basic optics - vutbr.czphysics.fme.vutbr.cz/~jirka/fp/l_02_basic_optics.pdf · 2017. 2. 20. ·...
TRANSCRIPT
Basic optics
• Geometrical optics and images
• Interference
• Diffraction
• Diffraction integral
we use simple models
that say a lot!
more rigorous approach
Basic optics
• Geometrical optics and images
• Interference
• Diffraction
• Diffraction integral
Images
Édouard Manet: A Bar at the Folies-Bergère (1882)
http://epod.usra.edu/blog/2012/10/inferior-mirage-on-a-desert-road.html
Mirage
light rays are bent to
produce a displaced image
of distant objects or the sky
Plane mirrors
convention:
- light entering from the left
- positive distances: O, I on the left
- real image: i > 0
- virtual image: i < 0
object distance image distance
Point objects
(here, we have
virtual image)
Mirror
ImageObject
p > 0 i < 0
• virtual image
• the same orientation and
size (height) as object
Extended objects
Plane mirrors
Head
Eye
Foot
Mirror
r, f > 0
r, f < 0
Spherical mirrors
convex
concaveReal
focus
Central axis
Virtual
focus
Central axis
Focal lengthRadius
Axis
Mirror
Image formation
Spherical mirrors
4 raysRay tracing
Spherical mirrors
- real image: i > 0
- virtual image: i < 0
Concave mirror:
p > f : real image
p = f : image at infinity
p < f : virtual image
Spherical mirrors
4 raysRay tracing
- real image: i > 0
- virtual image: i < 0
Convex mirror:
image is always
- virtual
- erect
- minified
Magnification m
erect image: m > 0
inverted image: m < 0
Spherical mirrors
- real image: i > 0
- virtual image: i < 0triangles ABV and DEV are similar
Spherical refracting surfaces
- real image: i > 0
- virtual image: i < 0
r < 0
r < 0
r < 0
r > 0
r > 0
r > 0
Real
image
Real
image
Virtual
image
Virtual
image
Virtual
image
Virtual
image
Axis
Thin lens
0 (thin lens)(thick lens)
for both refracting surfaces
Air
Glass
Axis
p i
Thin lens
< 0
f > 0
> 0
> 0< 0
f < 0
converging lens
diverging lens
Extensions
f > 0
3 rays
Ray tracing: converging lens
Ray tracing: diverging lens
f < 0
3 rays
diverging lensconverging lens
Thin lens (bottom line)
Thin lens: magnification m
Simple magnifier
angular magnification:
To distant virtual image
Compound microscope
Objective
Eyepiece
Parallel
rays
To distant virtual image
The lateral
magnification produced
by the objective lens
The overall
magnification
Refracting telescope
Objective
Eyepiece
Parallel
rays
To distant
virtual image
Parallel
rays from
distant
object
(angular magnification
of the telescope)
Aberrations (image errors)
- aberrations can be balanced
- image fidelity is limited only by diffraction
examples
Basic optics
• Geometrical optics and images
• Interference
• Diffraction
• Diffraction integral
Interference
What will happen if we add waves?
Double-slit experiment (Young’s experiment, 1801)
Incident
wave
An interference
pattern
Superposition
of waves
u – a suitable component
of E- or H- vector
assume
Incident
wave
Path length difference
(maxima)
(minima)
Different phases due to
different paths
(maxima)
(minima)
(two coherent sources)
(two incoherent
sources)
(one source)
(m for maxima)
(m for minima)
Inte
nsi
ty
Intensity in double-slit experiment
(missing – sign)
particles
http://www.feynmanlectures.caltech.edu/III_01.html
waves
Double-slit experiment with ...
Interference from thin films
Ray reflected at A
Ray reflected at B
Incident wave
For simplicity we assume
1. Normal incidence
2. Double beam interference
Phase difference
phase shift arising
from reflection at Bphase shift arising
from reflection at A
Ray reflected at A
Ray reflected at B
Incident wave
Example: air - n2 - air
(maxima)
(minima)
(maxima)
(minima)
Example: air - n2 - air
(Newton rings)
Example: glass - air - glass
Incident
light
Glass
Glass
Air
Temporal coherence
coherence length coherence time
Define:
monochromatic wave -
- perfectly coherent
pulse (wave-packet) -
- less coherent
white light -
- incoherent
Interference and temporal coherence
coherence length coherence time
Interference and temporal coherence
Spatial and temporal coherence
Michelson interferometer
Movable
mirror
incident wave
...
