Light Waves and Properties
In 1666, Isaac Newton split sunlight through a prism and found 7 colours, each bending by a different angle because of its wavelength.
● Know
- That light is a transverse electromagnetic wave
- The four main properties of light: absorption, reflection, refraction and scattering
- How to draw simple ray diagrams for reflection and refraction
● Understand
- Why light bends when it passes from one medium to another
- How different surfaces interact with light in different ways
- That the speed of light changes depending on the medium it travels through
● Can do
- Draw simple ray diagrams showing reflection and refraction
- Predict how light will behave at a boundary between two materials
- Design a practical investigation to observe light properties
Hold a glass prism up to a shaft of sunlight and a full rainbow fans across the wall: red on one side, violet on the other, with every colour of the visible spectrum in between. Light is an electromagnetic wave that our eyes can detect. The visible spectrum ranges from about 400 nm (violet) to 700 nm (red), and each wavelength produces a different colour sensation. White light is a mixture of all those wavelengths at once, the prism separates them because each wavelength bends by a slightly different angle.
Rayleigh scattering explains why the sky is blue. When light encounters particles much smaller than its wavelength (like air molecules), some light is scattered in all directions. The intensity of scattering is inversely proportional to the fourth power of wavelength: I ∝ 1/λ⁴.
This strong wavelength dependence means blue light (short wavelength) is scattered much more than red light (long wavelength). As sunlight travels through the atmosphere, blue light is scattered in all directions, including down to our eyes. This makes the sky appear blue from every direction except directly toward the Sun.
At sunrise and sunset, sunlight travels through much more atmosphere. Most blue light has been scattered away, leaving predominantly red and orange light to reach our eyes directly.
On the Moon, the sky is black even during the day because there is no atmosphere to scatter light. Astronauts see stars and the Sun simultaneously, and shadows are pitch black. In contrast, on Earth, scattered skylight fills in shadows and makes the sky appear bright blue. The redness of sunsets is most vivid after volcanic eruptions, which inject fine particles into the upper atmosphere and enhance scattering. After the 1883 Krakatoa eruption, sunsets worldwide were extraordinarily red for months, inspiring paintings and scientific studies.
Australian atmospheric optics: Australia clear skies and low pollution make it an excellent location for studying atmospheric optics. The Australian Bureau of Meteorology monitors aerosol levels that affect scattering. After major bushfires, Australian skies display spectacular red sunsets due to smoke particles enhancing scattering. Australian researchers at the ANU Research School of Earth Sciences use lidar (light detection and ranging) to study atmospheric composition by measuring how laser light scatters off particles and molecules at different altitudes.
The sky is blue because it reflects the ocean. This is false. The sky would be blue even over landlocked regions far from any ocean. The blue colour is due to Rayleigh scattering by air molecules, not reflection from water. The ocean appears blue for a different reason: water absorbs red light more strongly than blue, so deep water appears blue. These are separate phenomena with separate explanations. The similarity in colour is coincidental, not causal.
Sunlight enters Earth atmosphere. Blue light has shorter wavelength than red light. Predict which colour is scattered more strongly by air molecules.
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
When light encounters a boundary between two media, several things can happen:
Reflection: Light bounces off the boundary. The angle of incidence equals the angle of reflection (measured from the normal). Smooth surfaces produce specular reflection (clear images); rough surfaces produce diffuse reflection (no clear image).
Refraction: Light bends as it crosses the boundary because its speed changes. When entering a denser medium (air to water), light slows down and bends toward the normal. When exiting, it speeds up and bends away. The amount of bending depends on the refractive indices of the two media.
Absorption: Light energy is converted to other forms (usually heat) in the medium. Coloured objects absorb some wavelengths and reflect others.
Transmission: Light passes through the medium. Transparent materials transmit most light; translucent materials scatter it; opaque materials block it.
A swimming pool appears shallower than it actually is because of refraction. Light from the pool bottom travels from water (slower) to air (faster), bending away from the normal. Your brain interprets light as travelling in straight lines, so it projects the bottom to a shallower position. A pool that is 2 metres deep may appear to be only 1.5 metres deep. This is why spear fishers must aim below where the fish appears to be. Similarly, mirages on hot roads occur because air near the road is hotter and less dense, causing light from the sky to bend upward, creating the illusion of water on the road.
