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Forces, Energy & Light
Motion, energy and light, described without the equations.
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The speed of light in a vacuum is closest to which value?
Light, Sound and Waves
Light and sound behave nothing alike, but both are waves, and the same handful of ideas explains most of what they do. Ten questions on the two waves you meet constantly.
About this topic
Sound is a pressure wave running through matter, squeezing and stretching the air as it goes. Light is an electromagnetic wave and needs no matter at all, which is the single biggest difference between them and the source of most of the questions here. The rest is about what happens at boundaries. Waves bend when they change speed, reflect when they cannot escape, and shift in frequency when the source is moving. Those three behaviours cover a bent straw in a glass, a fibre-optic cable and an ambulance going past.
Questions & answers
Light, Sound and Waves: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1Why can sound not travel through the vacuum of space?
- A Space is too cold for vibrations to continue
- B The Sun's radiation cancels sound waves out
- C Gravity is too weak to hold a wave together
- D There are no particles to carry the pressure wave Correct
Answer: D. There are no particles to carry the pressure wave
Why: Sound needs matter to compress and rarefy, so with no medium there is nothing to pass the vibration on. Astronauts on a spacewalk talk by radio, which is electromagnetic and needs no medium at all.
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2You see a lightning flash several seconds before you hear the thunder. Why?
- A Sound waves take a longer, curved path through the air
- B Your ears respond more slowly than your eyes
- C Light travels enormously faster than sound Correct
- D The thunder is produced a moment after the flash
Answer: C. Light travels enormously faster than sound
Why: Sound covers about a third of a kilometre each second while light crosses that distance almost instantly. Counting three seconds between flash and rumble puts the strike roughly a kilometre away.
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3Which property of a sound wave determines the pitch you hear?
- A The distance it has travelled
- B Its frequency Correct
- C Its amplitude
- D Its speed through the air
Answer: B. Its frequency
Why: More vibrations per second means a higher note, measured in hertz. The A that orchestras tune to sits at 440 hertz, a standard only fixed internationally in the twentieth century.
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4What is the approximate range of frequencies a healthy young human can hear?
- A 20 hertz to 20,000 hertz Correct
- B 1 hertz to 1,000 hertz
- C 200 hertz to 200,000 hertz
- D 20 hertz to 200 hertz
Answer: A. 20 hertz to 20,000 hertz
Why: The upper limit falls steadily with age, which is why some people can hear a device that others in the room cannot. Dogs hear well beyond 40,000 hertz, and bats far higher still.
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5Sound in air is a longitudinal wave. What does that mean?
- A The air moves at right angles to the wave's direction
- B The wave travels in a straight line only
- C The wave carries matter along with it as it goes
- D The air moves back and forth along the same line the wave travels Correct
Answer: D. The air moves back and forth along the same line the wave travels
Why: Longitudinal waves work by compressing and stretching the medium in the direction of travel, rather than shaking it sideways. Light is transverse instead, vibrating across the direction it moves, which is what makes polarised sunglasses possible.
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6An ambulance siren sounds higher as it approaches and lower once it has passed. What is this effect called?
- A Resonance
- B Diffraction
- C The Doppler effect Correct
- D Refraction
Answer: C. The Doppler effect
Why: Motion towards you bunches the waves up and raises the frequency, and motion away stretches them out. Astronomers use the same shift in light to work out how fast distant galaxies are moving.
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7A straw in a glass of water looks bent at the surface. What causes this?
- A Light is absorbed by the water and re-emitted at an angle
- B Light changes speed as it passes between water and air, so it bends Correct
- C Water magnifies the lower part of the straw
- D The straw genuinely flexes under the water pressure
Answer: B. Light changes speed as it passes between water and air, so it bends
Why: The change of speed at the boundary is refraction, and it also makes a swimming pool look shallower than it is. Light slows to about three-quarters of its vacuum speed inside water.
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8Light stays inside a fibre-optic cable over long distances because of which effect?
- A Total internal reflection Correct
- B Magnetic confinement
- C Diffraction around the cable's core
- D Absorption by the outer coating
Answer: A. Total internal reflection
Why: Beyond a certain angle light striking the boundary is reflected entirely back into the fibre rather than escaping. The glass used is so clear that a block of it kilometres thick would still be see-through.
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9A red apple looks red in white light. What is happening?
- A It generates red light of its own
- B It bends white light into red as it passes through the skin
- C It absorbs red light and reflects everything else
- D It reflects red light and absorbs most other wavelengths Correct
Answer: D. It reflects red light and absorbs most other wavelengths
Why: The colour you see is whatever the surface fails to absorb and sends back to your eye. Under a pure blue lamp the same apple looks almost black, because there is no red light for it to return.
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10Which colour of visible light has the longest wavelength?
- A Green
- B Blue
- C Red Correct
- D Violet
Answer: C. Red
Why: Red sits at the long-wavelength end of the visible range, just before infrared. Longer waves scatter less in air, which is one reason red is used for brake lights and distant warning lamps.
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