cách tử quang

Team:
Dinh Cong Huy
Nguyen Thai Long
Bui Viet Hung
Nguyen Duc Nam
HUT- 17/10/2008
Gratings and application
Mechatronics Engineering- Advanced Training Program
•
Grating: a system of multi close equidistant and
parallel lines or bars, especially lines ruled on a
polished surface, used for producing spectra by
diffraction called also diffraction grating.
•
Diffraction grating: is an optical device used to learn
the different wavelength or color contained in a
beam of light. The device usually consist of thousand
of narrow, closely spaced parallel slits (or grooves).
Figure 1:
- Diffraction grating: is an
optical device used to learn
the different wavelength or
color contained in a beam of light.
The device usually consist of thousands of
narrow, closely spaced parallel slits (or grooves).
- Used for producing spectra.(Fig 1)
I- INTRODUCTION
Fig 2: The first
diffraction
grating was
made by

Fraunhofer
(which is called
Fraunhofer
diffraction).
II- Background of topic

1. Single – slit diffraction.

2. Double- slits diffraction.

3. Multi- slits diffraction.

4. Diffraction gratings.

5. Quiz.
II- Background
The smaller the slit, the broader the
entire the diffraction pattern.
1. Single slit diracon:
Phenomena: the beam spreads out
vercally aer passing through the slit.
Denote:
a- the width of the slit
d- the distance between slit and
screen
θ - the angle between the
perpendicular to the slit a line
from the center of the slit to a
point P on the screen
Condition for dark fringe:

sin m
a
λ
θ
=
m= 1,2,3…
m
m
y d
a
λ
=
2
sina
π
β θ
λ
=
( )
( )
2
2
0 0
sin sin /
sin( / 2)
/ 2 sin /
a
I I I
a
π θ λ

β
β π θ λ
 
 
 
 
 
= =
 
 
 
 
 
Posion of the mth dark fringe:
The phase di'erence
between the phase of
light of the upper and
lower trip:
The intensity :
2.Double slits diffraction:
Diffraction
envelope
Inference fringe
- must consider not only diffraction due to the individual slits but also the
interference of wave coming form different source.
-intensity:
2
2
0
sin( / 2)

cos
2 / 2
I I
φ β
β
 
=
 
 
2
sin
d
π
φ θ
λ
=
Where: is the phase
di'erence at point P of 2
incident lights, and is
calculated by the
formula:
φ
2
sina
π
β θ
λ
=
And : is the phase
di'erence of wave at P

from the top trip with
respect to the wave
received from the
bo*om trip:
β
3.Multiple slits diffraction:
-
Constructive interference occurs for rays
at angle θ satisfy: d.sinθ=mλ
(m= 0,±1,±2,±3…)
- The maxima are the same position as
for two slits diffraction with the same
spacing d.
Q: Why occurs such this
coincidence ?
Interference patterns for N equally spaced very narrow slits.
 The greater the value of N, the narrower the principle maxima
become and the smaller the intensity of secondary maxima is.
4.Diffraction grating:
-
Thousands of very narrow and close slits.
-
The phenomena seen on the screen is similar to multi slits
diffraction.
-
Positions of the maxima are given by: d.sinθ= mλ
(m = 0,±1,±2, )
m=±1: first order lines
m=±2: second order lines
…

5. Quiz:

1. The pattern of double diffraction.

2. The pattern of interference.
Q:What is the difference?
A: No fundamental distinsion
III- APPLICATION OF DIFFRACTION

For instrument analytical chemistry.

In laser system

In astronomical application

In synchrotron radiation beam lines

Some special uses for grating : gratings as filters, gratings in fiber-
optic telecommunication, gratings as beam splitters, gratings as
optical couplers….

Resolving power, scattering and stray.
1. Resolving power:

Disperse a spectrum into its
wavelength components.

2 wavelengths:
The resolving power:
Where:

And:
1 2
λ λ
≈
1 2
2
λ λ
λ
+
=
2 1
λ λ λ
∆ = −
N: the number slits of Diffraction
grating, m-th order diffraction.
R= N.m
R =
2. Spectrograph
Structure of a slit spectrograph
Sunlight spectrum

The important of this technique is to
astronomy.For example,to measure
sunlight spectrum.

With sunlight, people obtain
spectrum produced by a diffraction
grating has dark absorption lines.

The resolution of grating

spectrograph:

The greater the number of slits
N,the better the resolution;also,the
higher the order m of the diffration-
pattern maximum tthat we use,the
better the resolution.
.R N m
λ
λ
= =
∆
3. Spectrometer:
Explanation about spectometer:

The light to be analyzed passes through a slit, and a collimated beam
of light is incident on the grating.

The diffracted light leaves the grating at angles that satisfy
Equation:

a telescope is used to view the image of the slit.

The wavelength can be determined by measuring the precise angles
at which the images of the slit appear for the various orders
sind m
θ λ
=
4. Some pictures in real life:


Rain-bow colors reflection and diffraction on the surface of
CD/DVD.

Fresnel mirror.
Dispersion through a prism.
Data soures:
1. University Physics with mordern physics 12
th

edition- Young
2.Fundamentals of Physics 7
th
edtion– Halliday,
Resnick, Walker.
3.
4.
Thanks for your attention!!!
Have a nice day!