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Wavelength of red light

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Introduction

Quality of Measurement Coursework

Wavelength of red light

The Aim

The target is to take measurements to calculate the wavelength of red laser light by using the diffraction grating formula. Therefore I will use a variety of diffraction gratings. To improve accuracy I will always do the experiment with and in absence of two lenses. With this step I hope to get closer to the real wavelength.

The Set-up

Equipment list:

  1. red laser
  2. two metre rulers
  3. wall or projector screen
  4. double slit
  5. slit holder
  6. variety of diffraction gratings
  7. diverging lens
  8. converging lens
  9. two lens holders
  10. graph paper
  11. cello tape, blue tack
  12. marker pens

image00.png

The light from the laser passes through the diverging lens and splits up. Afterwards the converging lens concentrates the light. This process gives a more focused and smaller dot on the wall which leads to higher accuracy. The grating causes the concentrated light to break up again.

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Middle

To use the formula for the diffraction grating we need to find the spacing between each gap in the grating.

image10.png

Then we use the diffraction grating formula.

image11.png

Where d is the distance between each slit, θ the angle calculated above, n the order of the maxima and λ the wavelength to be calculated. We rearrange the formula:

image12.png

Example

Without lenses

Diffraction grating:                                                         80 lines per mm

Order of the maxima:                                                        1

Distance between central maximum and 1st order spectrum:        10.4cm

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Conclusion

The grating was not always aligned at right angles to the laser beam.

Interpretation

I proved that the method using lenses increases accuracy. So for my final wavelength I will use the results with lenses. I also showed that increasing the lines per mm on a diffraction grating gets closer to the actual wavelength. So I can use a weighted average:

image07.png

If we have a look on the visible spectrum of the electromagnetic spectrum we can see that 634nm is in the red section.

image08.jpg

In total I can say that there is a very small uncertainty included in my result. As stated in the uncertainty section, the uncertainty is always under 1%. There are very few errors which are exercisable, too.

I am satisfied with this experiment, because increasing the lines per mm would not give me a significant change in the wavelength. All in all I am very close to the actual wavelength.

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Here's what a teacher thought of this essay

3 star(s)

The method used gives a good value for the wavelength of laser light. Greater attention to the use of measurement techniques to reduce uncertainty would improve the grade. Also the final value should be compared to the stated value using a percentage deference calculation. 3 Stars

Marked by teacher Pete Golton 06/06/2013

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