Transcript Intro Optics Presentationx - Workspace

Introduction to
Experiments in Optics
Short Tutorial on Optics (PowerPoint)
Safety & Good working practices
A. Lens Imaging (Ray Optics)
B. Single-slit diffraction (Wave Optics)
1st Yr Laboratory, Physics, Imperial College London
Damzen 09/07
Technological Revolution in Optics
Communication
by photons
METRO
WAN
TRANSOCEANIC
V
WAN
METRO
Telephony/data/internet
Massive optical
data storage
CD/DVD
Blu-ray disc (25GB)
Precision laser
machining
Laser writing on
human hair
Laser cutting
Photolithography for manufacture
of computer chips
Medical laser therapy
& optical imaging
Corrective laser eye surgery
3-D laser imaging of cell
Optics has an important place in history
Optics, light &
vision has been vital
for human survival
Telescope observations forged our
understanding of the Universe
Microscopes revealed a microuniverse
Today, Optics
remains a key
scientific diagnostic
technique (e.g.
imaging).
A new revolution in
Optics has emerged
with the birth of the
laser, fibre optics,
integration of optics
and electronics, etc..
Historical debate on nature of light
Particles
or
Waves
Light = EM waves
c
1
 0 0
What is Light? - Revisited
LASER
Wave-particle
duality
Quantum Optics
Lasers
Stimulated
emission
Paradoxes in physics (BB radiation/ photoelectric effect)
Quantisation of light (photons) E=hn
Bohr model of atom
Wavefunctions / Probability
Diffraction of electrons
Planck / Einstein
Michelson
Maiman (Laser)
…
Fundamentals of Optics
REFLECTION
Mirror
i
r
REFRACTION
IMAGING
Refractive index boundary
1
Imaging Lens
ho
n1
2
n2
F’
O
hI
F
s’
s
r=i
DIFFRACTION
Snell’s Law
f
1 1 1
 
s s f
n1sin1=n2sin2
INTERFERENCE
Aperture
m
Linear polarised
Continuum of waves
Elliptically polarised
b

 cos( ) 
E  E0 
 cos(t )
 sin( ) 
double-slits
E
E 0  cos(t ) 


2  cos(t   / 4) 
screen
Finite no. of waves
s
s
POLARISATION
a
beam spread
I
EM-theory
Safety and Lab-book Practices
Safety
•Laser Safety
NEVER LOOK DIRECTLY INTO THE LASER OR POINT LASER AT OTHER PERSONS
•Electrical Safety
•Trip Hazards
Your Laboratory Notebook

DATE, TIME, TITLE OF EXPERIMENT.

CLEAR WRITTEN RECORD AS YOU GO ALONG.

DESCRIBE & DRAW WHAT YOU ACTUALLY SEE.
Lenses –Ray Diagrams and Formulae
CONSTRUCTING RAY DIAGRAMS
F’
O
Thin lens formula
I
F
s’
s
LENS
CALCULATIONS
f
O object; I image
s object distance; s’ image distance; f focal length
PRINCIPLE RAYS: (Any 2 are sufficient to construct image)
•Ray passing through the centre of the lens is undeviated.
•Ray parallel to the optical axis passes through a focal point.
•Ray passing towards, or away from, a focal point emerges
parallel to the axis.
1 1 1
 
s s f
Magnification
formula
s
m
s
In later lab-work: you’ll explore issues of real lens (e.g. finite aperture; lens aberration)
Before we proceed to first experiment…..
•
Find a lab-partner & Sit at one of the Optical Set-ups
1. Open your lab-book and write date and time
2. Write heading “Introduction to Experiments in Optics”
3. Write sub-heading: “A. Thin Lens Imaging”
Aligning an Optical Bench
A good rule of optical alignment is to:
• place one item at a time on bench (starting at light source)
• ensure light propagates parallel to bench (rotate post of light source if necessary)
• optical components are centred (by adjusting post height) and
• optical components are at right-angles to beam path (by rotating post).
lens
object
image, observed on
ground-glass screen
Light
source
s
Optical rail
s’
Observe
from behind
ground-glass
screen
Expt 1.1 Imaging with a Lens
1. Switch on light source (supply
at ~ 5V preset, do not adjust)
2. As object place slide
of letter L, in slotholder on light source
Object, L
3. Place f=100mm lens at
object distance s=150mm.
Measure s with ruler from
object to lens centre
4. Adjust position of
ground-glass screen
for sharpest image.
Measure s’.
ground-glass screen
f=100mm
Observe
from behind
ground-glass
screen
Light
source
s~150mm
s’
Optical rail
hO
5. Measure a dimension of object (hO)
and corresponding size in image (hI).
hI
Deduce magnification |m|=hI/hO
• Estimate an error for all experimental values measured s, s’, hO, hI.
Errors?
4 measured
quantities
hO  hO   O
hI  hI  I
s  s s
s  s   s 
Experimental measurement
hI
| m |
hO
m  m m
Theoretical prediction
s
| m |
s
m  m m
x
equation of form : z 
y
x
z
y
 y
 z  z.   x   
2