...
transmitted
wave
reflected
wave
Interference from thin films (again)
Now we consider
1. Arbitrary incident angle
2. Multiple beam interference
incident wave
...
...
transmitted
wave
reflected
wave
geometrical series
The amplitude of the resultant transmitted wave
Interference from thin films
incident wave
...
...
transmitted
wave
reflected
wave
geometrical series
The amplitude of the resultant reflected wave
Interference from thin films
incident wave
reflected wave transmitted wave
For simplicity assume symmetric structure
and pure real numbers
Interference from thin films
(relative transmitted intensity)
integer
Spectral response (thin film, FP etalon)
(relative transmitted intensity)
The free spectral range, FSR
Spectral response (thin film, FP etalon)
(relative transmitted intensity)
with loss
Spectral response (thin film, FP etalon)
Spectral analyzer
0
const.
Basic optics
• Geometrical optics and images
• Interference
• Diffraction
• Diffraction integral
Huygens-Fresnel principle
Every point of a wavefront at a given instant in time,
serves as a source of spherical secondary waves. The
amplitude of the optical field at any point beyond is
the superposition of all these wavelets.
A wavefront
at t = 0
The new wavefront
at t = ∆t
Huygens-Fresnel principle
Every point of a wavefront at a given instant in time,
serves as a source of spherical secondary waves. The
amplitude of the optical field at any point beyond is
the superposition of all these wavelets.
Diffraction
Every point of a wavefront at a given instant in time,
serves as a source of spherical secondary waves. The
amplitude of the optical field at any point beyond is
the superposition of all these wavelets.
Diffraction
Every point of a wavefront at a given instant in time,
serves as a source of spherical secondary waves. The
amplitude of the optical field at any point beyond is
the superposition of all these wavelets.
Incident
wave
Diffracted
wave
Screen
Diffraction from a single slit
z
x
z
radiates a wavelet
The superposition of all these wavelets:
a source at x
x
?
some “constant”
Incident
wave
Screen
Path length difference
in the Fraunhofer region (far-field region)
Amplitude of diffracted wave
Fourier transform and diffraction
The amplitude of diffracted wave is proportional
to the Fourier transform of the field distribution
across the aperture ( = the aperture function).
z
x
?Incident
wave
Screen
aperture function
(we will prove it later)
... back to diffraction from a single slit
z
x
?Incident
wave
Screen
Diffraction from a single slit (results)
z
x
?Incident
wave
Screen(minima)
Diffraction from a single slit (results)
(minima)
Diffraction from a circular aperture
diameter
Airy rings
first minimum
Diffraction from a circular aperture
Resolution of imagining systems
Rayleigh’s criterion for the minimum
resolvable angular separation
Diffraction from a double slit
z
x
substitution
Diffraction factor – due to the
diffraction by a single slit
Interference factor – due to the
interference between two slits
Diffraction factor – due to the
diffraction by a single slit
Interference factor – due to the
interference between two slits
Diffraction from a double slit
diffraction by
a single slit
interference
between two slits
diffraction from
a double slit
Diffraction factor – due to the
diffraction by a single slit
Interference factor – due to the
interference between two slits
Diffraction from a double slit
diffraction by a
single slit
interference fringes for
a double slit system
Diffraction gratings (multiple slits)
(maxima)
Path length difference
(grating orders)
Diffraction gratings (multiple slits)
Path length difference
Diffraction factor – due to the
diffraction by a single slit
Interference factor – due to the
interference from N slits
Diffraction gratings (multiple slits)
Diffraction factor – due to the
diffraction by a single slit
Interference factor – due to the
interference from N slits
X-ray diffraction
(Bragg’s law)
Incident
x rays
Davisson, C. J., "Are Electrons Waves?,"
Franklin Institute Journal 205, 597 (1928)
Electron diffraction
(Bragg’s law)
Incident
electron beam
Basic optics
• Geometrical optics and images
• Interference
• Diffraction
• Diffraction integral
Angular spectrum representation
arbitrary wave = superposition of plane waves
=
in homogeneous medium
+z
Angular spectrum representation (more details)
Plane wave
Wave function:
real complex
Superposition of plane waves:
For
(IFT)
(FT)
we choose + sign, i.e., we
assume propagation in +z
(possible reflections are
neglected)
for EM waves – scalar
approximation
Propagation of waves
+z
?
paraxial approximation
known
Propagation of waves
paraxial approximation
(calculation of the integral)
Fresnel-Kirchhoff diffraction formula
Fraunhofer approximation:
only for
+z
?known
Diffraction integral