# Unit 3: Physics ## Light - `Light`: Electromagnetic radiation/waves, as light interacts with both electricity and magnets - Light travels at ~**3.0*108<\sup>** - `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 ### Properties of electromagnetic waves - `Amplitude`: Height from centre to crest/trough - `Crest`: Peak of wave - `Trough`: Base of wave - `Wavelength`: Distance between two points on wave on the same plane - `Frequency`: Waves passing per (e.g., hertz (waves per second)) - Visible light wavelengths are between 400-700 nm long - Light always travels in a straight line - **Longer** wavelength = **smaller** frequency = **less** energy - **Shorter** wavelength = **higher** frequency = **more** energy - **Higher** energy, **lower** penetration (e.g., 2.4 GHz vs 5 GHz Wi-Fi) - `Luminous`: Emits light - Non-luminous objects do not emit light - `Colour`: Reflected parts of white light from non-luminous objects - Blacks absorb all visible light while whites do the opposite ### Luminescence - Things that emit light fill in here plz thanks ### Rays - 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) ## 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) ### Plane mirrors - `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 and convex mirrors - `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** - 2. Any incident ray that would pass through the **focus** will reflect **parallel** to the principal axis - 3. Any incident ray that would pass through the **centre** of curvature will reflect **back on the same path** - 4. 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 | ## Refraction - 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: $`\frac{sin\theta_{2}}{sin\theta_{1}} = \frac{v_{2}}{v_{1}} = \frac{n_{1}}{n_}2} ## Total internal reflection - `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