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# Light PowerPoint PPT Presentation

Chapter 16. Light. Light. The Ray Model of Light was introduced as a way to study how light interacts with matter Ray= a straight line that represents the linear path of a narrow bean of light Rays can change direction if reflected or refracted. Light Sources.

Light

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Chapter 16

## Light

### Light

• The Ray Model of Light was introduced as a way to study how light interacts with matter

• Ray= a straight line that represents the linear path of a narrow bean of light

• Rays can change direction if reflected or refracted

### Light Sources

• There are MANY different sources of light but there are only two TYPES of sources

• 1. Luminous Source = an object that emits light (such as the sun or a candle)

• 2. Illuminated Source = object that becomes visible as a result of the light reflecting off it (such as the Moon)

### Properties of Light

• The illuminance produced by a point source is proportional to 1/r2 (the inverse square law)

### More “Stuff”

• Speed of Light (c) = 3.00 x 108 m/s

• Diffraction = the bending of light around a barrier

### Electromagnetic Spectrum

• As the wavelength of visible light decreases, the color changes from red to violet

• As wavelength decreases, the frequency increases, and the energy of the wave increases

### Primary Colors of Light

• Primary colors of light = red, green, and blue

• Secondary colors=yellow, cyan, and magenta

### Color

• Complementary colors = 2 colors of light that can be combined to make white light

• Objects appear a certain color because they reflect that color light and absorb all the others

### Polarization

• Polarization is the production of light in a single plane of oscillation

### Doppler Shift

• Doppler Shift= the difference between the observed wavelength of light and the actual wavelength

• A positive change in λ = red shift

• The relative velocity of the source is away from the observer

• A negative change in λ = blue shift

• The relative velocity of the source is towards the observer

### Doppler Shift

• Stellar motion

• take the spectrum of a star

• compare observed wavelengths of absorption lines to lab values (H, Fe, Na, etc.)

• calculate star’s radial motion (need distance and tangential angular motion to get space motion)

• NO, you won’t have to calculate this!!!

### Reflection

• Reflection is the change in direction of a wave at an interface between two different media so that the wave returns into the medium from which it originated.

• Law of reflection: the angle of reflection=the angle of incidence

• θr=θi

### Reflection

• Specular reflection = when light hits a smooth surface the rays are reflected in parallel

• Diffuse reflection = when light hits a surface that is rough (on the level of the wavelength of light) the light scatters

### Reflection

• Reflected rays of light that enter the eye appear to originate at a point behind the mirror

• Virtual image= a type of image formed by diverging light rays

• Always on the opposite side of the mirror from the object

### Refraction

• Refraction= the bending of light as it passes into a new medium

• Index of Refraction= the ratio of the speed of light in a vacuum to the speed of light in that medium

• The index of refraction

determines how much the

light bends/refracts

### Total Internal Reflection

• Phenomenon that occurs when light traveling from a region of a higher index of refraction to a region of lower index of refraction strikes the boundary at an angle greater than the critical angle such that all light reflects back into the region of higher index

• Critical angle = the angle of incidence above which total internal reflection occurs

• This is how fiber optic cables work

### A. Images

• Light reflecting from an object to your eye

• Real image

• When light rays converge to form an image

• Virtual image

• An image your brain perceives though no light passes through it

### C. Plane Mirrors

• Flat, smooth, reflecting surface

• Upright image

• Image is same distance as you are from mirror

• Image is virtual

### D. Concave Mirrors

• Surface of mirror is curved inward

• Forms real and virtual images

• Distance of object from mirror determines size and type of image formed

• Object is closer than focal length upright virtual image

• Object is further than focal length  upside down real image

### E. Convex Mirrors

• Curves outward like the back of spoon

• Forms a virtual image

• Image is upright

• Image is smaller than actual object

### F. Lenses

• Transparent material with at least one curved surface that refracts light rays

### G. Concave Lenses

• Thinner in middle and thicker at edges

• Rays diverge

• Image is virtual, upright and smaller

• Used in some eyeglasses and telescopes

### H. Convex Lenses

• Thicker in middle and thinner at edges

• Refracts rays toward center of lens

• Rays converge

• Image depends on location of object

## Wave-Particle Duality of Light

### Atoms and light-Bohr Model

• Electrons only orbit in certain shells.

• Electrons jump from shell to shell when the atom absorbs and gives up energy.

• The ground state is the state the electron is in when it has the smallest allowable amount of energy

• The excited state is any energy level about the ground state, where the electron has more energy

• When the electron transfers from a higher energy level to a lower energy level energy is given off in the form of light

• This is how we get the emission spectra

### Energy a la Einstein

• Mass can be converted into energy with a yield governed by the Einstein relationship:

E=mc2

• E=energy (Joules)

• M=mass (kg)

• c=speed of light

### Particle Model of Waves

• The idea of light being simply a wave caused problems for physicists because the wave nature of light could not explain several important phenomenon

• The absorption and emission of electromagnetic radiation could not be explain using this “wave nature”

• These phenomena would eventually be explained by the “particle nature” of light

### Photoelectric Effect

• Heinrich Hertz first observed this photoelectric effect in 1887.

• Hertz had observed that, under the right conditions, when light is shined on a metal, electrons are released.

• Light falling on a metal can

cause electrons to be ejected

from the metal. This is known

as the photoelectric effect.

### Photoelectric Effect

• Einstein proposed that the energy in the light was not spread uniformly throughout the beam of light. Rather, the energy of the light is contained in "packets" or quanta

• He said that each quanta has a specific amount of energy found by

E = h f

• h is Planck's constant 6.62606957(29)×10−34

• f is the frequency of the light.

• From the conservation of energy, we would expect the electron to leave with kinetic energy KE given by

KE = h f – W