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Physics - Reflection (AQA Syllabus A)

One part of GCSE Science is the study of waves, including both mechanical waves (such as sound) and electromagnetic waves (such as light). This is the first of six quizzes on waves and it looks at reflection.

Waves travel in straight lines unless they are diffracted, refracted or reflected and this quiz deals with the last of those three. Reflection is completely predictable, all you need to know is the law of reflection. This simply states that 'the angle of incidence is equal to the angle of reflection'. But what are these angles and how are they measured?

The angle of incidence is the angle at which a wave hits the reflecting surface. The angle of reflection is the angle at which the wave bounces off the reflecting surface. It doesn't matter what the surface is or how smooth or bumpy it is, the law of reflection is ALWAYS obeyed.

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This is where it gets tricky. The angle is not measured from the surface, it is measured from the normal. The normal is an imaginary line at right angles to the reflecting surface where the ray hits. When you have been doing reflection experiments at school you will have probably used ray boxes and plane mirrors. When you have traced out the path the light rays took you will have made this imaginary line real by drawing it so that you could measure the angles.

Smooth surfaces produce good reflections and can act as mirrors when light waves hit them. This can happen with other waves from the electromagnetic spectrum. You can feel the infrared radiation from the sun by reflecting sunlight onto the back of your hand using a small, flat mirror. WARNING: if you do try that, be very, very careful not to reflect the sunlight anywhere near your eyes as it ould easily damage them.

Longitudinal waves can be reflected too. They follow exactly the same rule of reflection as electromagnetic waves. Sound waves are longitudinal and when a sound wave is reflected from a cliff face or the wall of a building, you hear the reflection as an echo.

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  1. Which of the following statements about reflection is not true?
    Echoes occur in towns and cities too but you are not usually aware of them
  2. What is an incident ray of light?
    You can also use the phrases 'incident wave', 'incident wavefront', 'incident sound wave' etc. In terms of waves, incident simply means the wave that is approaching the reflecting surface
  3. Electromagnetic radiation travels in straight lines, so how come certain radio waves can be detected on the other side of the world without using artificial satellites?
    The reflecting layer is called the ionosphere
  4. When you look at yourself in a mirror, your image is:
    Right and left are reversed in a reflection
  5. The location of a virtual image in a plane mirror appears to be where?
    A reflection always appears to be as far behind the mirror as the image is in front. If you haven't already learnt about Pepper's Ghost in your science lessons, look it up when you have finished on this website
  6. On ray diagrams, what is the normal?
    It is drawn from the point at which the incident ray hits the reflecting surface
  7. A sound wave hits a brick wall at 36o from the normal. At what angle does it reflect?
    Remember the Law of Reflection...
  8. A hiker shouts to their friend and 4 seconds later hears an echo of their shout. If sound travels at 330 m/s, how far away was the cliff that reflected their shout?
    You could use the equation speed = distance ÷ time or simple logic. The key thing is to remember that an echo is a 'there and back' journey
  9. What is a virtual image?
    Plane mirrors give virtual images, they cannot be projected onto a screen. Concave mirrors give real images, they can be reflected and seen on a screen
  10. A solar furnace can be used to cook food in sunny places where it is difficult to obtain other fuels. It works by reflecting and focussing the Sun's heat. What would be the best shape and material for a solar furnace?
    Since it needs to focus the Sun's thermal radiation (infrared) onto a cooking vessel, the only option is concave. The others all produce vurtual images

Author: Kev Woodward

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