Ch7 light(1)

•Light

Light, sounds, and odors can identify the pleasing

environment of this garden. But light provides the most

information. Sounds and odors can be identified and

studied directly, but light can only be studied indirectly, that is, in terms of how it behaves. As a result, the

behavior of light has stimulated thinking scientific investigations and debate for hundreds of years. Perhaps you have wondered about

light and its behaviors. What is light?

• Sources of Light

• Luminous– When something produces light it is said to be luminous

• Incandescent– When light is given off as a result of high temperatures

an object is said to be incandescent.– The electromagnetic wave model is used to explain

incandescent light.

• Black Body Radiation– The radiation given off by a mass at any temperature.

Three different objects emitting blackbody radiation at three different temperatures. The intensity of blackbody radiation increases with increasing temperature and the

peak wavelength emitted shifts toward shorter wavelengths.

Sunlight is about 9 percent ultraviolet

radiation, 40 percent visible light, and 51

percent infrared radiation before it

travels through the earth's

atmosphere.

• Properties of Light

Light rays are perpendicular to a wave front. A wave front that has traveled a

long distance is a plane wave front, and its

rays are parallel to each other. The rays show the direction of

the wave motion.

• Light interacts with matter– A ray of light travels in a straight line from a source until

it encounters some object or particles of matter.• What happens to the light depends on:

– the smoothness of the surface– the nature of the material– the angle that the light strikes the surface.

– If a surface is perfectly smooth, rays of light undergo reflection

– If a surface is not smooth the light rays are scattered in many random directions as diffuse reflection takes place.

– Materials that allow transmission of light are called transparent.

– Materials that do not allow the transmission of light are called opaque.

– Vertical rays of light are mostly transmitted through a transparent material with some reflection and some absorption.

– If the rays strike the surface at some angle much more of the light is reflected.

Light that interacts with matter can be reflected, absorbed

or transmitted through transparent

materials. Any combination of these interaction

can take place, but a particular substance is

usually characterized by

what it mostly does to light.

• Reflection– Most of the objects that we see are visible from diffuse

reflection– The rate at which groups of rays are spreading or their

essentially parallel orientation carries information about distances.

– Light rays that are diffusely reflected move in all directions while light rays reflected from a smooth surface move out in definite directions.

(A)Rays reflected from a perfectly

smooth surface are parallel to each

other. (B) Diffuse

reflection from a rough surface

causes ray to travel in many random

directions.

– When an object is reflected in a mirror there are three lines of importance.• The incident ray which is a line representing the

original ray.• A reflected ray, which represents the reflected ray.• A normal line which is a reference line that is

perpendicular to the reflecting surface.• The angle between thee incident ray and the normal is

the angle of incidence.• The angle between the reflected ray and the normal is

the angle of reflection.• The Law of Reflection states that the angle of

incidence equals the angle of reflection.

Bundles of light rays are reflected

diffusely in all directions from every

point on an object. Only a few light rays are shown from only one point on a tree in this illustration. The lights that move to

your eyes enable you to see a particular point from which

they were reflected.

Adjacent light rays spread farther and farther apart after reflecting from a point. Close to the point, the rate of

spreading is great. At a great distance, the rays are almost parallel. The rate of ray spreading carries information

about distance.

The law of reflection states that the angle of incidence ((i) is equal to the angle of reflection ((r). Both angles are

measured from the normal, a reference line drawn perpendicular to the surface at the point of reflection.

Light rays leaving a point on the block are reflected according to the law of reflection, and those reaching your eyes are seen. After reflecting, the rays continue to spread apart at the same rate. You interpret this to be a block the

same distance behind the mirror. You see a virtual image of the block, because light rays do not actually move from the

image.

• Refraction– When a light ray moves from one transparent material to

another, such as from water through air, the ray undergoes a change in the direction of travel at the boundary.

• This change in direction is called refraction

– Refraction results from a change in speed when light passes from one transparent material into another.

– Different materials transmit light at different speeds.• When light moves from one material to another with a slower

speed of light, the light is refracted toward the normal

• When light passes from one material into another with a faster speed of light, the light is refracted away from the normal

– The magnitude of refraction depends on:• The angle at which the light strikes

• The ratio of the speed of light in the two materials.

• A ray of light that is perpendicular to the surface (90O) is not refracted at all.

A ray diagram shows refraction at the boundary as a ray moves from air through water. Note that (i does not equal

(r in refraction.

(A) A light ray moving to a new material with a slower speed of light is refracted toward the

normal ((i(r). (B) A light ray moving to

anew material with a faster speed is

refracted away from normal ((i<(r).

When the angle of incidence results in an angle of refraction of 90o, the refracted light ray is refracted along the water

surface. The angle of incidence for a material that results in an angle of refraction of 90o is called the critical angle. When the

incident ray is at this critical angle or greater, the ray is reflected internally. The critical angle or water is about 49o,

and for a diamond it is about 25o.

