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: frequency: For any wave, speed of propagation equals the wavelength time frequency:
c Pter Tarjn 2 The electromagnetic spectrum 1 nm = 10-9 m=1 billionth of a meter = 1 millionth of a millimeter visible range ener
gy Pter Tarjn 3 Before we go on Optics Geometric optics Wave optics complicated relatively easy deals with rays of light
deals with wave equations Pter Tarjn 4 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 = 3108 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
Pter Tarjn 5 Dispersion A dispersive prism refracts light (changes its direction); 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. resolves light into components of different Pter Tarjn color. 6 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! Pter Tarjn 7 Rough surfaces Light incident on non-reflective, matte surfaces is scattered in every direction thats 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 thats why mountains in the distance seem hazy. Pter Tarjn
But why is the sky blue and why are the clouds white? Research for yourself 8 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: = . This type of reflection is called specular reflection. Pter Tarjn
9 Plane mirror Image is upright virtual same size as object same distance behind mirror as object before it Pter Tarjn 10 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 Pter Tarjn 11 Principal rays Pter Tarjn 12 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 Pter Tarjn 13 Concave mirror aka converging mirror
Pter Tarjn 14 Refraction When light enters a new medium, its direction, wavelength and speed optically less changes. Wavelength and dense speed are highest in vacuum. medium Def.: index of refraction of a medium: n = c/v (v is speed in medium). sin 1 n1
Snells Law: sin 2 optically more dense medium n2 (also called Descartes Law, Law of Refraction) consequences: mirage, different apparent size in water, etc. Pter Tarjn 15
Total internal reflection light moves from a dense to a less dense medium 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.
p://dev.physicslab.org/asp/applets/javaphysmath/java/totintrefl/default.asp Pter Tarjn 16 Plane-parallel plate Light rays traveling through a plane-parallel plate (e.g. window glass) are shifted but their direction remains unchanged. Pter Tarjn
reduced closer to the lens than the object in front of the lens http://dev.physicslab.org/asp/applets/javaphysmath/java/dlens/ Pter Tarjn default.asp 19 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.
d I 1 D = 1/m M i f is positive for convex lens Magnification of the lens: f is negative for concave lens I :Pter image size Tarjn
do O 20 Optical illusions The eye and the brain has the tendency not to see whats actually there but what it thinks is there this makes judging color, brightness and perspective especially difficult when taking photos Pter Tarjn 21
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! Pter Tarjn
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