Introduction to Experiments in Optics
Damzen 09/07
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
METRO WAN
METRO WAN
TRANS-OCEANIC
V
Technological Revolution in Optics
Communication by photons
Massive optical data storage
Precision laser machining
Medical laser therapy & optical imaging
Blu-ray disc (25GB)CD/DVD
Telephony/data/internet
Laser cutting
Laser writing on human hair Photolithography for manufacture
of computer chips
Corrective laser eye surgery 3-D laser imaging of cell
Optics has an important place in history
Telescope observations forged our understanding of the Universe
Microscopes revealed a micro-universe
Optics, light & vision has been vital for human survival
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..
Particles Waves
Light = EM waves
Historical debate on nature of light
00
1c
or
Wave-particle duality
Quantum OpticsLasers
Planck / EinsteinMichelson Maiman (Laser)…
What is Light? - Revisited
LASER
Paradoxes in physics (BB radiation/ photoelectric effect)Quantisation of light (photons) E=hBohr model of atomWavefunctions / ProbabilityDiffraction of electrons
Stimulated emission
i
r
Mirror
r=
i
1
2
Refractive index boundary
Snell’s Law
n1sin1=n2sin2
n1 n2
Aperture
beam spread
double-slits screen
REFLECTION REFRACTION
DIFFRACTION INTERFERENCE
Fundamentals of Optics
Continuum of waves Finite no. of waves
s s’
O
I
f
F
F’ ho
hI
Imaging Lens
f
1
s
1
s
1
s
sm
IMAGING
)cos()sin(
)cos(0 tEE
)4/cos(
)cos(
20
t
tEE
a b
L i n e a r p o l a r i s e d E l l i p t i c a l l y p o l a r i s e d
POLARISATION
EM-theory
Safety
•Laser SafetyNEVER LOOK DIRECTLY INTO THE LASER OR POINT LASER AT OTHER PERSONS
•Electrical Safety
•Trip Hazards
Safety and Lab-book Practices
Your Laboratory Notebook
DATE, TIME, TITLE OF EXPERIMENT.
CLEAR WRITTEN RECORD AS YOU GO ALONG.
DESCRIBE & DRAW WHAT YOU ACTUALLY SEE.
s s’
O
I
f
F
F’
fss
111
s
sm
Thin lens formula
Magnification formula
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.
Lenses –Ray Diagrams and Formulae
CONSTRUCTING RAY DIAGRAMS
O object; I images object distance; s’ image distance; f focal length
LENS CALCULATIONS
In later lab-work: you’ll explore issues of real lens (e.g. finite aperture; lens aberration)
• 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”
Before we proceed to first experiment…..
Aligning an Optical Bench
s
object
Light source
s’
image, observed on ground-glass screen lens
Observe from behind ground-glass screen
Optical rail
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).
s~150mm
Object, L
Light source
s’
ground-glass screen f=100mm
Observe from behind ground-glass screen
Optical rail
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 slot-holder on light source
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’.
5. Measure a dimension of object (hO)
and corresponding size in image (hI).
Deduce magnification |m|=hI/hO
• Estimate an error for all experimental values measured s, s’, hO, hI.
hO hI
Errors?
s
s
III
OOO
ss
ss
hh
hh
s
s|m|
O
I
h
h|m|
mmm
Theoretical prediction
mmm
Experimental measurement
2
y2
xz yx
.z
y
xz
y
xz :form ofequation
Calculate the magnification (inc. standard error) for the two methods.
Do they agree / are they consistent given the errors?
Why is s’ >s?
How might you estimate s’?
4 measured quantities
Experiment 1.2 Measuring focal length of lens
f
Pin-hole object
Light source
mirror f=100mm
Figure 3: Simple method for estimating focal length of a positive lens.
1. Use pin-hole slide as object
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)
3. Measure focal length f by finding the position for minimum
reflected spot sizeIs focal length f=100mm?
B. Wave-Optics : Single-slit Diffraction
Aperture (width a) causes light to
spread
(diffraction)
a
Near-field (z<a2/) = Fresnel diffraction
(complex mathematical form)
Far-field (z>>a2/)= Fraunhofer diffraction(simpler mathematical form
=Fourier Transform)
z
Light pattern at any plane z is the sum of secondary wavelets of the unobstructed aperture (including
phases)
z
secondary wavelets
f
diffracting object Lens, f observing
screen
a) b) x
very distant observing screen
~
diffracted rays at angle meet at infinity
parallel rays meet at a point in focal plane
input light
L
x
Figure 4: The far-field diffracted pattern can be visualised in the focal plane of a lens.
Far-field at focal plane of lens
Problem: Far-field z>> a2/may not be convenient for lab bench. Solution: Use lens.
f
x
DIODE LASER
f
Diffracting object = variable slit
bi-convex lens f=1000mm
observation screen ‘white card’
Expt.2 Visual Observation of Single-Slit Diffraction Pattern
• 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)
x
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
2
fax
fax
0
sinI)x(I
-10 -5 0 5 10 0.0
0.2
0.4
0.6
0.8
1.0
Distance
Inte
nsity
xm = m(f/a)
.... 3 ,2 ,1 m
Positions of zeroes
f
x
Far-field Single-Slit Diffraction Pattern
Is this what you see?
x
-10 -5 0 5 10
0.01
0.1
1
Distance
Log
Inte
nsity
-10 -5 0 5 10 0.0
0.2
0.4
0.6
0.8
1.0
Distance
Inte
nsity
Logarithmic Response of the Eye
bi-convex lens f=500mm
Photodiode/slit assembly on translation stage
voltmeter
LASER
f
diffracting object = single slit slide
Measurement with a Photo-detector
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 = 2f/a
1. Switch on photodiode power
supply and set voltmeter to 200mV
setting
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
f/a f/a
I0
I1
X1X-1
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