Australian optical engineering: The Australian Synchrotron in Melbourne produces intense beams of light from infrared to X-rays. Scientists use these beams to study the structure of materials at the atomic scale. The facility relies on precise control of reflection, refraction, and diffraction of light using specialised mirrors, lenses, and crystals. Australian researchers have used synchrotron light to study everything from protein structures to ancient artefacts, contributing to medical research, materials science, and archaeology.
Refraction is the same as reflection. This is false. Reflection involves light bouncing off a surface; refraction involves light bending as it passes through a boundary. They are distinct phenomena with different causes and different laws. A mirror relies on reflection; a lens relies on refraction. Both can occur simultaneously at a boundary - some light reflects while some refracts - but they are separate processes. The Fresnel equations describe how light energy divides between reflection and refraction at a boundary.
Total internal reflection (TIR) occurs when light travelling in a denser medium hits a boundary with a less dense medium at an angle greater than the critical angle. Instead of refracting out, all light reflects back into the denser medium.
The critical angle depends on the refractive indices: sin(critical angle) = n2/n1, where n1 > n2.
Optical fibres exploit TIR to transmit light over enormous distances with minimal loss. A glass core (high refractive index) is surrounded by cladding (lower refractive index). Light entering the core at shallow angles undergoes TIR at the core-cladding boundary, bouncing along the fibre like a pipe. Modern optical fibres can carry terabits of data per second across oceans.
Dispersion occurs because refractive index depends slightly on wavelength. Shorter wavelengths (blue) bend more than longer wavelengths (red) when passing through a prism. This separates white light into its constituent colours - the spectrum.
The NBN (National Broadband Network) in Australia uses optical fibre technology extensively. Fibre-to-the-Premises (FTTP) connections run optical fibres directly to homes, providing speeds up to 1 Gbps. The fibres are about the thickness of a human hair but can carry thousands of simultaneous phone calls, video streams, and data connections. Light pulses from lasers at one end travel through the fibre via total internal reflection, emerging at the other end where photodetectors convert them back to electrical signals. The signals can travel tens of kilometres without amplification because optical fibre has extremely low light loss (about 0.2 dB per kilometre).
Australian photonics industry: Australia has a significant photonics industry developing optical fibre technologies, laser systems, and optical sensors. The Australian Centre for Field Robotics at Sydney University uses fibre optic sensors for structural health monitoring of bridges and buildings. Companies like Redfern Integrated Optics produce advanced fibre components for telecommunications and sensing. The CSIRO Manufacturing division researches next-generation optical materials, including hollow-core fibres that could reduce signal loss even further. These technologies support Australia telecommunications infrastructure and export industry.
Total internal reflection only works with mirrors. This is false. TIR requires no mirrors, coatings, or special surfaces - just a boundary between two transparent media where light hits from the denser side at a steep enough angle. It is a purely geometric-optical phenomenon that occurs at every glass-air boundary. Diamonds sparkle because of TIR: their high refractive index gives a very small critical angle, so most light entering a cut diamond undergoes TIR and exits through the top facets, creating brilliance. No mirrors are involved.
Tap each card to flip. Mark Got it when you can recall the answer without flipping.
Wrong: "Light always travels in straight lines, so it cannot bend." No � light travels in straight lines in a uniform medium, but it bends (refracts) when it passes from one medium to another because its speed changes. This is why a swimming pool looks shallower than it really is.
Right: Light travels in straight lines within a uniform medium, but it bends, refracts, when it crosses a boundary between two media of different densities. The speed change causes the change in direction. This is why objects underwater appear displaced or distorted when viewed from above.
Wrong: "Reflection only happens in mirrors." No � all objects reflect light. If they did not, you would not be able to see them. Mirrors produce clear images because their surfaces are smooth, causing regular reflection. Rough surfaces produce diffuse reflection, which is why most objects do not produce clear images.
Right: All surfaces reflect light, if they didn't, they would be invisible. Mirrors look special because their flat, polished surface causes regular (specular) reflection, with all light rays reflecting at the same angle. Most everyday objects have rough surfaces that scatter reflected rays in all directions (diffuse reflection), which is how we see their shape and colour.
Wrong: "The colour of an object is painted onto it." No � the colour we see depends on which wavelengths of light the object reflects and which it absorbs. A red shirt looks red because it reflects red light and absorbs most other colours.
Right: The colour we perceive depends on which wavelengths of white light an object reflects back to our eyes. A red object reflects red wavelengths and absorbs the rest; a black object absorbs nearly all wavelengths; a white object reflects nearly all. The "colour" is a property of how the object interacts with light, not a paint sitting on its surface.
