Chapter 14: Light and ReflectionObjectives:

• Identify the components of the electromagnetic spectrum. • Calculate the frequency or wavelength of electromagnetic radiation. • Recognize that light has a finite speed. • Describe how the brightness of a light source is affected by distance. • Distinguish between specular and diffuse reflection of light. • Apply the law of reflection for flat mirrors. • Describe the nature of images formed by flat mirrors. • Calculate distances and focal lengths using the mirror equation for concave and convex spherical

mirrors. • Draw ray diagrams to find the image distance and magnification for concave and convex spherical

mirrors. • Distinguish between real and virtual images. • Describe how parabolic mirrors differ from spherical mirrors. • Recognize how additive colors affect the color of light. • Recognize how pigments affect the color of reflected light. • Explain how linearly polarized light is formed and detected.

Assignment

– Study vocabulary- 9 terms P. 549• Assignment– P. 550-553 Review Questions– 1-5, 14-16, 23-27, 42-44

Electromagnetic Spectrum

Electromagnetic Wave• a transverse wave consisting of oscillating electric

and magnetic fields at right angles to each other.• All electromagnetic waves move at the speed of

light • The currently accepted value for light traveling in a

vacuum is 2.99792458 × 108 m/s. We will use 3.00 × 108 m/s.

• c = fλspeed of light = frequency × wavelength

The AM radio band extends from 5.4 × 105 Hz to 1.7 × 106 Hz. What are the longest and shortest wavelengths in this frequency range? c = fλ and c= 3.00 × 108 m/s

Brightness decreases by the square of the distance from the source

• The farther you are from a light source, the less bright the light appears to be.

• The farther the light is from the source, the more spread out the light becomes. Therefore, less light is available per unit area at a greater distance from the source than at a smaller one.

• If you move twice as far away from the light source, one-fourth as much light falls on the book.

Reflection• Reflection- the turning back of an electromagnetic

wave at the surface of a substance.• Most substances absorb at least some incoming

light and reflect the rest. A good mirror can reflect about 90 percent of the incident light, but no surface is a perfect reflector.

The texture of a surface affects how it reflects light

• Light that is reflected from a rough, textured surface, such as paper, cloth, or unpolished wood, is reflected in many different directions. This type of reflection is called diffuse reflection.

• Light reflected from smooth, shiny surfaces, such as a mirror or water in a pond, is reflected in one direction only. This type of reflection is called specular reflection.

Incoming and reflected angles are equal

If a straight line is drawn perpendicular to the reflecting surface at the point

where the incoming ray strikes the surface, the angle of incidence and the angle

of reflection can be defined with respect to the line.

Flat Mirrors• Flat mirrors are the simplest mirrors• The relationship between the object distance from

the mirror, which is represented as p, and the image distance (that is, the distance the image appears to be behind the mirror’s surface), which is represented as q, is such that the object and image distances are equal. Simi- larly, the image of the object is the same size as the object.

The image formed by rays that appear to come together at the image point behind the mirror—but never really do—is called a virtual image. A flat mirror always forms a virtual image, which can only be seen “behind” the surface of the mirror. For this reason, a virtual image can never be displayed on a physical surface.

Ray diagrams are used to show the movement of light.

Concave Spherical Mirrors

• Concave Spherical Mirror- an inwardly curved, mirrored surface that is a portion of a sphere and that converges incoming light rays

• Concave spherical mirrors are used whenever a magnified image of an object is needed, as in the case of a dressing-table mirror.

One factor that determines where the image will appear in a concave spherical mirror and

how large that image will be is the amount by which the mirror is curved. This in turn

depends on the radius of curvature, R, of the mirror. The radius of curvature is the same as

the radius of the spherical shell of which the mirror is a small part; R is therefore the

distance from the mirror’s surface to the center of curvature, C.

