Unit 3: Physics

Light

Light: Electromagnetic radiation/waves, as light interacts with both electricity and magnets
Light travels at ~**3.0*10^8<>**
Energy: Ability to do work
Work: Ability to move matter in space
Energy can be transferred and transformed, but not destroyed
Light behaves as a particle and/or a wave
- Behaves as particle when travelling through a vacuum, which waves cannot do
- Behaves as wave by forming “interference patterns”, properties of light waves are also measurable
Photon: Light particle

Light path can be tracked via arrrows
Normal: Perpendicular line to an interface (e.g., mirror, medium boundary), intersecting where light reflects off
Angle of incidence: Angle of light hitting reflective surface, relative to the normal
Angle of reflection: Angle of light leaving reflective surface, relative to the normal
Laws of reflection
- Angle of incidence = angle of reflection
- Light rays are on the same plane
Types of reflection
- Specular reflection: All normals are parallel (e.g., reflection off mirror)
- Diffuse reflection: Not all normals are parallel (e.g., paper, not-mirrors)

A mininum of two incident rays are required to find an image
Where rays converge describe image
Dotted lines are used for light going beyond a mirror (as light does not actually travel there)
SALT: Describes image
- Size: Relative to object
- Attitude: Orientation relative to object
- Location: Relative to mirror and/or object
- Type: Virtual (behind mirror) or real (in front of mirror)

Object-image line: Line perpendicular to plane mirror
- Distance is equal on both sides of mirror
- Describes location of object without requiring 2+ incident rays
- Banned

Concave mirror: Curved mirror curving inwards in the direction of incident rays, like a cave
Convex mirror: Curved mirror curving away from incident rays, like back of a spoon

Principal axis: $`PA`$, line perpendicular to mirror when it hits it
Centre of curvature: $`C`$, point where the centre of the circle would be if mirror was extended to a full circle
Focus: $`F`$, point where all light rays focus on if incident rays are parallel to principal axis
Vertex: $`V`$, point where principal axis meets mirror
Imaging rules for curved mirrors:
- 1. Any incident ray parallel to the principal axis will reflect directly to or away from the focus
- 1. Any incident ray that would pass through the focus will reflect parallel to the principal axis
- 1. Any incident ray that would pass through the centre of curvature will reflect back on the same path
- 1. Any incident ray that reflects off the vertex reflect as if it were a plane mirror

Characteristics of concave mirror images

Object location	Size	Attitude	Location	Type
Farther than C	Smaller than object	Inverted	Between C and F	Real
At C	Same as object	Inverted	On C	Real
Between C and F	Larger than object	Inverted	Farther than C	Real
At F	N/A, lines do not converge
Between F and V	Larger than object	Upright	Behind mirror	Virtual

Characteristics of convex mirror images

Object location	Size	Attitude	Location	Type
Anywhere	Smaller than object	Upright	Between F and V/behind mirror	Virtual

Speed of light depends on its medium
Light bending while transitioning from a slower to faster medium or vice versa
Greater the change in speed, greater than change in direction
Turns in direction of leading edge
- Analogy: Sleds slowing from one runner first when transitioning from snow to pavement
Slow -> fast medium: Refracts away from normal
Fast -> slow medium: Refracts towards normal
Angle of refraction: Angle of light after interface, relative to normal
Index of refraction: speed of light in vacuum / speed of light in medium
- $`n = \frac{c}{v}`$
$`n_{1}sin\theta_{incidence} = n_{2}sin\theta_{refraction}`$
- Where $`n_{1}`$ and $`n_{2}`$ are the refractive indexes of two different media
Snell’s law: $` = = 2}

Critical angle: Angle of incidence that causes refracted ray to be perpendicular to normal
TIR occurs when angle of incidence exceeds critical angle, causing near-100% reflection
Happens only when refracting from slow to fast
Refraction is not perfect; some light is reflected during refraction
- Reflected ray grows brighter as we reach critical angle, and refracted ray grows dimmer
Higher index of refraction = lower critical angle