Light in Australia
The Great Barrier Reef: Light penetration in seawater is critical for coral survival. As sunlight enters the ocean, water absorbs red and orange light first, leaving mainly blue-green light at depth. This is why corals that live deeper in the reef tend to be less colourful, there is less red light available to reflect back.
Bushfire smoke and scattering: During severe bushfire seasons, such as the 2019–20 Black Summer, smoke particles in the atmosphere cause increased scattering of light. This can turn the sky an eerie orange or red colour because the smoke scatters shorter blue wavelengths and allows longer red wavelengths to pass through.
Indigenous astronomical knowledge: Aboriginal and Torres Strait Islander Peoples have sophisticated knowledge of light and observation, using starlight for navigation, seasonal calendars and ceremony. Understanding that light travels from distant sources and can be observed and interpreted is foundational to both traditional and modern astronomy.
✍ Copy Into Your Books
▾Light as a Transverse EM Wave
- Light is a transverse electromagnetic wave
- It can travel through a vacuum at ~300 000 km/s
- Oscillations are perpendicular to direction of travel
Four Properties of Light
- Absorption: light energy taken in and converted to heat
- Reflection: light bounces off a surface
- Refraction: light bends when changing medium
- Scattering: light deflected in many directions by particles
Reflection and Refraction Rules
- Angle of incidence = angle of reflection
- Light bends toward the normal when entering a denser medium
- Light bends away from the normal when leaving a denser medium
Light Property Detective
Ray Diagram Practice
At the start of this lesson you were shown a swimming pool that appears 2 m deep but looks only 1.5 m deep from the edge because light bends as it crosses the water surface.
Now that you've worked through the lesson, how has your thinking shifted? Can you explain that hook idea more precisely using what you've learned today?
Q1. 1. Explain why a pencil appears to bend when placed in a glass of water. In your answer, include the terms refraction, medium and speed. 4 MARKS
Q2. 2. Compare absorption and reflection. Include in your answer: (i) what happens to the light energy in each case, and (ii) one everyday example of each. 4 MARKS
Q3. 3. During severe bushfires in Australia, the sky can appear orange or red. Using your knowledge of scattering, explain why this happens and why the effect is stronger during bushfire seasons. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain why the pencil looks bent using the term refraction?
- Can you explain mirror reflection and sky colour using the properties of light?
Model answers (click to reveal)
Answers
▾MCQ 1
BLight is a transverse electromagnetic wave. This means its oscillations are perpendicular to the direction of travel, and it does not need a medium, it can travel through a vacuum.
MCQ 2
CWhen light passes from air into water, it slows down because water is denser. This causes the light ray to bend toward the normal.
MCQ 3
AAccording to the law of reflection, the angle of incidence equals the angle of reflection. Both angles are measured relative to the normal, so the reflected ray leaves at 40 degrees to the normal.
MCQ 4
DAir molecules in the atmosphere scatter shorter-wavelength blue light more effectively than longer-wavelength red light. This scattered blue light reaches our eyes from all directions, making the sky appear blue.
MCQ 5
BThe claim is incorrect because it uses an incorrect mechanism. Light bends (refracts) because its speed changes when entering a different medium, not because of a "pulling" force. Glass is denser than air, so light slows down and changes direction.
Short Answer 1
Model answer: The pencil appears to bend because of refraction. When light travels from the pencil through the water and into the air, it passes from one medium (water) to another (air). Light travels more slowly in water than in air, causing the light rays to bend at the boundary. Our brain assumes light travels in straight lines, so it interprets the bent rays as coming from a different position, making the pencil look bent at the water's surface.
Short Answer 2
Model answer: During absorption, the material takes in light energy and converts it to heat. For example, a black road surface absorbs sunlight and becomes hot. During reflection, light bounces off a surface without being absorbed. For example, a mirror reflects light in a predictable direction, allowing us to see a clear image. White surfaces reflect most light, while dark surfaces absorb most light.
Short Answer 3
Model answer: During bushfires, smoke particles enter the atmosphere and cause increased scattering of light. Normally, air molecules scatter shorter blue wavelengths more than red wavelengths, making the sky blue. During bushfires, the larger smoke particles scatter and absorb more of the shorter blue and green wavelengths, allowing the longer red and orange wavelengths to pass through and dominate what we see. The effect is stronger during bushfire seasons because there are far more particles in the air than usual, increasing the scattering and absorption of shorter wavelengths.