Chapter 27
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Chapter 27. Properties of Light. Electromagnetic Radiation. Light. Photons. Diffraction. Diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening.

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Chapter 27

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Chapter 27

Chapter 27

Properties of Light


Chapter 27

Electromagnetic

Radiation

Light

Photons


Diffraction

Diffraction

  • Diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening.


Reflection

Reflection

Regardless of the angle at which the wavefronts approach the barrier, one general law of reflection holds true: the waves will always reflect in such a way that the angle at which they approach the barrier equals the angle at which they reflect off the barrier.


These three are the same

These three are the same…

  • Light

    • pure energy

  • Electromagnetic Waves

    • energy-carrying waves emitted by vibrating electrons

  • Photons

    • particles of light


  • Electromagnetic wave velocity

    Electromagnetic Wave Velocity

    • The speed of light is the same for all seven forms of light.

    • It is 300,000,000 meters per second or 186,000 miles per second.


    Michelson s 1878 rotating mirror experiment

    Michelson’s 1878 Rotating Mirror Experiment

    • German American physicist A.A. Michelson

    • Using this method, Michelson was able to calculate c = 299,792 km/s

    Picture credit


    The electromagnetic spectrum

    The Electromagnetic Spectrum

    • A range of light waves extending in wavelength from radio waves to gamma rays


    The electromagnetic spectrum1

    The Electromagnetic Spectrum

    • Radio Waves - communication

    • Microwaves - used to cook

    • Infrared - “heat waves”

    • Visible Light - detected by your eyes

    • Ultraviolet - causes sunburns

    • X-rays - penetrates tissue

    • Gamma Rays - most energetic


    Chapter 27

    The Visible Spectrum

    A range of light waves extending in wavelength from about 400 to 700 namometers.


    Questions

    Questions

    • Is it correct to say that radio wave is a low-frequency light wave?

    • Is a radio wave also a sound wave?

    • See question on page 455.

    *


    Transparent materials

    Transparent Materials

    • Transparent - the term applied to materials through which light can pass in straight lines


    Transparent materials1

    Transparent Materials

    • The following are transparent materials, which does light travel through fastest?

      • Water at 0.75c

      • Glass at 0.67c

      • Diamonds at 0.40c


    Opaque materials

    Opaque Materials

    • Opaque - the term applied to materials that absorb light


    Example questions

    Example Questions

    • Are clouds transparent or opaque to visible light?

      • Answer: opaque

  • Are clouds transparent or opaque to ultraviolet light?

    • Answer: transparent


  • Opaque

    Opaque

    • Why do metal objects reflect light back?

      • The electrons are loose in metals (remember conductors) and do not spring energy from atom to atom. Rather they remit visible light


    Shadows

    Shadows

    • Umbra - the darker part of a shadow where all the light is blocked

    • Penumbra - a partial shadow

    • These terms also apply to Solar Eclipses and Lunar Eclipses.


    Solar eclipse

    Solar Eclipse

    Umbra

    Sun

    Full Shadow

    Earth

    Moon

    Partial Shadow

    Penumbra

    • A solar eclipse occurs when the Moon passes in front of the Sun.


    Lunar eclipse

    Lunar Eclipse

    Sun

    Earth

    Moon

    • A lunar eclipse occurs when the Moon passes into the Earth's shadow.


    Chapter 27

    Electric field only going

    up and down – say it is

    linearly polarized.

    Light can have other types of polarizations such as circularly polarized

    or elliptically polarized. We will only look at linearly polarized light.

    Polarization

    Net electric field is zero – Unpolarized light!


    Chapter 27

    Polarizers are made of long

    chained molecules which absorb

    light with electric fields

    perpendicular to the axis.

    How do we make Polarized Light?

    I. Polarizers-


    Chapter 27

    • Light is totally polarized when an object with a vertical Polarization axis and another object with a horizontal polarization axis are placed next to each other.

    • This causes no light to be able to get through the lens.


    Questions1

    Questions

    • Which type of eclipse is dangerous to view with the unprotected eye?

    • Why are lunar eclipses more commonly seen than solar eclipses?

    • See question on page 460.

    *


    Chapter 27 review questions

    Chapter 27 Review Questions


    You will observe a total eclipse of the sun when

    You will observe a total eclipse of the sun when...

    (a) you stand in the penumbra of the moon’s shadow

    (b) you stand in the umbra of the moon’s shadow

    (c) sunlight diffracts around the moon

    (d) sunlight reflects from the moon to the earth

    (b) you stand in the umbra of the moon’s shadow


    The speed of light

    The speed of light...

    (a) has never been measured

    (b) is about the same as that of sound

    (c) is infinitely fast

    (d) is very fast, but not infinite

    (d) is very fast, but not infinite


    In the dark at late evening no color is seen because of lack of stimulation of

    In the dark at late evening, no color is seen because of lack of stimulation of

    a. rods.

    b. cones.

    c. cornea.

    d. crystalline lens.

    b. cones.


    End of chapter 25

    End of Chapter 25


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