Sec 1 Light — Reflection & Refraction
Created by Miss Clarissa Ng | www.clartutors.com
1 Introduction to Light
💡 What is Light?
Light is a form of energy that can be detected by human eyes. There are many properties of light that produce observable effects:
- Light rays can be "bounced" (reflected)
- Light rays can be "bent" (refracted)
- Light rays can be "split" (dispersed) into many colours
🔦 Sources of Light: Luminous vs Non-luminous
| Type | Definition | Examples |
|---|---|---|
| Luminous objects | Produce their own light, which directly enters our eyes | Light bulbs, fires, stars (including the Sun) |
| Non-luminous objects | Do not produce their own light — seen because they reflect light from a source into our eyes | The Moon, cups, books, grass |
Key Point: Objects can be seen when light rays are reflected off them into our eyes. This light can come from the objects themselves or be reflected from another light source.
🪟 Effects of Light on Materials
When light falls upon a material, it can:
- Pass through (be transmitted)
- Be absorbed by the material
- Be reflected by the material
| Type of Object | Light Behaviour | Examples |
|---|---|---|
| Transparent | All or most light is transmitted through | Glass, water, clear air |
| Translucent | Only some light is transmitted; the rest is absorbed or reflected | Tracing paper, stained glass, frosted glass |
| Opaque | No light is transmitted; light is absorbed or reflected | Wooden cube, mirror, metal sheet |
2 Principles of Light Propagation
📏 Rectilinear Propagation
Light rays travel in straight lines.
This is the principle of rectilinear propagation (rectilinear = straight line, propagation = travel).
🔄 Principle of Reversibility
Light will follow the same path if its direction of travel is reversed.
Hence, reflected or refracted light will retain its path even when its direction is reversed.
3 Shadows & Eclipses
🌑 How Shadows Are Formed
A shadow is formed when rays of light travelling in straight lines from a light source are blocked by a translucent or opaque object. Other rays that are not blocked continue to travel in straight lines past the object, onto a surface or screen.
Three Conditions for Shadow Formation
- Presence of a light source
- Presence of an object that can block the light
- Presence of a surface or screen on which light can fall
📐 Factors Affecting Shadow Properties
| Factor | Effect on the Shadow |
|---|---|
| 1. Transparency of the object | Translucent objects → light/faint shadow; opaque objects → dark shadow |
| 2. Relative positions of source, object and screen | The closer an object is to the light source (and further from the screen) → the greater the size of the shadow |
| 3. Orientation of object and light source | Changing orientation changes the shape of the shadow; changing the angle of the light source also affects shape and size |
🌘 Eclipses
| Type | Arrangement | What Happens |
|---|---|---|
| Solar eclipse | Moon between Sun and Earth | The Moon passes directly between Earth and the Sun, blocking the Sun's light. The Moon's shadow is formed on Earth. |
| Lunar eclipse | Earth between Sun and Moon | The Earth passes between the Sun and the Moon, casting its shadow on the Moon. |
4 Reflection of Light
🪞 The Two Laws of Reflection
- The angle of incidence equals the angle of reflection.
- The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
📖 Key Definitions
| Term | Meaning |
|---|---|
| Incident ray | The ray of light falling on a surface |
| Reflected ray | The ray that emerges after reflecting off a surface |
| Normal | 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 |
| Angle of reflection (r) | The angle between the normal and the reflected ray |
✨ Types of Reflection
| Type | Surface | Reflected Rays |
|---|---|---|
| Regular (specular) reflection | Smooth surface | If incident rays are parallel, the reflected rays will also be parallel |
| Diffuse reflection | Rough surface | Even if incident rays are parallel, the reflected rays scatter in different directions because the angle of incidence differs at each point |
Important: In both cases (regular and diffuse), the individual rays still obey the laws of reflection — angle of incidence = angle of reflection at each point.
5 Plane & Curved Mirrors
🪞 Characteristics of Images in a Plane Mirror
- The image is the same distance from the mirror as the object is from the mirror.
- The image is upright.
- The image is the same size as the object.
- The image is a virtual image (cannot be captured on a screen).
Lateral Inversion: The image is laterally inverted — left and right are swapped.