Mirages are caused by hot air near the ground refracting, or bending light rays upward into the eyes of a distant

observer. The observer believes he is seeing and upside down image reflected from water on the highway.

• Dispersion and color– Electromagnetic waves travel at the speed of light with a

whole spectrum of waves of various frequencies and wavelengths.

– The speed of an electromagnetic wave (c) is related to the wavelength () and frequency (f) by

• c=f

– Visible light is the part of the electromagnetic spectrum visible to humans

– Six colors• Red.• Orange.• Yellow.• Green.• Blue.• Violet.

Light travels in a straight line, and the color of an object depends in which wavelengths of light the object reflects. Each of these flowers absorbs the colors of white light and

reflects the color that you see.

A one-way mirror reflects most of the light that strikes.

It also transmits some light to a person behind the mirror in a darkened room.

The flowers appear to be red because the reflect light in the 7.9 x 10 -7 m to 6.2 x 10-7 range of wavelengths.

– Light is interpreted to be white if it has the same mixture as the solar spectrum.

– Light that is composed of several colors is called polychromatic light

– Light that is composed of only one wavelength is called monochromatic light.

– A glass prism separates light into a spectrum of colors because the index of refraction is different for different wavelengths of light.

– An transparent material in which the index of refraction varies with wavelength has the property of dispersion.

• Evidence for waves

• Diffraction– Diffraction is the bending of light around an opaque

object.– The determining factor is the size of the opening as

compared to wavelength.– A wave front passing through a large opening will

continue to generate wavelets, so the original shape of the wave front moves straight through the opening.

– If the opening is very small a single wavelet will move out in all directions as an expanding arc from the opening.

(A) Huygens' wave theory described wavelets that formed from an older

wave front from a light source. The wavelets then moved out to form a new

wave front. (B) As the wavelets spread, the wave front eventually becomes a plane wave, or straight wave, at some distance. This continuous process

explained how waves could travel in a straight

line.

Water waves are observed to bend

around an obstacle such as a wall in the water. Light appears to move straight past an

obstacle, forming a shadow. This is one reason that Newton thought

light must be particles, not

waves.

(A) When an opening is large as compared to the wavelength, waves appear to move mostly

straight through the opening. (B) When the

opening is about the same size as the wavelengths, the waves diffract,

spreading into an arc.

Huygens' wave theory explains diffraction of

light. When the opening is large as compared to

the wavelength, the wavelets form a new

wave front as usual, and the front continues to

move in a straight line. When the opening is the size of the wavelength, a

single wavelet moves through expanding in an

arc.

Young's double-slit experiment produced a pattern of bright lines and dark zones when light from a single source passed

through the slits.

An interference pattern of bright

lines and dark zones is produced because

of the different distances that waves must travel from the two slits. (A) When they arrive together,

a bright line is produced. (B) Light

from slit 2 must travel further than

the light from slit 1, so they arrive out of

phase.

• Polarization– Unpolarized light consists of transverse waves vibrating

in all random directions.– Light is polarized if it vibrates in only one direction.– There are three ways to produce polarized light

• Selective absorption is the process that takes place in certain crystals where light in one plane is transmitted and all the other planes are absorbed

• Reflected light with an angle of incidence between 1O and 89O is partially polarized as the waves parallel to the reflecting surface are reflected more that other waves.

• Scattering occurs when light is absorbed and reradiated by particles about the size of gas molecules.

(A)Unpolarized light has transverse waves

vibrating on all possible directions

perpendicular to the direction of travel. (B) Polarized light vibrates only in one

plane. In this illustration, the wave

is vibrating in a vertical direction

only

(A) Two crystals that are aligned both transmit

vertically polarized light that looks like any other light. (B) When the crystals are

crossed, no light is transmitted

Light that is reflected becomes partially or fully polarized in a horizontal direction, depending on the incident angle

and other variables.

• Evidence for particles

• Photoelectric effect– Light is a form of energy and it gives off its energy to

matter when it is absorbed.– Some materials acquire the energy in electrons which can

sometime acquire enough energy to jump out of the material• This is called the photoelectric effect.

A setup for observing the photoelectric effect. Light

strikes the negatively charged plate, and

electrons are ejected. The ejected electrons move to

the positively charged plate and can be measured as a

current in the circuit.

• Quantization of energy– Vibrating molecules can have energy only in multiples of

certain amounts canned quanta.– When a photon interacts with matter, it is absorbed and

gives up all of its energy.– The energy given up by each photon is a function of the

frequency of the light.

• The present theory

• Light has a dual nature.– Wave properties.– Particle properties

• The actual nature of the photon is not actually describable in terms that are very descriptive.– The duality model does provide a conceptual framework

to at least begin talking about light.

It would seem very strange if there were not a sharp distinction between objects and waves in our everyday

world. Yet this appears to be the nature of light.