Light

Reflection of Light

• The **law of reflection** states: angle of incidence $(i) =$ angle of reflection (r).
• Angles are measured between the ray and the **normal** (an imaginary line perpendicular to the boundary).
• In a **plane mirror**, the image is **virtual**, **same size**, **same distance behind** the mirror as the object is in front, and **laterally inverted**.
• A **virtual image** is formed where extended reflected rays appear to meet; it cannot be projected onto a screen.
• When drawing ray diagrams, use arrows to show direction: incident ray points toward the boundary, reflected ray points away.

Investigating Reflection

• **Aim**: To investigate reflection by a plane mirror.
• **Independent variable**: angle of incidence; **dependent variable**: angle of reflection.
• **Method**: Use a ray box, protractor, and plane mirror; mark incident and reflected ray positions on paper.
• **Analysis**: The law of reflection $i = r$ should be verified; results are recorded in a table.
• **Safety**: Avoid looking directly at the ray box light; run burns under cold water for at least five minutes.

Refraction of Light

• **Refraction** is the change in direction of a light ray when passing from one medium to another.
• From less dense to more dense (e.g., air to glass), light bends **towards** the normal.
• From more dense to less dense (e.g., glass to air), light bends **away** from the normal.
• When light enters a rectangular glass block, it bends towards the normal at entry and away at exit.
• Only **speed** and **wavelength** change during refraction; **frequency** remains constant.

Refractive Index

• **Refractive index** $n =$ speed of light in vacuum / speed of light in material $(n > 1)$.
• Also given by Snell's law: $n = \sin i / \sin r$, where $i =$ angle of incidence, $r =$ angle of refraction.
• Denser materials have higher refractive indices (e.g., diamond ≈ 2.4, glass ≈ 1.5).
• Use the inverse sine function (sin⁻¹) to find an angle from its sine value.

Total Internal Reflection

• **Total internal reflection (TIR)** occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the **critical angle**.
• The **critical angle** is the angle of incidence for which the angle of refraction is 90°.
• When $i >$ critical angle, all light is reflected back into the denser medium.
• **Optical fibres** use TIR to transmit light signals for communications, endoscopes, and decorative lamps.

Ray Diagrams and Lenses

• A **converging (convex) lens** brings parallel rays to a focus at the **principal focus** (focal point).
• **Focal length** is the distance from the lens centre to the principal focus.
• Three key rays: through optical centre (undeviated), parallel to principal axis (refracts through focus), through focus (emerges parallel).
• For an object beyond 2f, the image is **real**, **inverted**, and **diminished**.
• For an object between f and 2f, the image is **real**, **inverted**, and **magnified**.
• For an object at 2f, the image is **real**, **inverted**, and **same size**.

Virtual Images

• A **real image** is formed where light rays converge; it can be projected onto a screen and is inverted.
• A **virtual image** is formed where light rays appear to diverge from; it cannot be projected and is upright.
• When an object is placed **closer than the focal length** to a converging lens, a **virtual**, **upright**, **magnified** image is formed (magnifying glass).
• In ray diagrams for virtual images, use dashed lines to trace back the diverging rays to where they appear to meet.

Dispersion of Light

• **Dispersion** is the separation of white light into its component colours by a prism.
• Violet light is refracted **most**; red light is refracted **least**.
• The visible spectrum in order of increasing wavelength: violet, indigo, blue, green, yellow, orange, red.
• Each colour corresponds to a different wavelength and frequency; frequency does not change during refraction.

Reflection of light showing the normal, angle of incidence (i) and angle of reflection (r).

Refraction of light from air into glass: light bends towards the normal.