🔍 Curved Mirrors
| Mirror Type | Image Properties | Uses |
|---|---|---|
| Convex mirror (curves outwards) | Virtual, upright, laterally inverted and smaller than the object | Security mirrors at blind corners — provide a wider field of vision for drivers |
| Concave mirror (curves inwards) | When close: virtual, upright, laterally inverted and magnified | Dental mirrors (magnified images of teeth), car headlights |
🔭 Applications
- Periscope — uses two plane mirrors inclined at 45° to the stem, allowing viewing over obstacles.
- Pinhole camera — works because light travels in a straight line. The image formed is inverted (upside down).
- Flash photography (red-eye effect) — red eyes occur when the flash unit is very close to the camera lens: light from the flash enters the eye and reflects along its original path back into the lens.
6 Refraction of Light
🌊 What is Refraction?
Refraction is the bending of light rays as they pass from one transparent medium to another of different optical density. This occurs because light travels at different speeds in different materials.
Why Does Light Bend?
- Light travels faster in air (less optically dense) and slower in glass/water (more optically dense)
- When light enters a denser medium at an angle, one side of the ray slows down first → the ray bends
- If light enters perpendicularly (along the normal), it does NOT bend — only its speed changes
✅ Conditions for Refraction
- The two media must have different optical densities
- The incident ray must not be perpendicular to the boundary (i.e. it must not travel along the normal)
- Both media must be transparent or translucent
⚠️ Important
If light travels along the normal (perpendicular to the boundary), it does not bend — even though its speed changes. Refraction only occurs when light enters at an angle.↩️ Direction of Bending: The Two Cases
| Direction of Travel | Bending | Speed |
|---|---|---|
| Air → glass/water (less dense to denser) | Bends towards the normal | Decreases |
| Glass/water → air (denser to less dense) | Bends away from the normal | Increases |
📜 The Two Laws of Refraction
- The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.
- The ratio of sin(i) to sin(r) is constant for a given pair of media — Snell's Law: n₁sin(i) = n₂sin(r)
⚡ Speed of Light in Different Media
| Medium | Optical Density | Speed of Light |
|---|---|---|
| Air (vacuum) | Least dense | Fastest (~3 × 10⁸ m/s) |
| Water | Moderately dense | Slower (~2.25 × 10⁸ m/s) |
| Glass | More dense | Slowest (~2 × 10⁸ m/s) |
Remember: The more optically dense the medium, the slower light travels through it. This change in speed is what causes refraction (bending).
7 Apparent Depth & Real Depth
🏊 Why Does a Pool Look Shallow?
When you look at an object underwater from above, the light rays coming from the object bend as they pass from water into air. Your brain assumes light travels in straight lines, so it perceives the object to be at a different position than where it actually is.
- Real depth = actual distance of the object below the surface
- Apparent depth = how deep the object appears to be (always less than real depth)
- The relationship: apparent depth < real depth
Key Takeaway: This is why a swimming pool looks shallower than it actually is!
🥤 Why Does a Straw Look Bent in Water?
When you place a straw in a glass of water, the part underwater appears to be at a different position than the part above water. This is because light from the submerged part bends as it exits the water into air, making the straw appear "broken" or bent at the water surface.
Try this: Place a pencil in a glass of water and look from the side. You will see the pencil appears to be broken at the water surface — refraction in action!
8 Dispersion & Colours
🌈 What is Dispersion?
Dispersion is the splitting of white light into its component colours (the spectrum) when it passes through a prism or other transparent medium. This happens because different colours of light travel at slightly different speeds in the same material, so they refract (bend) by different amounts.
- White light is a mixture of all seven colours: Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV)
- Red bends the least (travels fastest in glass) → appears at the top
- Violet bends the most (travels slowest in glass) → appears at the bottom
Rainbow formation: A rainbow forms when sunlight passes through water droplets in the atmosphere. Each droplet acts as a prism, dispersing white light into its component colours. The observer sees the rainbow when facing away from the Sun with rain in front of them.
🎨 Colour Absorption & Reflection
The colour we see an object to be depends on which wavelengths of light it reflects and which it absorbs:
| Object Colour | Reflects | Absorbs |
|---|---|---|
| White object | All colours | None |
| Black object | None | All colours |
| Red object | Red only | All other colours |
| Green object | Green only | All other colours |
Key Point: A red apple looks red because it reflects red light and absorbs all other colours. Under blue light, the red apple would appear black because there is no red light to reflect!