pHYSICS

Experiment 6

Aim of the Experiment

To study the emission spectra of Hydrogen, Neon and mercury vapors. (Virtual Lab.)

Principle

When an atom or ion in substance got excited, they emit radiations of particular frequencies. These radiations are seen in the form of spectra. There are several types of spectra mainly,

• Continuous spectra-continuous band of colors is obtained.

• Absorption spectra-dark lines on a bright background.

• Emission spectra-bright lines on dark background.

In this experiment a diffraction grating optical spectrometer is used to study the spectra.

The diffraction grating provides the simplest and most accurate method for measuring wavelengths of light. It consists of a very large number of equidistant narrow parallel rectangular slit of equal width separated by equal opaque portion. The ruled widths are opaque to light and space between any two successive lines is transparent and act as parallel slits.Number of rulings per cm of grating used in visible region varies from 5,000 to 12,000 lines per mm.

The slits of a grating give rise to diffraction and the diffracted light interferes so as to set up interference patterns. Complete constructive interference occurs when the phase or path difference is equal to some whole number of the wavelength. In general the grating equation for constructive maxima is,

Sin θ = Nm λ

Where, m is called the order of the spectrum, λ the wavelength, N the number of lines per cm and θ the diffraction angle measured with respect to the direction of the light incident on the grating.

Procedure

For doing simulation

1. Move the slider in 'Calibrate Telescope’ and click ‘START’ button.

2. Click on Combo box to select lamp.

3. Click ‘Switch On Light’.

4. Vernier reading is set to 0⁰ and telescope at 90⁰ by clicking on sliders of both.

5. Click the ‘Place grating button’.

6. Turn the telescope to left. Coincide vertical cross wire with the green line on the pattern.

7. Note the readings of vernier 1 and vernier 2.

8. Telescope is moved to right side of direct image and vertical wire is made to coincide with green line of pattern.

9. Note the readings of vernier 1 and vernier 2.

10. Click on slider under Fine angle to get readings more precisely.

11. Difference between the two readings on the same vernier is taken.

12. Mean value of this difference gives 2Θ, twice the angle of diffraction. From this, the value of Θ is obtained for green line.

13. Assuming the wavelength of green line, 546nm, the no. of lines per mm is calculated using equation, N=SinΘ/mλ where m is the order.

Observations

Result

The wavelength of the prominent lines of the mercury spectrum are given in nanometre in the tabular column.

Number of grating per metre=................/m

Reference Material

1. University Physics, F. W. Sears, M. W. Zemansky and H. D. Young. 13/e, 1986, Addison-Wesley

2. PHeT Virtual Lab 1 for studying Hooke's law systems of springs (Click this Link)

3. PHeT Virtual Lab 2 for studying the motion of spring-mass systems (Click this Link)

Questions

1. What is emission spectra?

2. What is absorption spectra?

3. Why we get discrete lines in Emission spectra?