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Investigate how the speed of light differs in air and in Perspex.

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Introduction

Gcse Physics coursework: Refractive Index

Planning

Aim: I am going to investigate how the speed of light differs in air and in Perspex.

Background info: The refractive index is a ratio for working out the speed of light. The ratio varies for different substances, it indicates the extent to how light refracts through different substances. On passing from a less dense medium to a more dense medium, light is refracted towards the normal, and thus the angle of incidence, i, is larger than the angle of refraction, r., Willebrord van Roijen Snell (1591-1626), came up with a law explaining the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for any given pair of medium.  so a simple statement of his law is:

sin i / sin r  =  a constant, n

So in this equation the constant n is equal to the refractive index.

N defines the speed of light in Perspex as a decimal of the speed of light in air

Example of refractive index values are: water (1.33); perspex (1.49); window glass (1.51); different glasses (between 1.46 and 1.69); and diamond (2.42). Diamond has a very high refractive index this is responsible for it having such a aparkle.

Apparatus

Lab pack

D-block

Ray Box

Glass lens

Collimator

Prediction

...read more.

Middle

10.000

15.500

17.000

0.174

0.267

0.293

0.280

0.620

15.000

24.500

24.500

0.259

0.415

0.415

0.415

0.624

20.000

32.500

32.500

0.342

0.537

0.537

0.537

0.637

25.000

39.500

40.000

0.423

0.636

0.643

0.639

0.661

30.000

49.000

50.000

0.500

0.755

0.767

0.761

0.657

35.000

60.500

61.000

0.574

0.870

0.871

0.870

0.659

40.000

74.500

76.000

0.642

0.964

0.970

0.967

0.664

image10.png

 Sine I      image05.pngimage06.png

Sine r

I now have seven readings for N and

...read more.

Conclusion

If I were to repeat the experiment again improvements that I could make may be to have used a sharper pencil when drawing on the light rays and kept sharpening it after I had drawn each ray to get rays of consistent thickness. As well as this I would use A collimator with a smaller slit to get a sparser more accurate ray and finally I would secure the d-block down some how.

Another experiment I could do is measure the speed of light in water.

Measuring The Speed Of Light in Glass

Prediction

 I predict that the speed of light in water multiplied by the refractive index of water (1.33) will be equal to the speed of light in air.

Firstly I would fill a fish tank half full of water; next I would place a long metal rod into the fish tank at an angle near the side of the tank and get someone to draw the refracted image on the side onto the fish tank, I would then place the rod in the tank at different angles and note down the angles of incidence and refraction, and work out the speed at which light travels in exactly the same way as the Perspex experiment.  

image12.png

...read more.

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