave?
Yes – exhibits behaviors of BOTH

3. Light Can Act Like a Wave
In 1801 Thomas Young an English scientist did the Double slit experiment.
Passed a beam of light through two narrow openings and projected it onto a screen.
He found the light produced a striped pattern which meant the light was constructively and destructively interfering.
This meant that light is composed of waves.

4. Light can Behave Particle
Other observations indicated that light can also act like a particle:
When light hits metal it knocks electrons off the surface.
They found that red light cannot knock electrons off metal no matter how bright it is.
If light were a wave then the brighter light should have more energy.
Photons are light particles that contain certain amounts of energy based on their frequency and wavelength.
Blue light has a higher frequency and shorter wavelength thus contains more energy than red light.

5. What is Light?
The “particle” of light is called the photon. It has no mass and no charge, but it does carry energy.
For ‘optics’ lessons, we will focus on light as a wave
What type of wave is light?
Electromagnetic

6. Exhibits Behaviors of Waves
Reflection
Refraction
Diffraction

7. Reflection Law of reflection
Angle of ‘incidence’ is equal to angle of ‘reflection’
(as measured from the ‘normal’ line)

8. More on Reflection

9. Reflection is Important
Allows us to see as light bounces off objects
Important with mirrors
… more to come

10. EX 1
Sitting in her parlor one night, Gerty sees the reflection of her cat, Whiskers, in the living room window. If the image of Whiskers makes an angle of 40° with the normal, at what angle does Gerty see him reflected?
Solution: Because the angle of incidence equals the angle of reflection, Gerty must see her cat reflected at an angle of 40°.

11. EX 2
Answer: 40°

12. Electromagnetic waves
What are the characteristics of electromagnetic waves?
Travels at “speed of light”
c = 3x108 m/s
Transverse wave
Requires no medium (can travel via a vacuum)

13. Electromagnetic Wave Relationships
Frequency and wavelength are inversely related via wave speed
And for electromagnetic waves, velocity is equal to the speed of light (c)

14. EX 3
What is the frequency of infrared light that has a wavelength of 8x10–6 m?

15. How Frequency/Wavelength relates to color

16. Wave Characteristics
Wavelength and Frequency for electromagnetic waves determines the ‘type’ of wave

18. The Color of light is Determined By its Frequency and Wavelength

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

20. How The Eye Works Rods and Cones
Rods (~120 million) detect B&W images
Cones (~6 million) detect color.
Each cone has sensitivity to Red, Green OR Blue

21. Green, Red and Blue Cones
Cones have a sensitivity to Green, Red and Blue
Note that to a lesser extent, our cones can detect other colors

22. Primary Colors Primary colors of light are Red, Green and Blue
Additive Colors
Mixing lights adds
colors
Adding primary colors
gives ‘secondary’
color in between
Adding all three primary
colors gives white

23. Secondary Colors Secondary colors are Magenta, Yellow and Cyan
Colors that absorb (subtractive) colors are pigments
Absorbs a primary color, reflects two (2) primary colors
Primary “paint” colors (pigments)
Adding secondary colors gives ‘primary’ color in between
Mixing all secondary colors gives
Black

24. MIRRORS

25. Reflection Definitions
Object Distance:
the distance from the mirror to the object. Positive ‘in front of mirror’
Image distance:
the distance from the mirror to the image.
Nature of image
real - inverted and able to projected on a screen; in front of mirror
virtual - right-side-up and NOT able to be projected on a screen; in ‘back’ of mirror
Size of Image
enlarged – image is larger than object
reduced – image is smaller than object
true – image is same size as object
Orientation of Image
upright – image points in same direction as object
inverted - image points in opposite direction as object

26. Reflection on Plane Mirror
Image forms where all rays converge
Images are:
Virtual
Upright
Same Size

27. (more) Reflections Definitions
Focal Point:
The point where parallel rays meet (or appear to meet) after reflecting from a mirror. The distance from the focal point to a mirror is called the focal length.
The focal length of a converging mirror is always positive (in front of the mirror)
The focal length of a diverging mirror is always negative

28. Law of Reflection
Reflected Rays bounce off at the same angle as the Incident ray
(as measured from the normal)
Image formation is where the rays cross
i.e. Our eyes perceive the image at this location

29. Mirror Types
Plain/Flat
Converging/Concave Diverging/Convex

30. Mirror Types

31. Reflected Rays Parallel Rays go through focal point
Lines through focal point reflect parallel
Rays through center reflect at incoming angle

32. Optics Refresher Ray Model of Light
Light is either absorbed or reflected
When it is reflected, it is reflected in all directions
(expanding outwardly like a sphere – similar to sound)
We can think of this as individual ‘rays’ of light travelling in straight lines in all directions

33. Reflection Reflection Incident ray Reflected ray
When light bounces off a surface.
Rough surfaces reflect light rays in many directions.
Diffuse reflection
Causes a blurry image or no image.
Incident ray
Ray hitting the surface
Reflected ray
Ray bouncing off the surface

34. Reflection Normal Angle of Incidence Angle of reflection
An imaginary line that is perpendicular to the surface the light is reflecting from.
Angle of Incidence
Angle between incident ray and the normal.
Angle of reflection
Angle between the reflected ray and the normal.
The angle of incidence equals the angle of reflection.

35. About Diverging (convex) Mirror
These characteristics always true for diverging mirrors.
Virtual image
Upright
Smaller

36. Terms of Mirrors Principal Axis (A) Optical axis
Center of Curvature (C)
- Center of sphere
Radius of Curvature (R)
– distance of C to mirror
Focal Point (F)
Focal Length ( f )
R=2*f

37. Equations for Curved Mirrors
f = focal length
do = distance to object
di = distance to image
All distances are positive in front of mirror and negative behind mirror.
Magnification
hi = height of image
ho = height of object

38. Understanding Image d + Real - virtual f Concave convex m Upright
inverted
>1
enlarged
<1
reduced =1
Same size h

39. EX 4
Wendy the witch is polishing her crystal ball. It is so shiny that she can see her reflection when she gazes into the ball from a distance of 15 cm. a) What is the focal length of Wendy‘s crystal ball if she can see her reflection 4.0 cm behind the surface? b) Is this image real or virtual?

40. EX 5

41. EX 6
Answer: a) -10 cm
b) smaller

42. EX 7
Answer: cm

43. EX 8
Answer: a) -36 cm
b) Yes, it would change the image distance

44. Drawing Ray Diagrams
Helps predict characteristics of the image

45. Concave Mirror

46. Image is virtual, upright and reduced
Convex Mirrors
Image is virtual, upright and reduced

47. Convex Mirror Ex 1 (diverting)
Draw a ray parallel to the principal axis.
Where this ray crosses the mirror, draw a reflected ray through the focal point.
Draw a ray toward the focal point.
Where this ray hits the mirror, draw a reflected ray parallel to the principal axis
Where the two reflected rays cross, draw the reflected image.
Optional: Draw the midpoint ray through the center of the mirror. This ray will reflect at the same angle as it came in (law of reflection)

48. Ex 2
Concave
(converging)
Mirror

49. Ex 3
Concave
(converging)
Mirror
When an object is located at the focal point of a concave mirror NO image will form.

50. Ex 4
Concave
(converging)
Mirror