Reflection, Refraction and Absorption
In 1870, John Tyndall demonstrated light bending through a curved water jet — 150 years before fibre-optic cables carried 95% of Australia's internet traffic.
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Q1 · Why does a straw in a glass of water appear bent at the surface?
Q2 · Why do you think a black car gets hotter in the sun than a white car of the same make and model?
● Know
- Reflection is when light bounces off a surface.
- Refraction is when light changes direction as it passes from one medium to another.
- Absorption is when light energy is taken in by a material and not reflected or transmitted.
● Understand
- The law of reflection states that the angle of incidence equals the angle of reflection.
● Can do
- Draw ray diagrams showing reflection and refraction, and use the law of reflection to predict the path of light.
When light encounters a boundary between two materials, three things can happen: reflection, refraction and absorption. Reflection is the bouncing of light off a surface. A mirror reflects almost all light in an organised way, which is why you see a clear image. A wall reflects light in many directions, which is why you see a vague, diffuse patch of colour rather than an image.
Refraction is the bending of light as it passes from one medium to another. Light travels slower in water than in air, and this change in speed causes the light ray to bend at the boundary. This is why a pencil in water looks bent, why a pool looks shallower than it really is, and why lenses can focus light.
Absorption occurs when light energy is converted into other forms, usually heat. A black surface absorbs most visible light and warms up. A white surface reflects most light and stays cooler. These three processes determine how light interacts with every object you see.
When you look at a swimming pool from above, the bottom appears closer than it really is. Light from the bottom travels upward through water, refracts away from the normal as it enters air, and reaches your eye. Your brain assumes the light travelled in a straight line, so it places the bottom closer to the surface. The actual depth might be 2 metres, but it looks like 1.5 metres.
Australian optics research: Scientists at the University of Sydney develop new optical materials that control reflection and refraction at surfaces. Anti-reflective coatings on solar panels reduce reflection and increase absorption, improving energy capture. These coatings use thin-film interference, exploiting the wave nature of light to cancel reflections at specific wavelengths.
Refraction means light curves. Light travels in straight lines within any uniform medium. Refraction is a sudden change in direction at a boundary, not a gradual curve. Once inside the new medium, light resumes straight-line travel. The bending happens only at the interface between materials with different optical densities.
You place a pencil in a glass of water. Predict what the pencil will look like from the side.
The pencil appears bent at the water surface. The part underwater looks shifted and magnified compared to the part in air.
Use these terms in your explanation: refraction · medium · light slows down
Reflection occurs when light strikes a surface and bounces back. The behaviour depends on the surface texture. A smooth surface, like a mirror or still water, produces specular reflection: parallel light rays remain parallel after reflection, producing a clear image. A rough surface, like a wall or paper, produces diffuse reflection: light rays scatter in many directions, so no image forms.
The law of reflection states that the angle of incidence equals the angle of reflection. Both angles are measured from the normal, an imaginary line perpendicular to the surface. If light hits a mirror at 30 degrees to the normal, it reflects at 30 degrees on the other side. This law is why you see yourself at the correct height in a mirror and why periscopes work.
Most objects you see are not light sources; they are visible because they reflect light from the Sun or artificial lights. The colour of an object is determined by which wavelengths it reflects and which it absorbs. A red shirt reflects red light and absorbs other colours. A white shirt reflects all visible wavelengths.
When you look in a bathroom mirror, light from your face hits the mirror, reflects according to the law of reflection, and enters your eyes. Your brain traces the reflected rays back behind the mirror and constructs a virtual image that appears to be the same distance behind the mirror as you are in front. The image is laterally inverted.
Sydney Opera House sails: The iconic white shells of the Sydney Opera House are covered in over one million glossy Swedish tiles. These tiles were chosen partly for their reflective properties: they reflect sunlight during the day and artificial light at night, giving the building its luminous appearance. The smooth, curved surfaces create complex patterns of specular reflection.
A mirror reverses left and right but not up and down. A mirror does not actually reverse left and right. It reverses front and back. When you raise your left hand, the reflection also raises its left hand. The confusion arises because you intuitively imagine yourself rotated 180 degrees to face the mirror, which would swap left and right. But the mirror does not rotate you; it reflects you.
