Light as a Wave: Part 1

Light as a Wave: Part 1

SNC2D

What is a wave?

A wave is a disturbance which carries energy from one location to another.

The material the disturbance travels through is the medium.

The movement of the disturbance is referred to as propagation.

2 Types of Waves

A longitudinal wave is a wave in which the particles of the medium move in a direction parallel to the direction of propagation.

2 Types of Waves

A transverse wave is a wave in which the particles of the medium move in a direction perpendicular to the direction of propagation.

Snapshot of a Wave

The dashed line represents the rest positions of the particles.

Snapshot of a Wave

The positions of maximum displacement are referred to as crests (positive displacement) and troughs (negative).

The maximum displacement is the amplitude.

Snapshot of a Wave

The distance between one crest and the next crest (or one trough and the next trough) is the

Snapshot of a Wave

The distance between one crest and the next crest (or one trough and the next trough) is the wavelength, represented by

Snapshot of a Wave

The distance between one crest and the next crest (or one trough and the next trough) is the wavelength, represented by .

Snapshot of a Wave

The time it takes one complete wavelength to pass a single point is the

Snapshot of a Wave

The time it takes one complete wavelength to pass a single point is the period, represented by

Snapshot of a Wave

The time it takes one complete wavelength to pass a single point is the period, represented by T.

Snapshot of a Wave

The number of complete wavelengths that pass a single point in one second is the frequency, represented by f.

Tf

1

Frequency

Frequency is measured in units of s-1 or Hertz (Hz).

“I do not think that the wireless waves I have discovered will have any practical application.”

http://en.wikipedia.org/wiki/Image:Heinrich_Rudolf_Hertz.jpg

Wave Speed

The speed of a wave is therefore:

Wave Speed

The speed of a wave is therefore:

Practice Question: What is the wavelength of a wave with a speed of 344 m/s and a frequency of 256 Hz?

fTt

dv

Wave Speed


Wave Speed


?

:

256

344

:

Unknown

Hzf

v

Givens

sm

Wave Speed


?

:

256

344

:

Unknown

Hzf

v

Givens

sm

m

HzSolve

f

vfvSelect

sm

34.1

256

344:

:

Light is an “EM” wave

Light is actually a transverse wave

Light is an “EM” wave

Light is actually a transverse wave, but the perpendicular-to-propagation disturbance is not of particles of a medium but of the electric and magnetic (EM) fields

http://www.monos.leidenuniv.nl/smo/basics/light_anim.htm

The Speed of Light

Because light waves do not need a medium, they can travel through a vacuum (empty space) and do so at the speed of light: c =

The Speed of Light

Because light waves do not need a medium, they can travel through a vacuum (empty space) and do so at the speed of light: c = 3.0 x 108 m/s.

Light-years

A light-year is the distance light would travel in 1 year.

Light-years


Practice Question: How far (in m) is 1.0 light-years?

Light-years


Practice Question: How far (in m) is 1.0 light-years?

yr

m

yr

d

d

h

h

s

s

m

1

105.9

1

365

1

24

1

min60

min1

60

1

100.3 158

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 x 108 m/s and

c f

or

fcand

c

f

More Practice

p. 391 #1 and 2

Light as a Wave: Part 1

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