Using Two Kind of Spectrometer to Identify the Atomic Spectra of Different Atoms: An Experiment

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The aim of this experiment is to use two kind of spectrometer to identify the atomic spectra of different atoms. We learned to use the calibration curve obtained from a known spectrum or measure the angle of diffraction to find out the wavelengths of unknown spectral lines.


Emission Spectrum
As we know, every atom has a set of discrete energy levels occupied by its electrons. A photon is emitted when an electron makes a transition from a higher energy level to a lower energy level. The wavelength λ of the photon is related to the change in energy ΔE of the electron by ∆E=hcλ

Because there are many possible transitions between the energy levels in a given atom, photons of different frequencies will be produced. An atom’s emission spectrum is the set of all these photon frequencies. Since different elements have different atomic structures and different atomic energy levels, the spectrum will be different for each element. In this way, an emission spectrum can be used to identify an element.


Figure 1

A spectrometer is an instrument for analyzing the spectra of radiations. In its simplest form, a spectrometer consists of three basic components: a collimator, a refracting or diffracting element to separate light into its various components, and a telescope. By using the spectrometer, each constituent color of the atomic spectrum can be viewed and the angle can be measured. These angles can be used to determine the wavelengths that are present in the light. For more precise work and recording the relative intensity of each color of light, we use a diffraction grating with a spectrophotometer system with data logging capabilities. By interfacing the light sensor and a rotary motion sensor with the computer, relative light intensities across a whole range of angles can be recorded. Caution that all the recording of the spectra has to be done in the dark.

Diffraction grating
A diffraction grating consists of many equally spaced, parallel lines. Light rays diffracted from adjacent lines can interfere constructively to form an image of the light source. In this experiment, we use the first-order diffraction grating to let rays from adjacent lines differ in path length by one wavelength of the light. By using a grating spectrometer system with data-logging capabilities, we can record the relative light intensities across a whole range of angles. As shown in Figure 1 on Page 2, the relationship between the wavelengths of light λ, the diffraction line spacing d, and diffraction angle θ, is: λ=d sinθ

Using the angles recorded and the equation, we can calculate the wavelengths of light rays of different intensities.

Figure 2

Experimental procedure

Part 1: Obtaining a Calibration Curve for the Glass Prism Spectrometer 1. Turned on the mercury and sodium lamp once reached the lab to let it warm up. 2. Set up the glass prism spectrometer as Figure 3. To level the spectrometer table, adjusted the three thumbscrews.

Figure 3

3. In order to focus the telescope and collimator, brought the cross-hairs into sharp focus by moving the eyepiece, and made sure that one of the cross-hairs is vertical. Afterwards, focused the telescope on a distant object and turned it directly opposite the collimator. Finally, adjusted the focus knob on the collimator and bring the slit into sharp focus. 4. Placed the glass prism onto the spectrometer table and the mercury light source several centimeters behind the slit. 5. Darken the surroundings using curtains and the opaque cloth, then looked through the telescope to find the mercury line spectrum. 6. Found the minimum angle of refraction by rotating the spectrometer table to a point where the spectral line changed its moving direction. Used the fine adjust knobs to make sure that the vertical cross-hair precisely aligned with the fixed edge of the violet part of the slit image. Then read the angle on the vernier scale to be D2. 7. Removed the...
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