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Waves - Reflection and Refraction

This GCSE Physics quiz about waves looks at reflection and refraction. Reflection occurs when any wave reaches a boundary between two different media. The amount of reflection depends on the nature of the medium that the wave is hitting. At GCSE, you will have studied this only for light waves, but what you have learnt about light reflection is equally applicable to any wave. All waves follow the laws of reflection, so make sure you revise them well. In the exam, stay calm if you come across a question that asks you about the reflection of water waves, earthquake waves, radio waves or sound waves. Simply pretend they are light waves and that should help you find the answer.

OK, lets think about light waves now. When light hits an opaque surface, some of the energy it carries is reflected and some is absorbed.

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Smooth shiny surfaces reflect the most and you can see an image in them. Rough surfaces tend to scatter the light in all directions and no image is formed. Dark surfaces reflect less than light surfaces as they absorb more of the energy of the light wave. But what happens if the light hits a transparent surface?

The answer is that a small amount is reflected but the rest travels into the transparent object. If the light hits the surface at an angle of incidence of 0° (in other words, travelling along the normal) it will continue in a straight line. If it hits at an angle of incidence of between 0° and an angle called the critical angle, it will change direction (bend). This is what we call refraction. We say the light has been refracted. Just to complicate matters, if it hits the surface at the critical angle, it will travel along the surface and if the angle of incidence is greater than the critical angle, it will reflect. Phew, that's a lot more to remember than with reflection!

Refraction happens because light travels at different speeds in different media. When it enters a more dense medium, it slows down and bends towards the normal. If it enters a less dense medium, it speeds up and bends away from the normal. The amount of bending is predictable and depends on a property of a transparent object known as the refractive index. The refractive index can be calculated in two ways by carrying out some experiments. The first is to measure the critical angle of the medium. To do this, the angle of incidence of a light ray is changed to find the exact point that it neither reflects nor refracts. The refractive index is then calculated as being the reciprocal of the sine of the critical angle. The alternative method is to use several different angles of incidence and measure the corresponding angle of refraction. Dividing the sine of the angle of incidence by the sine of the angle of refraction gives the value of the refractive index.

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  1. A light ray is reflected off the surface of a plane mirror at 40o to the normal. What is the angle of reflection of the reflected ray?
    The angle of reflection is always equal to the angle of incidence. This is known as Snell's Law or the law of reflection
  2. What determines how fast light travels in a medium?
    The higher the refractive index, the slower the wave travels and the more it will bend on entering the medium
  3. What equation can be used to calculate the refractive index of a substance?
    Using two different types of glass that have different refractive indices to make compound lenses for optical instruments avoids the defect of chromatic aberration
  4. The critical angle of diamond is 24 degrees. What is its refractive index?
    Diamond has a small critical angle which means a lot of light is totally internally reflected - this is why diamonds are so sparkly
  5. Another equation for calculating the refractive index of a substance is ...
    Whichever way that you work it out, the refractive index has no units
  6. If light enters a medium with an angle of incidence of 23 degrees and has an angle of refraction of 18 degrees, what is the refractive index of the medium?
    Simple substitution into the equation
  7. If a shiny surface is not smooth, what will happen to light rays incident upon it?
    Each tiny section of the surface still follows Snell's law
  8. When a light ray travels from air into glass, what can you say about the size of the angle of refraction compared to the angle of incidence?
    Glass is a more dense medium than air therefore the light refracts towards the normal
  9. What direction does the incident ray bend when travelling from glass to air?
    This is exactly the opposite to the case in the previous question because the light is travelling from a more dense medium into a less dense medium
  10. Which of the following can reflect light?
    Light can be reflected by most surfaces. The above are examples of a few which can be regularly observed in our daily lives. It does not just have to be a mirror which reflects light

Author: Martin Moore

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