Digital Photography
Part 1
A crash course in optics
Light
Photo + graphy (greek) = writing with light
Light is an electromagnetic (EM) wave. EM waves are periodic
changes in an electromagnetic field.
Characteristics of light:
• speed of propagation:
c (speed of light)
• wavelength:
λ
• frequency:
ν
For any wave, speed of propagation equals the wavelength time
frequency:
c  
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The electromagnetic spectrum
1 nm = 10-9 m =
1 billionth of a
meter = 1
millionth of a
millimeter
visible range
energy
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Before we go on…
Optics
Geometric optics
Wave optics
relatively easy
complicated
deals with “rays” of
light
deals with wave
equations
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Ray of light
Ray of light/light ray: ideally, an infinitely thin beam of light
Propagation: light ray travels in a straight line at speed c. The
speed of light in vacuum is 300,000 km/s = 3×108 m/s –
almost the same in air.
Reflection: light ray is bounced back from a surface
Refraction: light ray enters a different medium, wavelength and
speed change
Dispersion: on entering a dispersive medium, the components of
light become spatially separated
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Dispersion
White light consists of waves of
various wavelengths
(=different color). Separated
components can be reunited
with a lens, regaining white
light.
Some colors exist as both
single-wavelength spectral
colors and composite colors;
some only exist as composite
colors.
A dispersive prism
• refracts light (changes its
direction);
• resolves light into components
of different color.
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Shadow
Light sources in real life are never point-like and objects also scatter
light, so shadows are never really black, not even full shadows,
much less partial shadows. Shadows can be of any color,
depending on the color of the light, the object and the surface!
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Rough surfaces
Light incident on non-reflective,
matte surfaces is scattered in every
direction – that’s how we see
objects from every angle. This
phenomenon is called diffuse
reflection.
Not only surfaces scatter light.
Seemingly transparent media, like
air, also do – that’s why mountains
in the distance seem hazy.
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But why is the sky blue and why are
the clouds white? Research for
yourself…
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Reflection
Angle of incidence, angle of
reflection are measured from the
normal, not the surface.
The Law of Reflection: the angle of
reflection equals the angle of
incidence: β = α.
This type of reflection is called
specular reflection.
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Plane mirror
Image is
• upright
• virtual
• same size as object
• same distance behind mirror as
object before it
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Spherical mirrors
C: center of mirror
V: vertex of mirror
F: focal point
CV line: optical axis
CV distance: radius of the
sphere
FV distance: focal length =
half the radius
http://dev.physicslab.org/asp/applets/opticsmirrors/default.asp
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Principal rays
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Convex mirror
aka diverging mirror
Image is
• upright
• virtual
• reduced
• smaller distance
behind mirror than
object before it
http://dev.physicslab.org/asp/applets/javaphysmath/java/dmirr/default.asp
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Concave mirror
aka converging mirror
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Refraction
optically less
dense medium
When light enters a new medium, its
direction, wavelength and speed
changes. Wavelength and speed
are highest in vacuum.
Def.: index of refraction of a
medium: n = c/v (v is speed in
medium).
Snell’s Law: sin 1  n1
sin  2
optically more
dense medium
n2
(also called Descartes’ Law, Law of
Refraction)
consequences: mirage, different apparent size in
water, etc.
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Total internal reflection
Light moving from a dense to
a less dense medium
“bends away” from the
normal; but the angle of
reflection can be maximum
90 degrees (light is
refracted along the
surface). If the angle of
incidence is increased
beyond that critical angle,
light is totally reflected
rather than entering the
new medium.
http://dev.physicslab.org/asp/applets/javaphysmath/java/totintrefl/default.asp
light moves from a dense to a less dense medium
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Plane-parallel plate
Light rays traveling
through a planeparallel plate (e.g.
window glass) are
shifted but their
direction remains
unchanged.
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Convex lens
aka converging lens
http://dev.physicslab.org/asp/applets/javaphysmath/java/clens/default.asp
http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html
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Concave lens
aka diverging lens
Image is
• upright
• virtual
• reduced
• closer to the lens
than the object
• in front of the lens
http://dev.physicslab.org/asp/applets/javaphysmath/java/dlens/default.asp
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Thin lens equation
1
1 1


f do di
d0 : distance from object to center of
lens
d0 is always positive
di : distance from image to center of
lens
di is positive if image is
behind the lens (real image)
f : focal length
di is negative if image is in
front of the lens (virtual
image)
1/f (f measured in m): power of the lens,
measured in diopters. 1 D = 1/m
f is positive for convex lens
Magnification of the lens:
f is negative for concave
lens
I : image size
O : Péter
object
Tarján size
M
di
I

do
O
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Optical illusions
The eye and the brain has the tendency
not to see what’s actually there but
what it thinks is there – this makes
judging color, brightness and
perspective especially difficult when
taking photos…
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Imaging without lenses
Possible!
The camera obscura (Latin: dark
room) or pinhole camera is a
box with a little hole on one
side. It creates a real, reversed
image on the opposite side of the
box. Image is less bright than
with a lens, but depth of field is
almost infinite – the smaller the
hole, the more so.
Needs long exposure, but free of
distortion. Large creative
potential!
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