Digital Photography

Part 1A crash course in optics

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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: νFor any wave, speed of propagation equals the

wavelength time frequency: c

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The electromagnetic spectrum

visible range

1 nm = 10-9 m = 1 billionth of a meter = 1 millionth of a millimeter

energy

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Before we go on…

Optics

Geometric optics

relatively easy

Wave opticscomplicated

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 surfaceRefraction: light ray enters a different medium,

wavelength and speed changeDispersion: on entering a dispersive medium, the

components of light become spatially separated

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DispersionWhite 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.

But why is the sky blue and why are the clouds white? Research for yourself…

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ReflectionAngle 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 mirrorsC: center of mirrorV: vertex of mirrorF: focal pointCV line: optical axisCV distance: radius of

the sphereFV 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 mirroraka 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 mirroraka converging mirror

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RefractionWhen 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:

(also called Descartes’ Law, Law of Refraction)

consequences: mirage, different apparent size in water, etc.

2

1

2

1

sin

sin

n

n

optically less dense medium

optically more dense medium

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Total internal reflectionLight 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.

light moves from a dense to a less dense medium

http://dev.physicslab.org/asp/applets/javaphysmath/java/totintrefl/default.asp

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Plane-parallel plate

Light rays traveling through a plane-parallel plate (e.g. window glass) are shifted but their direction remains unchanged.

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Convex lensaka converging lens

http://dev.physicslab.org/asp/applets/javaphysmath/java/clens/default.asphttp://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html

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Concave lensaka diverging lens

Image is• upright• virtual• reduced• closer to the lens

than the object• in front of the

lenshttp://dev.physicslab.org/asp/applets/javaphysmath/java/dlens/default.asp

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d0 is always positive

di is positive if image is behind the lens (real image)

di is negative if image is in front of the lens (virtual image)

f is positive for convex lens

f is negative for concave lens

d0 : distance from object to center of lens

di : distance from image to center of lens

f : focal length

1/f (f measured in m): power of the lens, measured in diopters. 1 D = 1/m

Magnification of the lens:

I : image size

O : object size

Thin lens equation

O

I

d

dM

o

i

io ddf

111

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Optical illusionsThe 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 lensesPossible!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!