Module 4.3 – Light: Reflection, Refraction & Lenses

Key Definitions

• Luminous object: emits its own light.

• Refractive index n: ratio of light speed in vacuum to that in the medium (n = \frac{c}{v}).

• Total internal reflection (TIR): complete reflection of light back into a medium when it strikes the boundary above the critical angle.

• Concave lens (diverging): thinner at centre, spreads parallel rays outward.

• Convex lens (converging): thicker at centre, brings parallel rays to a focus.

Laws of Reflection & Refraction

• Reflection: \angle i = \angle r (incident = reflection).

• Snell’s law: n1\sin\theta1 = n2\sin\theta2.

• Light bends toward the normal when entering a higher n; away when entering a lower n.

Plane Mirrors

• Image properties: upright, laterally inverted, same size, virtual, distance behind mirror equals object distance.

• Lateral inversion: left–right reversal of image.

• Smooth surfaces give regular reflection → sharp images.

Refraction Concepts

• Critical angle \thetac: \sin\thetac = \frac{n2}{n1} for n1 > n2.

• No TIR when light travels into a medium of higher n (cannot bend past 90°).

• Apparent depth: objects in water appear closer due to refraction toward the normal.

• Underwater vision blurs because cornea–water have similar n; goggles restore air–cornea interface.

Total Internal Reflection Uses

• Occurs only when light moves from high n to low n and \thetai > \thetac.

• Enables optical fibres, prisms, sparkling of diamonds.

Lenses & Images

• Focus (F): point where parallel rays meet/appear to meet after lens.

• Convex lens:
  • Distant object ⇒ real, inverted, diminished image at focus.
  • Can also give virtual, upright, magnified image when object inside focal length.

• Concave lens: always gives virtual, upright, diminished image.

• Real images can be projected on a screen; virtual cannot.

Light Speed Ranking (common media)

v \text{(diamond)} < v \text{(glass)} < v \text{(Perspex)} < v \text{(ice)} < v \text{(water)} < v \text{(air)} (inverse of n).

Quick Facts

• Refraction makes a submerged pencil appear ‘broken’ at the water surface.

• Coin-in-cup reappearance demonstrates refraction.

• Increasing object–lens distance shifts image position (for convex lens, image moves toward focus).

Key Definitions

• Luminous object: emits its own light.
• Refractive index n: ratio of light speed in vacuum to that in the medium (n = \frac{c}{v}).
• Total internal reflection (TIR): complete reflection of light back into a medium when it strikes the boundary above the critical angle.
• Concave lens (diverging): thinner at centre, spreads parallel rays outward.
• Convex lens (converging): thicker at centre, brings parallel rays to a focus.

Laws of Reflection & Refraction

• Reflection: \angle i = \angle r (incident = reflection).
• Snell’s law: n1\sin\theta1 = n2\sin\theta2.
• Light bends toward the normal when entering a higher n; away when entering a lower n.

Plane Mirrors

• Image properties: upright, laterally inverted, same size, virtual, distance behind mirror equals object distance.
• Lateral inversion: left–right reversal of image.
• Smooth surfaces give regular reflection \u2192 sharp images.

Refraction Concepts

• Critical angle \theta c: \sin\theta c = \frac{n2}{n1} for n1 > n2.
• No TIR when light travels into a medium of higher n (cannot bend past 90°).
• Apparent depth: objects in water appear closer due to light refracting away from the normal as it passes from water (higher n) to air (lower n) before reaching the eye.
• Underwater vision blurs because cornea–water have similar n; goggles restore air–cornea interface, allowing the eye's lens system to focus light properly.

Total Internal Reflection Uses

• Occurs only when light moves from high n to low n and \theta i > \theta c.
• Enables optical fibres (light signals are guided along the fibre by continuous TIR), prisms (used in binoculars and periscopes to deviate light by 90° or 180° through TIR), sparkling of diamonds (designed to maximize internal reflections).

Lenses & Images

• Focus (F): point where parallel rays meet/appear to meet after lens.
• Convex lens:
  \u2022 Distant object \u21d2 real, inverted, diminished image at focus.
  \u2022 Can also give virtual, upright, magnified image when object inside focal length.
• Concave lens: always gives virtual, upright, diminished image.
• Real images can be projected on a screen; virtual cannot.

Light Speed Ranking (common media)

v\text{(diamond)} < v\text{(glass)} < v\text{(water)} < v\text{(air)}