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Light refracts because waves travel at different speeds in different materials. Free GCSE Edexcel Physics notes with key definitions and 11 FAQs.
Refraction and Wave Velocity is part of Light rays, lenses and electromagnetic waves in GCSE Edexcel Higher Physics.
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Light refracts because waves travel at different speeds in different materials. Free GCSE Edexcel Physics notes with key definitions and 11 FAQs.
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Use this lesson to stay inside Light rays, lenses and electromagnetic waves while connecting the detail back to the wider Topic 5: Light and the electromagnetic spectrum topic.
Read the core explanations from Refraction and Wave Velocity before testing yourself from memory.
Picture a lifeguard running from the beach into the sea to rescue someone. On sand, they sprint at full speed. But the moment they hit the water, they slow down dramatically and their path bends towards the person they're rescuing.
Waves behave exactly like this lifeguard. When light travels from air into glass, or sound moves from air into water, something fascinating happens. The wave doesn't just continue in a straight line - it bends at the boundary.
This bending isn't random. It happens because waves travel at different speeds in different materials. The technical name for this bending is refraction, and it's responsible for everything from why swimming pools look shallower than they are to how your glasses help you see clearly.
Here's the key insight that catches many students out: refraction only happens when there's a velocity change. If a wave somehow maintained the same speed in both materials, it would travel in a perfectly straight line with no bending at all.
Every material has its own 'speed limit' for waves. Light rockets through a vacuum at 300,000,000 m/s, but slows down to about 200,000,000 m/s in glass. Sound travels at 340 m/s in air but speeds up to 1500 m/s in water.
The crucial point is this: when a wave hits a boundary and its speed changes, the wave must bend because different parts of the wavefront travel at different speeds at the boundary. Think of it like a car wheel hitting a patch of mud - the wheel that hits first slows down while the other keeps going at full speed, causing the car to turn.
For light waves, denser materials like glass and water typically slow the waves down compared to air. This is why light bends towards the normal (the imaginary perpendicular line) when entering these materials. The wave 'leans in' as it slows down.
Ray diagrams are the language of refraction. They show exactly how waves bend at boundaries, and examiners love testing your ability to draw them accurately. The key is following a systematic approach every single time.
Start with the boundary line - draw it as a solid horizontal line separating your two media. Label the materials clearly above and below this line. Then draw the normal as a dashed vertical line perpendicular to the boundary at the point where your wave will hit.
Worked example: Constructing a Refraction Ray Diagram
Draw a ray diagram showing light travelling from air into glass at 30° to the normal.
Answer: Complete ray diagram showing refraction towards the normal when light enters glass
The golden rule for ray diagrams: when waves slow down (entering a denser medium), they bend towards the normal. When waves speed up (entering a less dense medium), they bend away from the normal. This isn't just a rule to memorise - it's a consequence of wave physics.
The relationship between wave speed, frequency, and wavelength gives us powerful tools for understanding refraction. When a wave crosses a boundary, its frequency stays constant - this is a fundamental rule of wave physics.
Here's what happens during refraction: frequency stays the same, but velocity changes. Since $v = f\lambda$, the wavelength must change too. If the wave slows down, the wavelength gets shorter. If it speeds up, the wavelength gets longer.
Worked example: Calculating Wavelength Change During Refraction
Light with frequency 5.0 × 10¹⁴ Hz travels from air (v = 3.0 × 10⁸ m/s) into glass (v = 2.0 × 10⁸ m/s). Calculate the wavelength in each medium.
Answer: λ in air = 6.0 × 10⁻⁷ m, λ in glass = 4.0 × 10⁻⁷ m
Notice how the wavelength decreased when light entered the denser medium. This is always the pattern: denser medium means slower waves and shorter wavelengths. The frequency never changes because it's determined by the source, not the medium.
Refraction isn't just a physics curiosity - it shapes our daily experience in countless ways. Every time you look through a window, wear glasses, or see a rainbow, you're witnessing refraction in action.
Swimming pools appear shallower than they really are because light from the bottom bends away from the normal as it exits the water. Your brain traces the light rays back in straight lines, creating the illusion that the bottom is higher than it actually is.
Optical fibres use total internal reflection, a phenomenon related to refraction that occurs when light hits the boundary inside the fibre at an angle greater than the critical angle. This keeps the light trapped inside the fibre so signals can travel long distances with little energy loss.
Mirages in deserts occur because hot air near the ground has a different density than cooler air above. Light refracts as it passes through these layers, creating the illusion of water where there is none. The 'water' is actually an image of the sky, bent by refraction.
Refraction questions follow predictable patterns in GCSE exams. Master these approaches and you'll handle any refraction question with confidence. The key is recognising which type of question you're facing and applying the right method.
For ray diagram questions, always start by identifying the two media and which is denser. Remember: denser medium means slower waves, which bend towards the normal. Less dense medium means faster waves, which bend away from the normal.
Calculation questions usually involve the wave equation $v = f\lambda$. The golden rule is that frequency never changes during refraction - it's set by the source. Use this fact to find wavelengths in different media or to compare wave speeds.
Watch out for the classic exam trap: measuring angles from the boundary instead of the normal. Examiners deliberately include this mistake in mark schemes because so many students fall for it. Always measure from the dashed perpendicular line.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Sound travels at about 1500 m/s in water, much faster than 340 m/s in air. Different media have different wave velocities for the same type of wave.
Total internal reflection happens when light inside a material hits the boundary at an angle greater than the critical angle, so it is all reflected back inside. Optical fibres use it to keep light trapped so signals travel long distances with little energy loss.
Light refracts because waves travel at different speeds in different materials. When a wave crosses a boundary and its speed changes, different parts of the wavefront travel at different speeds, so the wave bends. Refraction only happens when there is a velocity change.
Light travels at about 200,000,000 m/s in glass, compared with 300,000,000 m/s in a vacuum. Slowing down as it enters glass makes light bend towards the normal.
A mirage is caused by refraction: hot air near the ground has a different density from the cooler air higher up, so light bends as it passes through these layers. The water that seems to be there is actually an image of the sky.
When light slows down entering a denser medium such as glass or water, it bends towards the normal. When it speeds up entering a less dense medium, it bends away from the normal.
During refraction the frequency stays the same, because it is set by the source, but the velocity changes. Since v = fλ, the wavelength must change too: if the wave slows down, the wavelength gets shorter.
Use λ = v ÷ f with the speed in glass. For light of frequency 5.0 × 10¹⁴ Hz, the wavelength in air is 3.0 × 10⁸ ÷ 5.0 × 10¹⁴ = 6.0 × 10⁻⁷ m, and in glass (v = 2.0 × 10⁸ m/s) it is 4.0 × 10⁻⁷ m.
Swimming pools look shallower because light from the bottom bends away from the normal as it leaves the water. The brain traces the light rays back in straight lines, so the bottom appears higher than it actually is.
Refraction only happens when there is a velocity change. If a wave kept the same speed in both materials, it would travel in a perfectly straight line with no bending at all.
Glasses use refraction to help people see clearly: light changes speed as it passes into and out of the lens material, so it bends. Refraction is the bending of waves when they pass from one medium to another due to a change in wave velocity.