4/21 Chapter 27 Color Each wavelength in the visible part of the spectrum produces a different color. Additive color scheme RGB Red Green Blue Any color can be produced by adding the appropriate amounts of each color. Equal amounts of blue and green produce cyan; blue and red, magenta; red and green, yellow. Equal amounts of all three produces white. These are the additive primary colors. TV/computer screens. Subtractive Color Scheme CMYK Cyan, Magenta, Yellow, black Subtractive because upon reflection from a surface, some wavelengths are absorbed from the white light and subtracted from it. Used in printing. In modern computers must translate from the RGB screen to the CMYK printer using the capability of the printer. Include black because mixing CMY gives a muddy brown color. Chapter 28 Reflection and Refraction Geometrical Optics Treats light as traveling on rays like a laser beam. It applies when the wavelength of the light is small compared with the size of the optical system. Reflection Two types: specular (like from a mirror), and diffuse. Law of Reflection: The angle of reflection is equal to the angle of incidence. In the picture, the symbol is the Greek letter theta and is commonly used for angles. The subscript i means incident and the r means reflected.
For specular reflection, light rays parallel to each other are reflected parallel to each other. Bumpiness of surface small compared to wavelength of the light. The same law holds for diffuse reflection, but the surface is bumpy with bumpiness large compared to wavelength. Since the surface is bumpy, parallel rays are not reflected parallel any image information is lost. Refraction The bending of a light ray as it crosses the interface between two media. We ll consider an air/glass or air/water interface. If a light ray passes from air into glass or air into water, it is bent toward the perpendicular. Angle of refraction is less than the angle of incidence. If a light ray passes from glass or water into air, the ray is bent away from the perpendicular. The angle of incidence can be increased until the angle of refraction is 90 degrees without reaching 90 degrees itself. We have reached total internal reflection. Physics behind light pipes. Lenses focusing light. A lens possesses a characteristic called its focal point. Two types of lenses: converging (fat in the middle, skinny around the edge) and diverging (skinny in the middle and fat round the edge). The focal length of a lens is the distance from the focal points to the lens. The far focal point of a lens is the focal point on the side of the lens opposite the direction from which the light is coming. The near focal point of a lens is the focal point on the same side of the lens from which the light is coming. Light rays that come in parallel to the axis of a converging lens are focused at the far focal point of the lens. Note in the pictures at right, the light is coming in from the left. Light rays that come in parallel to the axis of a diverging lens bent directly away from the near focal point of the lens. Three special rays for a converging lens: 1. A light ray that comes into the lens parallel to the axis of the lens is deflected through the far focal point of the lens.
2. A ray light that passes through the near focal point of the lens is bent parallel to the axis of the lens. 3. A light ray passing through the center of a lens is undeflected. A simple lens diagram. This diagram shows the set up for the eye, the camera, and the movie projector. For the eye and the camera, the object is usually much farther away from the lens. For the projector, the object is usually just outside the focal point, which moves the image very far away. Applications to Vision Correction. Near-sightedness myopia Far-sightedness hyperopia Astigmatism Near sighted eye shape kind of like a football prolate spheroid. Image formed in front of the retina not focused when reaches the retina. The picture shows light from very far away focusing in front of the retina Put a diverging lens in front of the eye, which moves the image back to the retina. The light diverges after the lens, and the optical system of the eye brings it to focus at the retina. Far-sighted eye is shaped like a doorknob oblate spheroid When the eye is completely relaxed, image is formed behind the retina, which means unfocused on the retina. Eye must be under constant tension to bring the image back to the retina. Can lead to headaches. Put a converging lens in front of the eye to bring the image of an object at infinity to the retina. After the lens, the rays converge a bit, and the optical system of the eye can cause these converging rays to focus at the retina. Astigmatism Different focal points for vertical and horizontal if the horizontal part of an image is focused, the vertical won t be. Use cylindrical lenses to adjust one of the focal points to the location of the other. Chapter 29 Light waves
Interference and Diffraction and Polarization Interference add waves from individual sources to get the total wave from all of them principle of superposition. Diffraction wave traveling into a shadow. Polarization restricting the direction of the electric field in a light wave. Huygens Principle allows us to discuss interference and diffraction. Ripples in a pond wave fronts. We can think of each point on a wave front as a source of spherical waves (in a pond, circular waves) Interference between these wavelets produces the next wave front. Two-slit Interference Shine a laser beam on a plane containing two slits. If the width of the slits is small compared to a wavelength, they will behave like two separate sources with the same frequency and oscillating in phase. In the picture, each slit is a source of circular ripples. Each ripple corresponds to a crest. Where to ripples cross, a maximum appears due to constructive interference. Halfway between two ripples is a trough. Where a ripple passes a point halfway between the ripples of the other source, a minimum occurs due to destructive interference. Alternate bands of bright and dark will appear on a screen. Diffraction Wave creeping into a shadow. The source on the edge of a slit produces wavelets that travel into the shadow. There are no sources to produce wavelets that will interfere with these wavelets, so they just propagate on into the shadow. Polarization limiting the direction of the electric field in a light wave. Unpolarized Light Double arrows represent oscillation of the electric field in that direction. Light can be polarized using polarizing lenses such such as found in some types of sunglasses. Such lenses contain long chain molecules in which electric charges can easily oscillate in the direction of the molecules but no in the perpendicular direction.
If unpolarized light strikes a polarizing lens such as above, light parallel to the molecules can easily move charges in the molecules it does work and is absorbed. The part of the electric field perpendicular to the molecules, doesn t do work no motion field is not absorbed. Only polarizations perpendicular to the molecules gets through the lens. If we try to send horizontally polarized light through the lens, it won t get through it will be blocked by the lens. This is how polarizing sunglasses work glare comes from reflection off horizontal surfaces. For nonmetallic horizontal surfaces the light tends to be horizontally polarized. The glare is reduced by the sunglasses. This is also how 3D movies work Two images, the left and right, are displayed on the screen. One is polarized vertically and the other, horizontally. The 3D glasses have on side allowing vertical polarization and the other, horizontal.