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Is there any rule governing the angle light is refracted through?

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Is there any rule governing the angle light is refracted through?

Aim:  To find a connection or rule governing that angle light is refracted through.

Introduction: Visible light is s form of energy that we can detect with our eyes. Light moves in waves.

The light that we can see has a wavelength of about 1/2000of a millimetre. The speed of light in air is 300 million metres per second. This is about a million times faster than the speed of sound.

Light travels more slowly in glass than it does in air. It goes about two-thirds as fast through glass than it does in air. This is the reason why light is refracted. Since the light is being slowed down by the optically more dense medium, the ray of light travelling from the air into the glass will bend towards the normal. As below:

A number called the refractive index measures how quickly light travels through a certain material. To work this out you would need this following equation:

  Refractive Index of a substance =  Speed of light in air

                                                          Speed of light in the substance

The refractive index of glass and Perspex is 1.5. The greater the number, the slower light passes through the substance. Here are some other refractive indexes for other materials:

SubstanceRefractive IndexSpeed of light

 Air                                        1.0                                 300,000,000

 Water                                1.33                                 225,000,000

 Perspex                                1.5                                 200,000,000

 Glass                                        1.5                                 200,000,000

 Diamond                                2.4                                 120,000,000

There is another way to find the refractive index of a substance. The formula for this is:

                        Refractive Index = Sine i

                                                Sine r

This rule is called Snell’s Law.

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Experiment Design:

      The equipment needed for this experiment is:

                                          1       Ray box + singleslit plate

                                          2       Power box

                                          3       Rectangular Prism (Perspex)

                                            4      Ready prepared sheets (see below)

Step one: Choose between 5 to 10 angles of incidence, making sure that they have a wide enough range to easily compare results. E.g. not 21, 22, 23, etc. Then draw this following diagram, putting each angle on it’s own page (preferably).

Angle i is the variable, you put the different angles of incidence at i.

Step Two: Plug the ray box into the power box putting red with red and black with black, then turn the electricity on. Set the power box on to 12 amps, or whatever brightness is required, just as long as the amps are kept the same throughout. Place the metal plate with the single slit pointing downwards into the ray box. As shown below:

Step Three: One at a time place the sheets with diagrams on in front of the ray box. Place the rectangular prism on the horizontal line, then draw another line beneath the prism, this will be where the ray of light exits the prism. Position the ray box/ sheet so that the ray of light travels down the centre of the angle i line, making sure it his the block at the normal.

Step Four: A ray of light will be refracted through the glass prism and will come out at the opposite side of where it was first directed. Draw crosses along the centre of the exiting ray of light.

Step Five:

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 Although some of my results aren’t as reliable as the rest I believe that I have enough evidence to support a conclusion, that the rule which governs the angle that light is refracted through is Snell’s law.

 To gain more accurate results all round I could repeat the whole experiment up to five times, which would give me a more reliable mean. Instead of using a ray of light, which spreads easily, I could use a laser, which is guaranteed not to spread. To measure the angle of r more accurately I could use a microscope, making the measurement more precise. If I carried the lines on I could use a larger protractor which would have accuracy up to 0.1 of a degree.

 To extend the experiment on further I could use different materials for my prisms, e.g. diamond or glass. So I could then see the differences in the angle of refraction, whether it is smaller or larger. I could change the shape of the prism, e.g. rounded or triangular, but that would also make it harder to measure the angles since the light would be dispersed in a different way. Could use different coloured lights, violet light and red light are at the opposite ends of the spectrum, the red light should have a larger angle of refraction since it travels faster than the violet light.

Bibliography:    ‘Physics for You’ by Keith Johnson, I obtained Snell’s law from this book and information on light and refraction.

      By Cheryl Gogin

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