#14 Spectra

Objective: 

The purpose of this experiment is to measure the wavelength of a few visible light and form a linear transformation of the derived wavelength, and use this to measure the spectra of hydrogen atom and an unknown to find out what the unknown is.

Procedures:

1.      Set up as instructed in the lab manual, put the light source as the white light.

2.      Record the position for each light seen through the diffraction grating. 

3.      Change the light source to hydrogen atoms and unknowns and perform same procedure.

Results:

       The acceptable visible light range is from 390 nm to 750 nm.

The wavelength is derived using formula:

L is 100 cm, and d is 1/500 mm.

For white light:

Color	Distance D (cm)	λ(nm)
Purple Begins	17.5	345
Purple -> Blue	22	430
Blue -> Green	24.5	478
Green -> Yellow	26	503
Yellow -> Red	28	539
Red	35	661
Red ends	40	743

           Using 345 and 743 to do the linear transformation, the equation is λ = 0.905λ'+78 in nm.

Then for the hydrogen atom:

Color	Distance D (cm)	λ(nm)
red	33.5	653
Yellow	28	566
Green	24	500
Purple	21	450

            

λ is derived from the early equation and linear transformation.

For the unknown, we derived following data:

Trial 1:
Color	Distance D (cm)	λ(nm)
Yellow	29	582
Green	27	550
Blue	21	450
Trial 2:
Color	Distance D (cm)	λ(nm)
Yellow	29.5	590
Green	28	566
Blue	21.5	458

            We suspect the unknown to be Mercury (Hg). The spectra for Mercury is:

          The spectra for Hg shows peak at 435, 545, and 580 nm, which is very close to the calculated data.

Discussion:

          The measurement is very inaccurate for the precise distance from diffraction grating to the light source is hard to measure, so does the displacement. The derived spectra for white light gives a fairly precise linear transformation to the system. The unknown is Hg as what professor told us, but the wacelength for blue light is comparably different from the theoretical one.