MODELS OF LIGHT

ELECTROMAGNETIC SPECTRUM

• Electromagnetic radiation is a form of energy • Source: many objects throughout Earth and Space

• Ex: the sun is a huge source of electromagnetic radiation• Travels as a transverse wave • Seven types of electromagnetic radiation, depending

on wavelength, frequency & energy

microwaves infrared ultravioletvisible light x rays

ELECTROMAGNETIC SPECTRUM

Low frequencyLong wavelengthLow energy

High frequencyShort wavelength

High energy

ELECTROMAGNETIC SPECTRUM

Rock

Music

Is

Very

Useful for

X-raying

Goats

Raging

Martians

Invaded

Venus

Using

X-ray

Guns

Radio

Microwaves

Infrared

Visible light

Ultraviolet

X-rays

Gamma

Grandpa

Xavier

Unfortunately

Vomited

In

My

Room

VISIBLE LIGHT AS A MODEL

• Our eyes can detect a very small part of the electromagnetic spectrum

VISIBLE LIGHT AS A MODEL

• Visible light is used as a model to study the rest of the electromagnetic spectrum

• WHY• Becomes visible when it interacts with matter• Safe to work with

• WHAT HAVE WE LEARNED?• Invisible as it travels• Has electrical and magnetic properties• Can travel in a vacuum (empty space)• Travels thru a vacuum at the speed of light

• 300 000 000 m/s or 7.5x around the equator per second

MODELS OF LIGHT

• Scientists use models to represent ideas and concepts• 3 different ways to model light, depending on what

property we are trying to study• Ray Model

• Wave Model

• Particle Model

RAY MODEL OF LIGHT

• Oldest model…from ancient Greece!• Light travels in straight lines called rays • A ray is an arrow used to show the direction of a

straight-line path of light

RAY MODEL OF LIGHT

• Light rays spread out as they travel away from a source• Explains why light gets dimmer farther from a source

RAY MODEL OF LIGHT

• Used to predict shape and size of shadows• Object farther from source = smaller shadow• Object closer to source = larger shadow

WAVE VS. PARTICLE MODEL OF LIGHT

• For a while, scientists argued about wave model vs. particle model

• One experiment proved one right first…double slit experiment

Isaac Newton

Thomas Young

I do declare that ye light beith a

particle!

Tush tush! Ye light beith a wave!

Thou are a poisonous bunch-backed toad. Not but a half-witted

pig!

Thou hast no more brain than I have in mine

elbows, ye fool!

WAVE VS. PARTICLE MODEL OF LIGHT

• What would you expect to see if the particle model was correct?

• Imagine spray paint…each drop represents one particle

WAVE VS. PARTICLE MODEL OF LIGHT

• What would you expect to see if the wave model was correct?

• https://youtu.be/Iuv6hY6zsd0?t=254

WAVE MODEL OF LIGHT

• 1. Why is it more useful or helpful to use visible light as a model for electromagnetic radiation, rather than X-rays or any other type?

• 2. At 2 metres from a light source, light is ¼ as bright as it was 1 metre from the source. Explain why this is the case.

• 3. Visible light travels through empty space at the speed of light (300 000 000 m/s). Would an astronaut in orbit around Earth see visible light from the Sun as it travels through outer space? Explain.

• 4. Like visible light, microwaves spread out from a source. How might this affect cell phone use?

• 5. Describe one way that a light wave is like a water wave, and one way that it is different.

1. WHY IS IT MORE USEFUL OR HELPFUL TO USE VISIBLE LIGHT AS A MODEL FOR

ELECTROMAGNETIC RADIATION, RATHER THAN X-RAYS OR ANY OTHER TYPE?

• Two reasons• Light becomes visible when it interacts with matter• Light is safe to work with

2. AT 2 METRES FROM A LIGHT SOURCE, LIGHT IS ¼ AS BRIGHT AS IT

WAS 1 METRE FROM THE SOURCE. EXPLAIN WHY THIS IS THE CASE.

• The ray model of light shows us that light spreads out as it travels away from a source

• At 1m from the source, the light is more concentratedand therefore brighter

• At 2m from the source, the light has spread out and is covering a larger area, and is therefore dimmer

3. VISIBLE LIGHT TRAVELS THROUGH EMPTY SPACE AT THE SPEED OF LIGHT (300 000 000 M/S). WOULD

AN ASTRONAUT IN ORBIT AROUND EARTH SEE VISIBLE LIGHT FROM THE SUN AS IT TRAVELS

THROUGH OUTER SPACE? EXPLAIN.

• No!• Visible light is INVISIBLE as it travels• Only becomes visible when it interacts with matter

4. LIKE VISIBLE LIGHT, MICROWAVES SPREAD OUT FROM A SOURCE. HOW

MIGHT THIS AFFECT CELL PHONE USE?

• Cell phone signals will be stronger closer to a cell phone tower because microwaves are concentrated

• Farther from the cell phone tower microwaves spread out and become weaker

5. DESCRIBE ONE WAY THAT A LIGHT WAVE IS LIKE A WATER WAVE, AND

ONE WAY THAT IT IS DIFFERENT.

• Similar:• Transverse wave• Can be described using amplitude, wavelength,

frequency

• Different:• Does not require a medium• Can travel through a vacuum • Light waves have magnetic & electrical properties

WAVE MODEL OF LIGHT

• We already know that waves can be described using amplitude, frequency, and wavelength

• Amplitude is the height• Wavelength is the distance of one complete wave

• Large wavelengths can be measured in metres or even kilometers

• Small wavelengths can be measured in nanometers (1 000 000 000 nanometers in one meter!)

• Frequency is the number of waves per second• Measured in Hertz (Hz)• Calculated by determining the unit rate

WAVE MODEL OF LIGHT

• Calculating frequency…• A surfer sitting on his surfboard counts 18 waves passing

by him in 9 seconds. What is the frequency?

• Frequency = waves ÷ seconds• Frequency = 18 waves ÷ 9 seconds• Frequency = 2 waves/second = 2 Hz

WAVE MODEL OF LIGHT

• If you’re sitting on the beach and you count that 20 waves splash your feet in one minute (60 seconds) whatis the frequency of the waves?

• Frequency = waves ÷ seconds• Frequency = 20 waves ÷ 60 seconds• Frequency = 0.33 waves/second = 0.33 Hz

WAVE MODEL OF LIGHT

• The wave model of light explains why there are different colours of light! • Prism activity

• Each colour has a different wavelength

• Red has the longest wavelength• Closest to infraRED radiation

• Violet (or purple) has the shortest wavelength• Closest to ultraVIOLET radiation

• Longest wavelength – ROY G BIV – Shortest wavelength

PARTICLE MODEL OF LIGHT

• Aka…how Albert Einstein won his Nobel prize • The “photoelectric effect”• When blue light is projected onto a metal surface,

electrons are released • When red light is projected onto a metal surface, NO

electrons are released• Blue light is a shorter wavelength = higher frequency =

higher energy…enough energy to free an electron from its orbit!

PARTICLE MODEL OF LIGHT

Long WavelengthLow FrequencyLow Energy

Short WavelengthHigh Frequency

High Energy

PARTICLE MODEL OF LIGHT

• If the wave model was right, even with low energy red light, enough energy would eventually build up to release electrons• Like a faucet filling up a cup

PARTICLE MODEL OF LIGHT

• But that never happened…so it must be individual particles that release electrons • Like a strength test at the PNE

PARTICLE MODEL OF LIGHT

A “particle” of light is called a photon