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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.

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Chapter 16

Chapter 16



  • 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 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
Properties of Light

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

More stuff
More “Stuff”

  • Speed of Light (c) = 3.00 x 108 m/s

  • Diffraction = the bending of light around a barrier

Electromagnetic spectrum
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

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

  • Secondary colors=yellow, cyan, and magenta


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

Doppler shift
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 shift1
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 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


  • 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


  • 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= 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
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
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
C. Plane Mirrors

  • Flat, smooth, reflecting surface

    • Upright image

    • Image is same distance as you are from mirror

    • Image is virtual

D concave mirrors
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
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
F. Lenses

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

G concave lenses
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
H. Convex Lenses

  • Thicker in middle and thinner at edges

    • Refracts rays toward center of lens

    • Rays converge

    • Image depends on location of object

Atoms and light bohr model
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
Energy a la Einstein

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


  • E=energy (Joules)

  • M=mass (kg)

  • c=speed of light

Particle model of waves
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
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 effect1
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