Dr Sushil Kumar
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LASER Wavelength Determination by Diffraction Grating Method
Last updated on Thursday, September 28th, 2023
Diffraction Grating
In this experiment, you determine the laser wavelength using a diffraction grating. To cover all the physical concepts associated with this experiment, I have created this quiz. You can attempt it and learn more about the experiment.
In the video, you can watch the demonstration of the experiment, including the practical lab session with the diffraction grating. The grating is a glass slab that contains a specific number of lines known as grating elements. You might have noticed the text on the glass slab, which reads ‘15000 LPI.’ This stands for ‘15000 lines per inch.
Find LASER Wavelength by a Diffraction Grating
You may know that one inch is equal to 2.54 cm. So, if you want to determine the grating element, you can calculate it mathematically. This can be done during a diffraction grating practical lab using any laser.
During this lab, you will not only learn about the applications of a diffraction grating but also gain insights into plane diffraction gratings. In the experiment, I’ve used the formula nλ = 2d sin θ, although in general, we don’t include the ‘2’ in the diffraction grating formula.
In this diffraction grating experiment, you will observe a pattern of diffracted light. If you’re using white light, you’ll see it split into seven colors. If it’s monochromatic light, you’ll observe only dark and bright regions. For more detailed information, you can refer to this video.
Diffraction grating definition:
A grating is a finely crafted glass slab upon which you can observe multiple slits. Each of these slits is referred to as a ‘grating element,’ and its width is also known as the ‘grating element.’
In the quiz provided below, you’ll find various viva questions related to the diffraction grating experiment. These questions will aid in your understanding of this phenomenon and its practical applications. If your question is ‘What is a grating element?’ it should now be clear to you.
1Q.
1Q. What is full form of the LASER?
- Light Amplification of Spontaneous Emission by Radiation
- Light Amplification of Simple Emitted Radiation
- Light Amplification by Stimulated Emission of Radiation
- Light After Source that is Emitted by Radiation
2Q. What is Interference?
- It is the interaction between two or more waves of the same or close frequencies emitted by the Coherent sources.
- It is an optical phenomenon, that can be seen everywhere.
- This occur when light distribute in random directions
- Interference when grating is placed in the path of laser light and light collect at a certain point.
Interference is the phenomenon based on the principle of superposition. Where intensity distribute of two or more than two waves in a defined manner. For the interference the two sources should emit waves in phase and light of same frequency.
3Q. What is diffraction grating?
- A rectangular type plate of glass on which ruled very close and parallel lines. When monochromatic light falls on it light get diffracted which provides a definite pattern of light.
- A mirror that can diffract the laser light
- A device which is helpful to take the readings in this experiment.
- An optical instrument or device which have several parallel lines.
In this experiment grating is a glass plate ruled with very fine, close and parallel lines. Each line is said to be slit. The number of lines may vary in one inch dimension of the grating. For an example 300 LPI (line per inches), 15000LPI, etc.
4Q. Step to calculate the percentage error:
- Determine the difference between the observed value and the standard value
- divided by the standard value
- now multiply by the 100 to change in percentage
- use label of % in result.
- Correct
- Wrong
5. What is grating Element?
- Grating element is the sum of slit width and opaque spacing
- The distance between two consecutive slits
- This is element by which grating is made
- Grating element is a device that we use in experiment for the determination of wavelength.
6. What is the condition for constructive interference?
- Grating should be clean and straight
- Path difference should be integral multiple of lembda (wavelength)
path difference = n x lembda - Grating should have at least two slits
- Light Source should be a Laser
In order to obtain constructive interference the two rays should arrive at a point in same phase. Also the path difference should be integral multiple of the wavelength.
