How to find the width of a single slit by He-Ne Laser

May 1 • General • 5657 Views • 2 Comments on How to find the width of a single slit by He-Ne Laser

A He-Ne Laser is a type of gas Laser whose gain medium consists of a mixture of helium and neon inside of a small bore capillary tube , usually excited by a DC electric discharge. To find the width of a single slit by He-Ne Laser is done by using the concept of Fraunhofer single slit Diffraction mechanism.The diffraction pattern at the right is taken with a  He-Ne Laser and a narrow single slit.

To find the width of a single slit by He-Ne Laser  is by visualizing the nature of the single slit diffraction. The phenomenon of diffraction involves the spreading out of waves past openings which are on the order of the wavelength of the wave. The spreading of the waves into the area of the geometrical shadow can be modeled by considering small elements of the wavefront in the slit and treating them like point sources.

Fig: the single slit diffraction mechanism :

The diffraction pattern were taken with Helium Neon Laser and a Laser slit.The slit widths used were on the order of 100 micrometers, so their widths were 100 times the laser wavelength or more. A slit width equal to the wavelength of the laser light would spread the first minimum out to 90° so that no minima would be observed. The relationships between slit width and the minima and maxima of diffraction can be explored in the single slit calculation.

To find the width of a single slit by He-Ne Laser is by using :

In this we will use the diffraction scheme which can be given as

tan θ=y/D

tan  θ = sin  θ~ y/d

Condition for minimum is

a sin θ=mλ

y~ mλd / a

Which shows :

Displacement y = ( Order m *Distance d) / Slit width a

The gain medium of the Laser is a mixture of  helium and neon gases, in approximately a 10:1 ratio, contained at low pressure in a glass envelope. The gas mixture is mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines.

A neon laser with no helium can be constructed but it is much more difficult without this means of energy coupling. Therefore, a HeNe laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.[5]The energy or pump source of the laser is provided by a high voltage electical discharge passed through the gas between electrodes (anode and cathode) within the tube. A DC current of 3 to 20 mA is typically required for operation.

The optical activity  of the laser usually consists of two concave mirrors or one plane and one concave mirror, one having very high (typically 99.9%) reflectance and the output coupler  mirror allowing approximately 1% transmission.


Q 1)A He-Ne laser (λ = 633 nm) shines through a single slit of unknown width D onto a screen 1.50 m away from the slit. The distance on the screen between the m=1 minima on either side of the central maximum of the diffraction pattern is measured and is found to be 2.3 cm. What is the width D of the slit?

Ans 1) he distance on the screen between the m=1 minima on either side of the central maximum of the diffraction pattern is WIDTH OF CENTRAL MAXIMA=w

w=2λS/D =2.3 cm=0.023 m
where S is screen distance and D is width of slit

S=1.5 m


D=2* 6.33*[10^-9]*1.5 / 0.023 =8.25*10^-7 m or 0.825 micrometer

width D of slit is 8.25*10^-7 m or 0.825 micrometer.

Q 2) A helium–neon laser (λ = 630 nm) is used in a single-slit experiment with a screen 3.6 m away from the slit. If the slit is 0.10 mm wide, what is the width of the central bright fringe on the screen? Measure this width using the locations where there is destructive interference. in m

Ans 2 ) α’ ≈ α
tan α= y/D ≈ sin α’
sinα’ = λm/d (where d is the width of the slit)
y ≈ λmD/d
y ≈ 630*10^-9*1*3.6/0.0001=0.02268 m
So, the width of the central bright fringe
is about 2*y


Ans 3)

the resolving power of your eyes to see how close your vision is to “perfect”, that is, diffraction-limited performance.

First, measure the diameter D of the pupil of your eye (under normal room-light conditions). With one eye open, look closely at the image of your pupil in a mirror and measure your pupil’s diameter with a clear plastic ruler placed on the mirror or over your eye.

Next, measure the angular resolution of your eye with the following procedure: Your partner will stand on a “zero position” mark on the floor some distance away and will hold up an eye-test chart for you to view with both eyes open. The test chart consists of an array of vertical bars and an array of horizontal bars, as shown below. Beyond a certain distance, the human eye cannot resolve the bars and the arrays appear to be gray blotches rather than black stripes on a white background.


Ans 4 ) Red HeNe laser have many industrial and scientific uses. They are widely used in laboratory demonstrations in the field of optics in view of their relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence  and long coherence  (however since about 1990 semiconductor lasers have offered a lower cost alternative for many such applications). A consumer application of the red HeNe laser is the  player, made by Pioneer. The laser is used in the device to read the optical disk.

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2 Responses to How to find the width of a single slit by He-Ne Laser

  1. Ritika Savita says:

    LASERs form an important part of the curriculum of Electrical & Electronics & Communication Branch of engineerng. Different kinds of LASERs are taught along with their characteristics. Here, we are elaborating on Single-Slit LASER>

  2. shweta singh says:

    This article of the He-Neon laser is quite beneficial…with all the related questions and answers the article can easily be understood with the related concepts of it

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