Physics: Light, Sound and Space
Light, sound and space, in 13+ Physics, applies the general wave ideas from Electricity, Magnetism and Waves to two specific examples, plus one topic that does not involve waves.
Before you start
Make sure you're comfortable with these topics first:
Method
- For reflection questions, remember the law of reflection: the angle of incidence always equals the angle of reflection, both measured from the normal (an imaginary line at right angles to the mirror's surface).
- For refraction questions, remember that light bends towards the normal when it enters a denser material (e.g. from air into glass) and bends away from the normal when it leaves a denser material and enters a less dense one.
- For colour questions, identify which colours of light are being absorbed and which are being reflected or transmitted by the object or filter, since the colour you see is the light that reaches your eye, not the light that is absorbed.
- For sound questions, identify the vibrating source first, then describe how the vibration passes from particle to particle through the medium, remembering that sound needs a medium and cannot travel through a vacuum.
- For echo questions, remember an echo is sound reflected off a surface, and use distance = speed x time (splitting the total time in half for the sound to travel there and back) when a calculation is needed.
- For space questions, learn the order of the eight planets outward from the Sun, and be able to state whether a described feature (e.g. day length, presence of rings, distance from the Sun) belongs to a rocky inner planet or a gas giant outer planet.
- For questions on gravity in space, remember that weight (not mass) changes with gravitational field strength, so an object's mass stays the same on the Moon but its weight is smaller because the Moon's gravitational field strength is weaker than the Earth's.
Worked example
A submarine sends out a sound pulse that reflects off the sea bed and returns as an echo 0.8 seconds later. The speed of sound in seawater is 1500 m/s. Calculate the depth of the sea bed below the submarine.
- The 0.8 seconds is the time for the sound to travel down to the sea bed AND back up again, so the time for one-way travel is 0.8 / 2 = 0.4 seconds.
- Write down the equation linking distance, speed and time: distance = speed x time.
- Substitute the one-way time: distance = 1500 x 0.4.
- Calculate: 1500 x 0.4 = 600.
- Final answer: the sea bed is 600 m below the submarine.
Practice questions
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Q1State the law of reflection.Show answer
Answer: The angle of incidence equals the angle of reflection.
Q2A ray of light travels from air into a glass block. State whether the ray bends towards the normal or away from the normal as it enters the glass.Show answer
Answer: Towards the normal.
Q3A red filter is held in front of a torch shining white light. State which colour of light passes through the filter.Show answer
Answer: Red.
Q4State why an astronaut in open space, far from any planet or star, cannot hear an explosion happening nearby.Show answer
Answer: Sound needs a medium (particles) to travel through, and space is a vacuum, so there is nothing to carry the sound.
Q5A cliff is 170 m from a person who claps and hears the echo. Use distance = speed x time and a speed of sound of 340 m/s to calculate how long after the clap the echo is heard.Show answer
Answer: 1 second (the sound travels 170 m to the cliff and 170 m back, a total of 340 m, at 340 m/s: 340 / 340 = 1).
Q6Put these bodies in order of their distance from the Sun, starting with the closest: Mars, Mercury, Earth, Venus.Show answer
Answer: Mercury, Venus, Earth, Mars.
Q7State whether the Earth's day (one full rotation on its axis) or the Earth's year (one full orbit of the Sun) takes longer.Show answer
Answer: The year - a day takes approximately 24 hours, while a year takes approximately 365 days.
Q8An astronaut has a mass of 70 kg on Earth. State her mass on the Moon, and explain why her weight on the Moon is less than her weight on Earth.Show answer
Answer: Her mass is still 70 kg (mass does not change with location); her weight is less on the Moon because the Moon's gravitational field strength is weaker than the Earth's.
Exam-style questions
Written in the style of a 13+ Common Entrance exam paper, with a full mark scheme.
A pupil investigates the reflection of light using a plane mirror, a ray box and a protractor. She shines a ray of light at the mirror so that it hits the mirror's surface at an angle of 35 degrees to the normal. (a) State the size of the angle of reflection. (b) Describe the path of the reflected ray in relation to the normal. (c) The pupil then moves the ray box so the incoming ray hits the mirror at a smaller angle to the normal. State what happens to the angle of reflection.
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A student compares how far sound travels through three materials: air, water and steel, and finds sound travels fastest through steel and slowest through air. (a) Suggest why sound travels faster through a solid such as steel than through a gas such as air, in terms of the particles in each material. (b) State what would happen to the sound if it had to cross a region of empty space (a vacuum), and explain why.
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Table 3 gives data about two planets in the solar system. These figures are for practice only. Planet | Average distance from Sun (million km) | Length of year (Earth days) Mercury | 58 | 88 Neptune | 4500 | 60190 (a) State which of these two planets has the longer year. (b) Suggest why a planet that is further from the Sun has a longer year than a planet close to the Sun. (c) State one other difference you would expect between a small, rocky inner planet like Mercury and a large outer planet like Neptune.
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Free printable worksheet
Want more practice on paper? Download the physics: light, sound and space worksheet pack - 17 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 13 of 13+ Science Workbook, the whole course as one free printable PDF.
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