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# Chapter 27 - PowerPoint PPT Presentation

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

Properties of Light

Electromagnetic

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.

### 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…

• Light

• pure energy

• Electromagnetic Waves

• energy-carrying waves emitted by vibrating electrons

• Photons

• particles of light

• ### 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

• German American physicist A.A. Michelson

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

Picture credit

### The Electromagnetic Spectrum

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

### The Electromagnetic Spectrum

• Microwaves - used to cook

• Infrared - “heat waves”

• Visible Light - detected by your eyes

• Ultraviolet - causes sunburns

• X-rays - penetrates tissue

• Gamma Rays - most energetic

The Visible Spectrum

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

### 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.

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### Transparent Materials

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

### 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 - the term applied to materials that absorb light

### Example Questions

• Are clouds transparent or opaque to visible light?

• Are clouds transparent or opaque to ultraviolet light?

• ### 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

• 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

Umbra

Sun

Earth

Moon

Penumbra

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

### Lunar Eclipse

Sun

Earth

Moon

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

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!

chained molecules which absorb

light with electric fields

perpendicular to the axis.

How do we make Polarized Light?

I. Polarizers-

• 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.

### 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.

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## Chapter 27 Review Questions

### 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...

(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

a. rods.

b. cones.

c. cornea.

d. crystalline lens.

b. cones.