Light and Reflection Chapter 13 Page 444

Characteristics of Light �  Let’s talk about the electromagnetic spectrum.

�  This includes visible light. What looks like “white” light can be split into many different colors.

�  Light possesses characteristics of both particles and waves. For our purposes, we will model light as a wave.

�  Just like other waves, electromagnetic waves vary depending on their frequency and wavelength.

Characteristics of Light �  Differences in frequency and wavelength account

for the different colors we see, as well as whether or not the wave is even visible.

�  Light is composed of oscillating electric and magnetic fields, which are perpendicular to each other, and perpendicular to the direction of the wave’s motion.

�  There’s a table of different waves on the EM spectrum on page 447. Wavelength, λ, is measured in units of length, like m, mm, cm. Frequency is measured in Hertz.

Characteristics of Light � 

Characteristics of Light �  ALL electromagnetic waves move at the speed of

light!

�  In a vacuum: 2.99792458 x 108 m/s.

�  In air: 2.99709 x 108 m/s.

�  There is a relationship between frequency, wavelength, and speed that we’ve seen before as regards waves. This applies to light waves, as well!

�  c=fλ Sample Problem A page 448 Practice A page 449

Characteristics of Light �  The motion of electromagnetic waves can be

approximated as rays.

�  In reality, light spreads out as it travels.

�  A laser is an example of a light source that spreads very little; it is concentrated and focused, unlike—for example—our fluorescent lights.

Characteristics of Light �  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.

�  My dad will talk for hours about lumens. He used to work for Sylvania. He once had to travel by air with a xenon lamp for Imax movies, but this was before 9/11 so security just asked some questions and didn’t hassle him about it. I’ve heard this story fifty times. Zzzzzzz…..............

Characteristics of Light �  The illuminance—the luminous flux divided by the

area of the surface—is measured in lm/m2 which is called lux. It decreases as the radius squared when you move away from a light source.

Section Review page 450

Flat Mirrors �  Light interacts with surfaces in two ways: some of the

light may be absorbed, and some of the light may be reflected.

�  No surface is a perfect reflector, though good mirrors can reflect around 90% of incident light.

�  A rougher surface reflects incoming light in many different directions. This is called diffuse reflection.

�  A smooth surface reflects incoming light in one direction. This is called specular reflection. (The direction depends on the direction of incoming light.)

Flat Mirrors �  Incoming and reflected angles are equal.

�  Imagine a straight line drawn perpendicular to the surface at the point where the incoming light is striking it. (Another word for this kind of line is normal.)

�  The angle of incidence is measured between the ray of incoming light and the normal line.

�  The angle of reflection is measured between the normal line and the ray of reflected light.

Flat Mirrors �  The simplest mirror is a flat mirror.

�  Light bounces off objects in front of the mirror and is reflected to an observer. To the observer, the light originates on the other side of the mirror.

Flat Mirrors �  An object’s image is said to be at a location behind

the mirror.

�  The image will be the same distance from the mirror as the object. The image is also the same size as the object.

�  This image is known as a virtual image. It can never be displayed on a physical surface.

Flat Mirrors �  We can predict an image location by using

geometric ray diagrams.

�  Sizes and distances will be reflected symmetrically in a flat mirror. The object’s distance from the mirror at any given point will be the same as the image’s distance from the mirror. All dimensions in the image will be the same as on the object.

Curved Mirrors �  While flat mirrors create images with the same

dimensions as the original object, curved mirrors create images with different dimensions compared to the original object.

�  A basic type of curved mirror is a concave spherical mirror—a mirror that is shaped like part of a sphere’s surface. Concave mirrors like these can magnify nearby objects, as needed, and are often used when applying makeup, for example.

Curved Mirrors �  As you get farther away from a concave spherical

mirror, the image will appear smaller and upside-down.

�  The location of the image is determined in part by the radius of curvature of the mirror’s surface, R. The radius of curvature is the distance from the mirror’s surface to the center of curvature, C.

�  A mirror’s principal axis extends from the center of the mirror’s surface through its center of curvature.

Curved Mirrors �  Rays that are farther away from the principal axis

don’t exactly intersect at the image point.

�  This is called spherical aberration.

�  Ray diagrams and the mirror equations are valid for rays that are near the principal axis of the mirror.

�  These are called paraxial rays. (Para—near, axial—axis.)

Curved Mirrors �  The Mirror Equation: If you know the mirror radius

and the distance the object is from the mirror, you can predict where the image will appear using the mirror equation.

�  Object distance: p

�  Image distance: q

�  Radius of curvature: R

�  Focal length: f

Curved Mirrors �  Consider light coming from very far away from the

mirror. From large enough distances, we say that the light is coming “from infinity.”

�  Light coming from infinity will be directed towards the same location, called the focal point—halfway between the mirror and its center of curvature.

�  Real images are formed on the front side of the mirror.

�  Virtual images are formed behind the mirror, on the back side.

Curved Mirrors �  The image from a curved mirror is rarely the same

size as the object. Thus, we can talk about the magnification of an image.

�  Once you know the object’s image location, you can determine its magnification.

�  Magnification: M

�  Object height: h

�  Image height: h’

Curved Mirrors �  For negative values of M, the image will be upside-

down.

�  For values of M between 0 and 1, the image will be smaller.

�  For values of M larger than 1, the image will be larger.

Curved Mirrors �  Another type of curved mirror is a convex spherical

mirror.

�  These are also called diverging mirrors, as all rays coming from infinity will bounce off at a greater angle away from the principal axis.

�  The focal length for a convex spherical mirror is negative; the focal point and center of curvature are behind the mirror.

�  Think: fisheye mirrors opposite driveways; smaller insets in rear-view mirrors.

Sample Problem C page 465 Practice C page 466

Curved Mirrors �  In our ray diagrams for spherical mirrors, you may

have noticed that our rays rarely intersect perfectly at the image’s location. This is a real phenomenon called a spherical aberration, and when you see images reflected in spherical mirrors they will appear blurry.

�  A mirror whose surface cross-section is parabolic eliminates this problem completely!

Curved Mirrors �  Parabolic mirrors are used in some telescopes!

Color and Polarization �  Objects can absorb certain wavelengths of light

while reflecting others.

�  We perceive the reflected wavelengths of light. For example, a fresh, green leaf reflects green light.

Color and Polarization �  Colors of light can be additive. Additive primary

colors of light—red, green, and blue—will combine to make white light.

�  Additive properties of colors are applied in television screens and projections.

�  Colors can be subtractive. Subtractive primary colors of light—cyan, magenta, and yellow—will combine to filter out all light.

�  Subtractive properties of colors can be applied in painting, coloring, etc.

Color and Polarization �  Light from a normal source will include various

waves that have electric fields which oscillate in many directions.

�  This light is unpolarized.

�  Polarization is when light passes through a filter which only allows waves with a certain orientation to pass through. These waves have linear polarization.

Color and Polarization �  Light is polarized along the transmission axis of

the substance it passes through.

�  If light is shone at two substances with perpendicular transmission axes, light will not get through.

Color and Polarization �  Polarization can also occur by reflection and by

scattering.

�  Light reflected off a surface will be polarized parallel to the surface.

�  Light can be scattered off molecules of atmospheric gas.