Real Image

• Real Image- an image formed when rays of light actually intersect at a single point

• With a concave mirror you can focus the image at one point. If you move that point back or forth you will get a distorted image.

Convex Spherical Mirror

• An outwardly curved, mirrored surface that is a portion of a sphere and that diverges incoming light rays

• Convex spherical mirrors take the objects in a large field of view and produce a small image, so they are well suited for providing a fixed observer with a complete view of a large area.

• Convex mirrors are often placed in stores to help employees monitor customers and at the intersections of busy hallways so that people in both hallways can tell when others are approaching.

Color and Polarization

• You have probably noticed that the color of an object can appear different under different lighting conditions. A plant that appears green in sunlight will appear black under a red light. These differences are due to differences in the reflecting and light-absorbing properties of the object being illuminated.

Primary colors produce white light when combined • When mixing all the colors of light, you get white

light.• Television and computer

screens take advantage of the additive primaries. Each picture element—or pixel—on a TV screen consists of three color dots. Lighting combinations of the color dots and varying the brightness allow the screen to display any desired color.

Color and the Electromagnetic Spectrum• The figure below shows what a pixel looks like as it

produces various colors. Notice that red and green color dots are lit to produce yellow, the red and blue dots produce magenta, and the blue and green dots produce cyan. Lighting all three dots in a pixel produces white, and lighting none of them produces black.

Subtractive primary colors filter out all light when combined

However, if you mix a blue pigment

(such as paint or the colored wax of

a crayon) with a yellow pigment,

the resulting color is green, not

white. This difference is due to the

fact that pigments rely on colors of

light that are absorbed, or

subtracted, from the incoming

light.

Primary and Secondary Colors

Polarization

• Linear Polarization- the alignment of electromagnetic waves in such a way that the vibrations of the electric fields in each of the waves are parallel to each other

• Light waves travel in all directions, if we filter out waves of a certain direction we can eliminate them.

• Polarization

Polarization and Scattering of Light• When looking into the blue sky of a crystal-clear

day, humans see light that is uniform. However, for some animals, like honeybees and pigeons, the light in the sky is far from uniform. The reason is that these animals are sensitive to the direction of the electric field in a beam of light.

• In general, the direction of the electric field in a light wave, or any other electromagnetic wave, is referred to as its polarization.

Polarization and Scattering of Light• To understand polarization more clearly, consider the

electromagnetic waves pictured in the figure below.

• Each of the waves has an electric field that points along a line. For example, the electric field in figure (a) oscillates up and down in the vertical direction. We say that this wave is linearly polarized in the vertical direction.

Polarization and Scattering of Light• Light is also polarized when it reflects from a smooth surface, like

the top of a table or the surface of a calm lake. The figure below shows a typical situation with unpolarized light from the Sun reflecting from the surface of a lake.

• As the figure indicates, the reflected light from the lake is polarized horizontally.

Polarization and Scattering of Light

• Polarizing sunglasses take advantage of this effect by using sheets of polarizing material with a vertical transmission axis. With this orientation, the horizontally polarized reflected light—the glare—is not transmitted.

Polarization and Scattering of Light• Why is the sky blue? The answer to this question has to

do with the way light scatters.• Light scatters most effectively when its wavelength is

comparable to the size of the scatterer. • The molecules in the atmosphere are generally much

smaller than the wavelength of visible light. But blue light, with its relatively short wavelength, is scattered more effectively by air molecules than red light, with its longer wavelength.

• Similarly, microscopic dust particles in the upper atmosphere also scatter the short-wavelength blue light more effectively. That is why we see a blue sky.

Polarization and Scattering of Light

• A sunset appears red because you are looking directly at the Sun through a long expanse of the atmosphere. Most of the Sun's blue light has been scattered off in other directions. This leaves you with red light.

Vocabulary Quiz Tomorrow

• 9 terms• Page 549.• Assignment– P. 550-553 Review Questions– 1-5, 14-16, 23-27, 42-44