Absorption is the process by which a material takes in light energy and converts it to other forms, usually heat. When light hits a surface, some wavelengths are reflected and some are absorbed. The absorbed energy excites electrons and atoms in the material, increasing their thermal motion. This is why dark objects heat up faster in sunlight than light-coloured objects.
A black surface absorbs most visible light across all wavelengths, reflecting very little. A white surface reflects most visible light, absorbing very little. A red surface absorbs all colours except red, which it reflects. These selective absorption and reflection patterns are what give objects their colours.
Absorption is not limited to visible light. Dark clothing absorbs infrared radiation from the Sun, making it hotter than light clothing. Solar panels are designed to absorb as much light as possible across the visible and near-infrared spectrum, converting that energy into electricity rather than heat.
On a sunny Australian summer day, the temperature inside a black car can reach 70 degrees C, while a white car in the same location might reach only 50 degrees C. The black paint absorbs about 90 percent of visible and infrared radiation, converting it to heat. The white paint reflects about 80 percent of radiation, keeping the interior cooler. This is why white cars are popular in hot climates.
Australian bushfire-resistant housing: Architects designing homes in bushfire-prone areas use light-coloured, reflective roofing materials. Dark roofs absorb radiant heat from approaching fires and can ignite even before flames reach the building. Reflective roofs bounce that heat away, buying precious time for residents to evacuate. Understanding absorption and reflection literally saves lives.
Black objects are black because they have no colour. Black objects are black because they absorb all visible wavelengths and reflect almost none. Colour is a perception caused by the wavelengths that reach your eye. If no wavelengths are reflected, no colour information reaches your eye, and you perceive black. It is the absence of reflected light, not the absence of colour property in the object.
When light hits a surface, most of it is and converted to . When light hits a surface, most of it is .
The rules of refraction are simple once you memorise them correctly. When light enters a denser medium (where it slows down), it bends toward the normal. When light enters a less dense medium (where it speeds up), it bends away from the normal. The normal is an imaginary line drawn perpendicular to the surface at the point where the light hits.
Air to water: light slows down, bends toward the normal. Water to air: light speeds up, bends away from the normal. Glass to air: speeds up, bends away. Air to glass: slows down, bends toward. These four cases cover most refraction situations you will encounter.
The amount of bending depends on the difference in optical density between the two materials. A large difference produces a large bend. A small difference produces a small bend. This is why a pencil in water looks more bent than a pencil in oil.
When you look at a fish underwater, the fish is not where it appears to be. Light from the fish travels through water, refracts away from the normal as it enters air, and reaches your eye. Your brain traces the light back in a straight line, placing the fish closer to the surface and further forward than it really is. Experienced spearfishers aim below the apparent position to compensate for this optical illusion.
Australian diamond optics: Argyle diamonds from Western Australia are prized for their brilliance, which depends on refraction. Diamond has a very high refractive index (about 2.4), so light entering a diamond slows dramatically and bends sharply toward the normal. Expert cutters shape diamonds so that light undergoes multiple internal reflections before exiting, maximising the sparkle.
Light always bends toward the normal. Light only bends toward the normal when entering a denser medium. When entering a less dense medium, it bends away. The direction of bending depends on whether the light is speeding up or slowing down. Memorising one rule and applying it universally will give wrong answers half the time.
Find the error in this student statement.
- Light does slow down in water.
- When light slows down, it bends away from the normal.
- Therefore the student statement is correct.
Reflection, refraction and absorption are not exclusive to light. All waves exhibit these behaviours. Sound reflects off hard surfaces, creating echoes and reverberation. Sound refracts when moving between air layers of different temperatures, because temperature affects air density and therefore sound speed. Sound is absorbed by soft materials like curtains and carpets, which is why recording studios are full of foam and fabric.
Water waves reflect off harbour walls, refract as they enter shallower water, and are absorbed by sandy beaches. Earthquake waves reflect off boundaries between rock layers, refract as they pass through regions of different density, and are absorbed by molten rock. The universality of these three processes is one of the great simplifying principles of wave physics.