x



y
Why is s’ >s?
How might you estimate s’?
Calculate the magnification (inc. standard error) for the two methods.
Do they agree / are they consistent given the errors?
2
Experiment 1.2 Measuring focal length of lens
2. Angle mirror so you can see reflected
spot of light on object slide.
(You may not be able to see this until
lens is near its focal length position)
1. Use pin-hole slide as object
f=100mm
mirror
Pin-hole object
Light
source
f
Figure 3: Simple method for estimating focal length of a positive lens.
3. Measure focal length f by
finding the position for minimum
reflected spot size
Is focal length f=100mm?
B. Wave-Optics : Single-slit Diffraction
secondary
wavelets
Aperture (width a)
causes light to
spread
(diffraction)
z
a
Light pattern at any
plane z is the sum of
secondary wavelets
of the unobstructed
aperture (including
phases)
z
Far-field (z>>a2/l)=
Fraunhofer diffraction
(simpler mathematical form
=Fourier Transform)
Near-field (z<a2/l) =
Fresnel diffraction
(complex mathematical form)
Far-field at focal plane of lens
Problem: Far-field z>> a2/lmay not be convenient for lab bench.
Solution: Use lens.
a)
diffracted rays at angle
 meet at infinity
input
light
~

parallel rays meet at a
point in focal plane
x
x


f
L
diffracting object
b)
very distant
observing
screen
Lens, f
observing
screen
Figure 4: The far-field diffracted pattern can be visualised
in the focal plane of a lens.
x

f
Expt.2 Visual Observation of Single-Slit
Diffraction Pattern
bi-convex lens
f=1000mm
observation screen
‘white card’
x
DIODE
LASER
f
Diffracting object = variable slit
1. Replace white light source by
Diode Laser
•
•
2. Visually observe diffraction
pattern of variable slit on white
screen placed at focal length of lens
Note in you lab-book the effect of changing the slit width.
With the central maximum peak of width ~10mm sketch the
diffraction pattern (to scale)
Far-field Single-Slit Diffraction Pattern
1.0
0.8
Intensity
x

f
0.6
0.4
0.2
0.0
-10
-5
0
Distance
 sin  laxf 
I( x )  I 0  ax 
  lf  
Is this what you see?
2
x
5
Positions of zeroes
xm = m(lf/a)
m  1,  2,  3 ....
10
Logarithmic Response of the Eye
1
1.0
Log Intensity
Intensity
0.8
0.6
0.1
0.4
0.01
0.2
-10
0.0
-10
-5
0
Distance
5
10
-5
0
Distance
5
10
Measurement with a Photo-detector
bi-convex lens
f=500mm
Photodiode/slit
assembly on
translation stage
1. Switch on
photodiode power
supply and set
voltmeter to 200mV
setting
LASER
f
diffracting object = single slit slide
I0
voltmeter
2. Position central maximum of diffraction
pattern to coincide with photodiode slit at
centre of its translation stage. You may
need to rotate laser and move
diffracting slit sideways to achieve this
I1
X-1
lf/a
X1
lf/a
Measurements:
1. Measure voltage of central maximum (V0) and first secondary maximum (V1).
2. Measure positions of the first minima (X1 and X-1). Hence calculate the slit width
using:
X1 – X-1 = 2lf/a
Final Comments
It is hoped that this introductory Optics session has given you:
• some useful practice in laboratory work (inc. lab notebook
and errors)
• provided some groundwork for more advanced Optics you
will perform in the lab later in the year.
• confidence in working in the UG laboratory