7. What are the Laser light characteristics ?
- Highly directional, low angle divergence, monochromatic, coherent, highly intense,
- For DJ nights, decorations, and experiments
- Spontaneous and stimulated emission, population inversion
- two energy levels, excited state life time 10^{-8} sec, remain focus
LASER characteristics are those which remain same for different lasers, whether that is Ruby Laser, Semiconductor laser or He-Ne Laser, etc. In all cases emitted light radiation will be highly direction. That you call uni directional, highly collimated beam, monochromatic, etc. as mention in option one.
8Q. How to find the grating element of a given diffraction grating having 1500 Lines Per Inch (LPI)?
- This is aim of the experiment
- by using the formula n lambda= d sin theta
- In one inch available lines are 15000 it means number of slits available. So one slit i.e. grating element will be=1 inch/15000
- This is standard value which we can determine by the formula
For a given diffraction grating, first you see what text is mentioned on that. This text tells about the number of slits in this glass plate. For an example in this case 15000 lines per inch given. So from the result we will get standard value of grating element that is single slit width d. Use it for percentage error.
1 inch= 2.54 cm
given
15000 lines=2.54 cm
so this is the standard result for grating element.
By this way you can determine the grating element for a given grating of like 300 LPI, or 500 LPI.
9Q. What is population Inversion?
- Large number of atoms in ground state as compared to the excited state
- Large number of atoms in the higher energy state as compared to the lower energy state
- When metastable state have large enough time to hold the atoms the ground state populated more by atoms, this is called population inversion
- In Ruby Laser we observe the population inversion, where meta stable state get more populated as compared to the excited state
For the LASER action population inversion is must. In an atom, the electrons absorbs the energy and goes in higher states and after spending the life time come back in its original state. This is transition of the electron from one state to the other. Because this reason electronic configuration changes of the atoms. The change in electronic configuration is termed as excited state of the atom correspond to the absorbed energy.
The distribution of the atoms in any state can be defined by the Maxwell-Boltzmann’s distribution law.
Like this one can say that N1 are the number of atoms are correspond to the E1 state. N0 are the total number of atoms.
When atoms stay for longer duration in higher energy state, the amount of atoms in ground state decreases, known as population inversion. This result is opposite to the general case for atoms, where more number of atoms in ground state and few atoms in the excited state.
10Q. Does diffracted Waves can interfere?
- Yes
- No
- Never
- May be
11Q. What is the standard value of Red Color wavelength in He-Ne gas LASER?
- 6328 x10^{-10} meter
- 4526 x10^{-10} meter
- 3598 x10^{-10} meter
- 5864 x10^{-10} meter
12Q. What is spontaneous and stimulated emission process?
- In spontaneous emission, atom remain in excited state for the definite time period and after that jumps in lower energy state without any induction
- Spontaneous emission process is that process in which atom jumps from one state to the other along with emission and absorption of the light
- During the period of atomic excitation (in higher energy state of atom) one spontaneously emitted photon or incident photon interact electromagnetically with it (induce excite atom). As a result a photon emits with same nature as of the incident one. This is stimulated emission.
- When atom is in excited state an incident photon interact with it and as a result it get induce. This induced atom emits two photon of same frequency.
In LASER there are two process one is of absorption and the other of emission. Further emission of photon may be either through spontaneously or by stimulated process. For the LASER light stimulate emission is required. In stimulated emission one photon emit from the excited atom and second photon is just the incident one.
13. Why do you use n lambda= d sin theta formula? What is the origin to derive this formula for this experiment?
- Constructive interference condition
- Destructive Interference condition
- Farting element condition
- None of these
This equation is condition for constructive interference i.e. maxima.
n is the order of maxima, n=1 first order; n=2 second order, etc.
λ is the wavelength of laser light
d is spacing between slit
is the angle for 1st, 2nd or 3rd ordered patterns. Which you determine by the geometry.
R.H.S. is the path difference between two waves which interfere at the screen which is D distance away from the grating.
L.H.S. is the condition for maxima i.e. constructive interference.
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Good questions.. looking forward for more on different topics and experiments as per the present bsc. Course