# Electromagnetic Waves Fig. 21.22, p.675 Chapter 13 Section 1 Characteristics of Light.

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Electromagnetic Waves

Fig. 21.22, p.675

Chapter 13 Section 1 Characteristics of Light

The most famous and conspicuous supernova remnant. The Crab Nebula is the centuries-old wreckage of a stellar explosion, or supernova, first noted by Chinese astronomers on July 4, 1054, and that reached a peak magnitude of -6 (about four times brighter than Venus). According to the Chinese records, it was visible in daylight for 23 days and in the night sky to the unaided eye for 653 days. Petroglyphs found in Navaho Canyon and White Mesa (both Arizona) and in the Chaco Canyon National Park (New Mexico) appear to be depictions of the event by Anasazi Indian artists. The Crab Nebula lies about 6,300 light-years away in the constellation Taurus, measures roughly 10 light-years across, and is expanding at an average speed of 1,800 km/s. Surprisingly, its expansion rate seems to be accelerating, driven by radiation from the central pulsar. Its luminosity at visible wavelengths exceeds 1,000 times that of the Sun

Fig. 21.23a, p.676 Crab Nebula—X-ray image

Fig. 21.23b, p.676 Crab Nebula—Optical image

Fig. 21.23c, p.676 Crab Nebula—Infrared image

Fig. 21.23d, p.676 Crab Nebula—Radio image

 Light has dual nature—Particles and Waves  “Particles” of light are called photons  Each photon has a particular energy ◦ E = h ƒ ◦ h is Planck’s constant  h = 6.63 x 10 -34 J s ◦ Encompasses both natures of light  Interacts like a particle  Has a given frequency like a wave  c = f

 Light travels in a straight-line path in a homogeneous medium until it encounters a boundary between two different media  The ray approximation is used to represent beams of light  A ray of light is an imaginary line drawn along the direction of travel of the light beams

Chapter 13  Illuminance decreases as the square of the distance from the source.  The rate at which light is emitted from a source is called the luminous flux and is measured in lumens (lm).

 A ray of light, the incident ray, travels in a medium  When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium ◦ This means it is directed backward into the first medium

 Specular reflection is reflection from a smooth surface  The reflected rays are parallel to each other  All reflection in this text is assumed to be specular

 Diffuse reflection is reflection from a rough surface  The reflected rays travel in a variety of directions  Diffuse reflection makes the road easy to see at night

 The normal is a line perpendicular to the surface ◦ It is at the point where the incident ray strikes the surface  θ i = θ r θi θ θr

Chapter 13

 The incident ray, the reflected ray, the refracted ray, and the normal all lie on the same plane  The angle of refraction, θ 2, depends on the properties of the medium

Section 3 Curved Mirrors Chapter 13  A concave spherical mirror is a mirror whose reflecting surface is a segment of the inside of a sphere.  Concave mirrors can be used to form real images.  A real image is an image formed when rays of light actually pass through a point on the image. Real images can be projected onto a screen.

Chapter 13 Section 3 Curved Mirrors

 The Mirror Equation relates object distance (p), image distance (q), and focal length (f ) of a spherical mirror.

Chapter 13  The Equation for Magnification relates image height or distance to object height or distance, respectively.

 p is the distance to the object ◦ + object in front of mirror ◦ - object behind mirror  q is the distance to the image ◦ - image behind mirror (virtual image) ◦ + image in front of mirror (real image)  f = 1/2C = 1/2R ◦ +for concave mirrors ◦ - for convex mirrors  M = image height/object height h i /h o = -q/p ◦ + for upright image ◦ - for inverted image ◦ If M<1, the image is smaller than the object ◦ If M>1, the image is larger than the object

 Three rays can always be drawn for curved mirrors. Where they intersect is where the image is located.  Ray 1: A ray drawn from the object through the focal point is reflected parallel to the principal axis  Ray 2: A ray drawn from the object through the center of curvature is reflected back on itself.  Ray 3: A ray drawn from the object parallel to the principal axis reflects through the focal point.

Lens TypeObject Beyond Focal Point Object At Focal Point Object Between Focal Point And Lens Converging (convex) Real Inverted Reduced Image No Image Formed Erect Virtual Magnified Image Diverging (concave) Virtual Erect Reduced Image

QuantityPositive WhenNegative When Object location (p)Object is in front of the lens Object is in back of the lens Image location (q)Image is in back of the lens Image is in front of the lens Image height (h’)Image is uprightImage is inverted R 1 and R 2 Center of curvature is in back of the lens Center of curvature is in front of the lens Focal length (f)Converging lensDiverging lens

 Ray diagrams are essential for understanding the overall image formation  Three rays are drawn ◦ The first ray is drawn parallel to the first principle axis and then passes through (or appears to come from) one of the focal lengths ◦ The second ray is drawn through the center of the lens and continues in a straight line ◦ The third ray is drawn from the other focal point and emerges from the lens parallel to the principle axis  There are an infinite number of rays, these are convenient

Fig. 22.16, p.697

Chapter 13  Additive primary colors produce white light when combined.  Light of different colors can be produced by adding light consisting of the primary additive colors (red, green, and blue).

 Subtractive primary colors filter out all light when combined.  Pigments can be produced by combining subtractive colors (magenta, yellow, and cyan).

 Each atom produces a wave with its own orientation of E  All directions of the electric field E vector are equally possible and lie in a plane perpendicular to the direction of propagation  This is an unpolarized wave

Chapter 13  Light can be polarized by reflection and scattering.  At a particular angle, reflected light is polarized horizontally.  The sunlight scattered by air molecules is polarized for an observer on Earth’s surface.

 A wave is said to be linearly polarized if the resultant electric field vibrates in the same direction at all times at a particular point  Polarization can be obtained from an unpolarized beam by ◦ selective absorption ◦ reflection ◦ scattering

Chapter 13 Section 4 Color and Polarization Crossed FiltersAligned Filters

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