The Mercury Spectrum

This experiment will determine the wavelengths of coloured light using high energy mercury in a discharge tube.  Using a 400 lines per millimetre diffraction grating and an optical spectrometer, the angle at which certain light colours appear in their fringes can be established.

Types of spectrum

There are two basic types of spectra: emission spectra and absorption spectra.  Emission spectra are where light is given out by a source, and absorption spectra are where light from a source is absorbed when it pass though another material, usually a liquid.

Both emission spectra and absorption spectra can be further classified as being line spectra, band spectra or continuous spectra.

The mercury spectrum used the experiment is a line spectra.  Line spectrum can be produces only by low-density monatomic gases and vapours.  Sources which give rise to line spectra include low-pressure discharge tubes, flames, gaseous nebulae and the tails of comets.

Colour

Diffraction order (n)

d sin θ (mm)

Wavelength (mm)

9.329

purple

1

 4.053x10-4

 4.053x10-4

18.834

purple

2

8.071x10-4

 4.035x10-4

29.067

purple

3

12.146x10-4

 4.049x10-4

40.325

purple

4

16.344 x10-4

 4.045x10-4

53.825

purple

5

20.180 x10-4

 4.036x10-4

76.250

purple

6

24.284 x10-4

 4.047x10-4

10.000

blue

1

 4.34 x10-4

 4.341x10-4

20.434

blue

2

8.728 x10-4

 4.364x10-4

31.467

blue

3

13.050 x10-4

 4.350x10-4

44.217

blue

4

17.434 x10-4

 4.359x10

43.900

yellow

3

17.339 x10-4

 5.778x10-4

67.517

yellow

4

23.02 x10-4

 5.775x10-4

I calculated the wavelength using the equation λ= d sin θ / n and I also plotted a graph on d sinθ against n, to find out the wavelength, I need to find out the gradient of each color using the formula  y2-y1 / x2-x1

Look up the accurate wavelength

Now I got the wavelength of each colour in both graphical way and mathematical way, I looked up the accurate wavelength books, and compare them to my table and graphs.

The wavelength I calculated out is very close to the accurate wavelength from books, which means, I used the right method and equation to find out the wavelength, the possible errors would be the data may not be very accurate, the angle are all bigger on one site than another, this may suggest that it was not centre.