Comparing Spectra of Tungsten and Energy-Saving Light Bulbs

Purpose: In this lab we plan to observe the spectra of both an energy-saving bulb and a tungsten filament bulb. We hope to compare these spectra, and from our observations, derive a sense of what makes each type of bulb unique. We also plan to calculate the wavelength of red, green and blue light using a diffraction grating.

Materials:

Shoe Box

Lamp

Tungsten-filament light bulb

Mercury vapor, energy saving light bulb

300 or 1000 diffraction grating

Scissors

Modeling Clay

String

Sewing pin and drawing pin

Electrical tape

Protractor

Procedure:

1. Firstly, we located a suitably dark location to conduct our experiment in. In our case, this ended up being a bath room.

2. Next, we prepared the shoe box for use. This was done by cutting off one end, and cutting a 1mm wide slit 5 cm long on the other side. This box was then taped to a table and covered with a light, non-synthetic towel to minimize light pollution.

3. Next, the protractor was placed 55 cm away from the shoe box, with its zero degree point aligned with the slit of the box. A drawing pin was inserting through its apex, with its sharp point exposed.

4. The diffraction plating was placed parallel to the flat edge of the protractor, and secured upright with modeling clay. Its center was aligned with the slit of the shoe box, and the tip of the drawing pin.

5. A black string was attached to the drawing pin, and at its other end a piece of modeling clay holding a sewing pin was attached. This served as our sighting mechanism.

6. A lamp with a energy-savings bulb was then inserted into the shoe box and turned on. Once observations and recordings were taken, the tungsten-filament bulb was inserted to compare qualitatively the differences between the two bulb type’s spectra.

Observations:

(Observations and Results based on data gathered from 300 diffraction grating)

We observed the energy-savings light bulb to produce four orders of diffraction, with a dimly visible 5^th order also noted. Interestingly enough, the color blue was not observed in the 3^rd or 4^th orders. Green was also not observed in the 4^th order.

The energy-saving light bulb also had very fine lines in between each color (spectral lines). These lines are produced because the light bulb only gives off specific wavelengths, rather than a whole mix across the range. It tells us that the energy-saving light bulb is energy-saving due to the fact that it only produces particular wavelengths of light.

This is further confirmed by our observations of the orders produced by the tungsten-filament bulb. This bulb only produced 2 full orders, with a third partial order (observable up to blue) also present. More importantly, the orders were blurred together, and no spectral lines were present. It is due to this that these bulbs consume more energy than energy-saving mercury bulbs, as they must expend the energy to produce the wavelengths observed where spectral lines were produced by the energy-saving bulb.

Data:

Order, n	θn/degrees	Uncertainty in θn/degrees	Sin θn	Uncertainty in sin θn
Blue Spectral Line
1	7.5	0.5	0.131	0.008727
2	15.8	0.5	0.272	0.008727
3	N/A	N/A	0	N/A
4	N/A	N/A	0	N/A
5	50	0.5	0.766	0.008727
Green Spectral Line
1	9.1	0.5	0.158	0.008727
2	19.6	0.5	0.335	0.008727
3	28.9	0.5	0.483	0.008727
4	N/A	N/A	0	N/A
5	83.6	0.5	0.804	0.008727
Red Spectra Line
1	11.9	0.5	0.206	0.008727
2	22.1	0.5	0.376	0.008727
3	31.3	0.5	0.52	0.008727
4	46.7	0.5	0.728	0.008727
5	68.3	0.5	0.909	0.008727

Wavelengths Calculated per instructions:

Blue: 465 nm

Green: 589 nm

Red: 692 nm

Note: Wavelengths were calculated by using gradients between two points on the graph (Pts 1 and 2 for Blue, 1 and 2 for Green, 3 and 4 for Red). This was done due to the lack of some colors from the orders.

Discussion/Conclusion:

Great uncertainties arose as we took measurements for higher orders, due to the degradation of clarity that occurs as higher orders are assayed. Measurements could be improved by using a larger diffraction plate so that 4^th orders are clearly viewable, and appear at less tangential angles. Multiple measurements are taken to assure the accuracy of the data. In our particular run of the experiment, both observers took separate observations, and averaged the two together.

It is useful in many cases to plot the data so that we can draw from it a general trend. However, given that we did not observe certain colors in numerous orders, the utility of graphing the data is diminished.

Our calculated wavelengths were upwardly shifted by roughly 30-50 nm. This may have been due to the light bulb or conditions we were operating in. Additionally, there was a crack in our diffraction plate that may have slightly jaded our results.