Light as a Wave Part 2 SNC2D. The Speed(s) of Light Light will travel slower in denser media (e.g. glass), but the speed in air is still effectively 3.0

Light as a Wave Light as a Wave Part 2Part 2

SNC2DSNC2D

The Speed(s) of LightThe Speed(s) of Light

Light will travel slower in denser media Light will travel slower in denser media (e.g. glass), but the speed in air is still (e.g. glass), but the speed in air is still effectively 3.0 x 10effectively 3.0 x 1088 m/s and m/s and

c f

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c

f

The SpectrumThe Spectrum

The wavelength (and therefore the The wavelength (and therefore the frequency) of a light wave will frequency) of a light wave will depend of the scale of the depend of the scale of the disturbance in the EM fields.disturbance in the EM fields.


Visible light is actually a very small Visible light is actually a very small part of the EM spectrum:part of the EM spectrum:


Very short (i.e. gamma ray) wavelengths Very short (i.e. gamma ray) wavelengths would be generated by very small-scale would be generated by very small-scale transitions (i.e. in the nucleus).transitions (i.e. in the nucleus).

Wavelength and Wavelength and FrequencyFrequency

Short wavelengths are associated with Short wavelengths are associated with high frequencies:high frequencies:

v f fv

Wavelength and Wavelength and FrequencyFrequency

Short wavelengths are associated with high Short wavelengths are associated with high frequencies:frequencies:

v f fv

fm

H zms

3 0 10

103 10

8

1220.

X-raysX-rays

Still-short Still-short wavelength x-wavelength x-rays are useful rays are useful for diagnostic for diagnostic imaging.imaging.

UltravioletUltraviolet

Ultraviolet light rays, originally called Ultraviolet light rays, originally called deoxidizing or chemical rays for their deoxidizing or chemical rays for their chemical reactivity, can cause skin chemical reactivity, can cause skin cancer.cancer.

InfraredInfrared

Infrared light is felt as heat. You are Infrared light is felt as heat. You are emitting infrared radiation.emitting infrared radiation.

Microwaves and Radio Microwaves and Radio WavesWaves

We use longer-wavelength microwaves We use longer-wavelength microwaves to cook food and still-longer-to cook food and still-longer-wavelength radio waves to wavelength radio waves to communicate.communicate.

Visible wavelengthsVisible wavelengths

In the visible spectrum, wavelength In the visible spectrum, wavelength translates to colour.translates to colour.


Red light has the longest Red light has the longest , ~700 nm., ~700 nm.

Violet light has the shortest Violet light has the shortest , ~400 , ~400 nm.nm.


Red light has the longest Red light has the longest , ~700 nm (700 , ~700 nm (700 x 10x 10-9-9 m) m)

Violet light has the shortest Violet light has the shortest , ~400 nm , ~400 nm (400 x 10(400 x 10-9-9 m) m)


Red light has the longest Red light has the longest , ~700 nm (700 x , ~700 nm (700 x 1010-9-9 m) m)

Violet light has the shortest Violet light has the shortest , ~400 nm , ~400 nm (400 x 10(400 x 10-9-9 m) m)

Practice QuestionPractice Question: What is the frequency : What is the frequency of red light?of red light?


Practice QuestionPractice Question: What is the : What is the frequency of red light?frequency of red light?

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ResonanceResonance

Different atoms and molecules have Different atoms and molecules have different natural frequencies of different natural frequencies of vibration and will emit and absorb vibration and will emit and absorb different frequencies of light. different frequencies of light.

(When light is absorbed, the wave (When light is absorbed, the wave energy is converted to the thermal energy is converted to the thermal energy of the particles.)energy of the particles.)

ResonanceResonance

e.g. hydrogen atoms, when excited, e.g. hydrogen atoms, when excited, will emit the frequencies:will emit the frequencies:

ResonanceResonance

e.g. hydrogen atoms, when excited, will e.g. hydrogen atoms, when excited, will emit the frequencies:emit the frequencies:

so when light is reflected from or so when light is reflected from or transmitted through hydrogen, it will transmitted through hydrogen, it will absorb those same frequencies:absorb those same frequencies:

These lines are created These lines are created by electrons jumping by electrons jumping between the second between the second and higher orbital and higher orbital shells.shells.

(An electron falling (An electron falling from the higher to from the higher to the second emits the second emits light; an electron light; an electron jumps up when light jumps up when light is absorbed.)is absorbed.)

ResonanceResonance

Gas discharge tubes produce Gas discharge tubes produce these characteristic frequencies these characteristic frequencies by exciting the gas/vapour with by exciting the gas/vapour with an electric current.an electric current.

The frequencies can The frequencies can

be viewed using a be viewed using a

spectroscopespectroscope..

White LightWhite Light

““White” light is the sum of all White” light is the sum of all frequencies (ROYGBIV), which can frequencies (ROYGBIV), which can be seen by viewing white light be seen by viewing white light through a spectroscope or, similarly, through a spectroscope or, similarly, by refracting light through a prism.by refracting light through a prism.

ColourColour

The perceived colour of an object The perceived colour of an object illuminated with white light illuminated with white light therefore depends on which therefore depends on which frequencies are reflected.frequencies are reflected.

Example:Example:

ColourColour

The perceived colour of an object The perceived colour of an object illuminated with white light therefore illuminated with white light therefore depends on which frequencies are depends on which frequencies are reflected.reflected.

Example:Example:

Reflects GREENReflects GREEN

andand

Appears GREENAppears GREEN

Black = HotBlack = Hot

A shirt that appears black is A shirt that appears black is absorbing all colours (and absorbing all colours (and therefore all that energy of therefore all that energy of the light waves).the light waves).

A shirt that appears white is A shirt that appears white is reflecting all colours.reflecting all colours.

Black = HotBlack = Hot

A shirt that appears black is A shirt that appears black is absorbing all colours (and absorbing all colours (and therefore all that energy of therefore all that energy of the light waves).the light waves).

A shirt that appears white is A shirt that appears white is reflecting all colours.reflecting all colours.

So wear white to stay cool.So wear white to stay cool.

Primary ColoursPrimary Colours

White light can also be White light can also be produced by combining produced by combining only three colours. These only three colours. These colours are called colours are called primary coloursprimary colours, and in , and in physics arephysics are

Primary ColoursPrimary Colours

White light can also be White light can also be produced by combining produced by combining only three colours. These only three colours. These colours are called colours are called primary coloursprimary colours, and in , and in physics are Red, Blue, physics are Red, Blue, and Green.and Green.

Secondary ColoursSecondary Colours

Secondary colours are Secondary colours are formed by the addition of formed by the addition of 2 primary colours:2 primary colours:



Blue + Red =Blue + Red =



Blue + Red = MagentaBlue + Red = Magenta




Blue + Green =Blue + Green =




Blue + Green = CyanBlue + Green = Cyan





Red + Green =Red + Green =





Red + Green = YellowRed + Green = Yellow

More PracticeMore Practice

p. 391 #3 – 5, 7 – 13p. 391 #3 – 5, 7 – 13

Tomorrow: ReflectionTomorrow: Reflection

Documents

Light as a Wave Part 2 SNC2D. The Speed(s) of Light Light will travel slower in denser media (e.g. glass), but the speed in air is still effectively 3.0