Understanding this universality allows scientists and engineers to transfer knowledge between domains. Acoustic engineers designing a concert hall use the same reflection principles as optical engineers designing a telescope. Oceanographers studying wave refraction use the same mathematics as physicists studying light through lenses.
On a hot day, the air near the ground is warmer and less dense than the air above. Sound travels faster in warmer air, so sound waves from a distant source refract upward, away from the ground. This is why sounds seem to carry poorly on hot days. Conversely, on a cold evening, the air near the ground is denser, sound travels slower, and waves refract downward toward the ground, making distant sounds seem clearer and closer.
Australian lighthouse optics: Historic lighthouses like Cape Leeuwin in Western Australia used Fresnel lenses to refract light into a powerful beam visible for dozens of kilometres. The lens design exploits refraction to bend light from a small source into a parallel beam. Modern lighthouses use similar optical principles with LED sources, demonstrating how centuries-old refraction physics still guides maritime safety around the Australian coastline.
Reflection, refraction and absorption are properties of light only. These are properties of waves. Light exhibits them most obviously because we can see light, but sound, water waves, seismic waves and every other wave type behave the same way. The underlying physics applies universally.
Refraction is not just a classroom curiosity; it has real consequences for anyone who works with or in water. Because light bends at the air-water interface, underwater objects appear closer to the surface than they really are. The apparent depth is about three-quarters of the actual depth for typical viewing angles. This affects divers, snorkelers, spearfishers, lifeguards and marine biologists.
The displacement is not just vertical; it is also horizontal. An object directly beneath you appears directly beneath you, but an object off to the side appears shifted toward you. This is because the light rays from the sides refract more dramatically than rays from directly below. The effect increases as you look more obliquely at the surface.
Understanding these optical effects is essential for safety. A diver who thinks the bottom is 3 metres away might actually be 4 metres away, risking decompression issues. A spearfisher who aims at the apparent position of a fish will miss every time. Training in underwater optics is part of professional diving and lifesaving courses.
A snorkeller looks down at a sea turtle resting on the sandy bottom. The turtle is actually 4 metres below, but due to refraction it appears to be only 3 metres down. The snorkeller dives down, expecting to reach the turtle quickly, but finds the distance is greater than it looked. This discrepancy can be dangerous for inexperienced divers who misjudge their breath-hold capacity.
Australian surf lifesaving: Lifeguards at Bondi and other Australian beaches are trained to account for refraction when scanning the water. A swimmer in distress may appear closer to shore than they really are because refraction compresses the visible depth. Lifeguards learn to overestimate distances and use reference points rather than visual estimates when planning rescues in choppy water.
Water magnifies underwater objects. Water does not magnify in the sense of a lens making things larger. It displaces the apparent position of objects, which can create a slight enlargement effect at certain angles, but the primary effect is positional shift, not magnification. A fish does not look bigger underwater; it looks closer and shifted. Diving masks include flat glass, not magnifying lenses.
You learned that light can be reflected, refracted, or absorbed when it interacts with materials.
Why does a mirror reflect your image clearly, while a white sheet of paper does not, even though both reflect light?
The hook showed you something you've probably seen a hundred times: a straw in a glass of water that looks snapped at the surface. That's refraction at work — the same principle that carries your internet along fibre-optic cables across the country.
Now that you understand reflection, refraction and absorption at wave boundaries, how would you explain the snapped-straw effect to a friend? Did knowing about fibre optics change how you think about everyday light behaviour?
1. According to the law of reflection, the angle of incidence equals the:
2. What happens to light when it passes from air into glass at an angle?
3. The normal to a surface is:
4. Which of these is an example of refraction?
5. What happens to most of the light that hits a black surface?
Explain the difference between reflection and refraction, and give one everyday example of each. (3 marks)
Hint: Consider what happens to the light ray in each case.
Draw a labelled diagram showing a light ray reflecting off a plane mirror. Include the incident ray, reflected ray, normal, and label the angles of incidence and reflection. (3 marks)
Hint: Remember that the angle of incidence equals the angle of reflection, both measured from the normal.
Explain how optical fibres use the principle of total internal reflection to transmit light signals over long distances. (3 marks)
Hint: Describe what happens when light hits the boundary between the core and cladding of the fibre.