LIGHT The Wave Model of Light Textbook Section

LIGHT The Wave Model of Light Textbook Section 10. 1

RECAP: Light • Light behaves like a wave and like a particle • The particles are electrons, called “photons” • The waves have a crest, a trough, an amplitude, a frequency, and a wavelength Draw wave on whiteboard

RECAP: The Wave Model of Light • The model that describes lights as traveling as a wave is called The Wave Model of Light • Describes light in terms that could be used to describe waves from a ripple in water

RECAP: The Wave Model of Light • The model that describes lights as traveling as a wave is called The Wave Model of Light • Describes light in ways that could be used to describe a ripple (wave) in water

RECAP: The Wave Model of Light • Wave: A disturbance in a medium (e. g. a fluid, such as water or air, or solid, such as a metal), and which transfers energy from one point to another without transferring matter.

RECAP: The Wave Model of Light

RECAP: The Wave Model of Light • When the photons oscillate (move up and down), they create an electromagnetic field, consisting of: • an electric “field” • a magnetic “field”

RECAP: The Wave Model of Light The electromagnetic field Electric Field

RECAP: The Wave Model of Light • Lights of different frequencies (number of waves in a period of time) creates a spectrum of lights, known as the electromagnetic spectrum

RECAP: The Wave Model of Light The Electromagnetic Spectrum

Of the visible light, red has the lowest frequency (and the longest wavelength), while violet has the highest frequency (and the shortest wavelength).

The Wave Model of Light • Shining white light onto a prism creates the visible light of the spectrum (the colours of the rainbow) • This is because white light is a combination of the frequencies of all the visible colours

The Wave Model of Light

Additive Colour Theory of Light Mixing equal proportions of red, green, and blue light (the primary additive colours) creates white light. In the process, magenta, yellow, and cyan (the secondary additive colours) are created.

Subtractive Colour Theory of Light Mixing equal proportions of magenta, yellow, and cyan (the primary subtractive colours) creates black light. In the process, red, green, and blue (the secondary subtractive colours) are created. As such, this is the opposite of additive theory.

Subtractive Colour Theory of Light Each of the primary subtractive colours reflects only the colours that do not lead to black. And these colours (the reflected colours) are the ones we see. We do not see colours that are absorbed.

RECAP: The Wave Model of Light • There is an inverse relationship between frequency and wavelength: As one increases, the other decreases. • This can be shown using the formula: • v = f x λ • Where…

Three-Variable Equations v = f x λ v = speed of wave, in metres per second, nanometres per second, etc. f = frequency of wave, in cycles per second, s-1, or Hertz (Hz) λ = wavelength, in metres, nanometres, centimetres, etc.

Three-Variable Equations You’ve seen this type of equation before! When we drive, we travel a certain distance (in kilometers) by traveling at a certain speed (in kilometers per hour) for a certain amount of time (in hours). Such that… distance = speed x time

Three-Variable Equations •

Three-Variable Equations

Three-Variable Equations •

In summary. . . distance = speed x time

Three-Variable Equations This is because any variable that is on the numerator (multiplying another value) on one side of the equal sign will transfer over to the other side as being in the denominator (dividing)! And vice versa…

Three-Variable Equations As such, whenever we have an equation with 3 variables, we will know 2 of them… And this allows us to solve for (find) the 3 rd one.

Three-Variable Equations Using the example of speed, time, and distance…

Three-Variable Equation A car travels a distance of 250 kilometres in 3 hours. Assuming that it always travels at the same (constant) speed, at what speed what is it traveling? We know: distance = 250 km time = 3 hours We can find: speed = ?

Three-Variable Equations •

Three-Variable Equations Now, a three-variable equation example for the formula v = f x λ where: v = speed of wave (in m/s) f = frequency of wave (in /s or s-1) λ = wavelength (in m)

Three-Variable Equations A light wave has a wavelength of 0. 5 m and a speed of 3, 000 m/s. What is the frequency of the wave? v=fxλ We know: v = 3000 m/s λ = 0. 5 m We can find: f = ? cycles/second, or Hz

Three-Variable Equations •

Three-Variable Equations f = 6, 000 cycles/s Therefore, a wave with a wavelength of 0. 5 m and a speed of 3, 000 m/s has a frequency of 6, 000 s-1, or 6, 000 cycles per second, or 6, 000 Hz. This means that, in one second, this wave completes 6, 000 cycles.

One of these is not like the other…

The Triangle “Trick”…

Some Practice Problems

1) A light wave with a wavelength λ = 3 m has a frequency f = 4 Hz. What is the speed of the wave? 2) “The Aurora Borealis is a night display in the Northern latitudes caused by ionizing radiation interacting with the Earth's magnetic field and the upper atmosphere. The distinctive green color is caused by the interaction of the radiation with oxygen and has a frequency of 5. 38 x 1014 Hz. What is the wavelength of this light? ”

1) We know: f = 4 Hz λ = 3 m We need to find: v = ? The equation: v = f λ v = ( 4 s-1 ) ( 3 m) v = 12 m/s Therefore, the speed of this wave is 12 m/s.

2) We know: f = 5. 38 x 1014 Hz v = speed of light in a vacuum! = c = 3 x 108 m/s We need to find: λ = ?

That is a very small value… As such, it would make sense to use a small unit, so we can describe it with a larger number. For example, we can use nanometers. 1 nm = 10 -9 m (i. e. : a nanometer is a very small fraction of a metre!)

RECAP: The Ray Model of Light • Even though light travels in waves, we can represent the way a wave travels by describing it in rays rather than waves • Ray: a straight line that has a point of origin and can travel infinitely

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