LIGHT

Fundamental Terms of Reflection:
- Incident Ray: The incoming ray that strikes a surface.
- Reflected Ray: The ray that bounces off the surface.
- Normal Line ( $N$ ): An imaginary line perpendicular to the surface at the point of incidence.
- Angle of Incidence ( $i$ ): The angle between the incident ray and the normal line.
- Angle of Reflection ( $r$ ): The angle between the reflected ray and the normal line.
- Point of Incidence: The specific location where the incident ray hits the boundary.
Characteristics of Reflection:
- The angle of incidence is equal to the angle of reflection: $i = r$ .
Image Characteristics Formed by a Plane Mirror:
- Size: The image is the same size as the object.
- Orientation: The image is upright.
- Type: The image is virtual (cannot be projected onto a screen).
- Orientation Note: Images in a plane mirror are also laterally inverted.

Definition: Refraction is the change in direction of a wave as it passes from one medium to another, caused by a change in speed. It depends heavily on the density of the medium.
Refraction Terms:
- Incident Ray: Ray traveling in the first medium toward the boundary.
- Refracted Ray: Ray traveling in the second medium after crossing the boundary.
- Angle of Incidence ( $i$ ): Angle between incident ray and normal line.
- Angle of Refraction ( $r$ ): Angle between refracted ray and normal line.
Directional Rules for Refraction:
- Bent Towards the Normal: Occurs when light enters a denser medium and slows down (i > r).
- Bent Away from the Normal: Occurs when light enters a less dense medium and speeds up (r > i).
- No Refraction: A ray traveling along the normal line (perpendicular to the boundary) is not refracted and continues in a straight line.
Physical Changes during Refraction:
- Constant: Frequency ( $f$ ) remains constant because it depends solely on the source.
- Variables: Speed ( $v$ ), wavelength ( $\lambda$ ), amplitude, and direction/angle all change.

Refractive Index ( $n$ ):
- Definition: $n = \frac{c}{v}$ , where $c$ is the speed of light in air/vacuum and $v$ is the speed of light in a specific medium.
- Speed of light in air: $c = 3 \times 10^8\,\text{m/s}$ .
Snell's Law and Related Ratios:
- Using angles: $n = \frac{\sin(i)}{\sin(r)}$ (when light travels from air to a medium).
- General form: $n_1 \sin(\theta_1) = n_2 \sin(\theta_2)$ .
- Relationship between speed and wavelength: \frac{n_2}{n_1} = \frac{\sin(i)}{\sin(r)} = \n \frac{v_1}{v_2} = \frac{\lambda_1}{\lambda_2} Where:
  - $n_1, n_2$ : Refractive indices of medium 1 and 2.
  - $v_1, v_2$ : Speed of light in medium 1 and 2 ( $\text{m/s}$ ).
  - $\lambda_1, \lambda_2$ : Wavelength in medium 1 and 2 ( $\text{m}$ ).

Condition for TIR: Light must be traveling from a denser medium to a less dense medium.
Scenarios Based on Angle of Incidence ( $i$ ) and Critical Angle ( $c$ ):
- 1) If i < c: Both reflection and refraction occur. Most of the light is refracted and bent away from the normal.
- 2) If $i = c$ : The refracted ray travels along the boundary between the two media. The angle of refraction ( $r$ ) is exactly $90^\circ$ .
- 3) If i > c: Total Internal Reflection (TIR) occurs. There is no refracted ray; all light is reflected back into the denser medium.
Equation for Critical Angle ( $c$ ):
- For light traveling from a medium to air ( $n_{\text{air}} = 1$ ): $n = \frac{1}{\sin(c)}$ or $\sin(c) = \frac{1}{n}$

Definition: Dispersion is the splitting of white light into its component colors based on their different wavelengths and frequencies.
Cause: White light is a mixture of all colors. Each color travels at a different speed when entering a medium like glass, causing them to refract at different angles.
The Spectrum of Colors:
- Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV).
Physical Properties of the Spectrum:
- Red: Has the longest wavelength and Lowest frequency. It is refracted the least.
- Violet: Has the shortest wavelength and Highest frequency. It is refracted the most.
Mathematical Relation: $v = \lambda \times f$ .
- Wavelength (λ) and Frequency (f) are inversely proportional for a given speed.

Reflection applied in: Plane mirrors.
Refraction applied in: Lenses (Converging and Diverging) and Optical fibers (employing TIR).
Dispersion applied in: Glass prisms and the formation of rainbows.

This material is endorsed by Cambridge Assessment International Education.
It supports the full Cambridge IGCSE and IGCSE (9-1) Physics syllabuses (0625/0972).
Valid for examination from 2023.
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