What do you know about light?Light carries energyTravels in straight linesDoes not need a medium to travel throughExists in a wide variety of forms, defined as the

Electromagnetic spectrum.Nothing can move faster than particles of light!

The Speed of Light

How long does it take for light to fill a classroom once the light switch is flipped?

Almost instantly!This is because the speed of light is 3.0 x

10^8 m/s in a vacuum! This is very fast!!How long does it take for light to reach Earth

from the Sun?Try it! The Earth is 1.5 x 10^11 meters from

the Sun

Speed of light: constant?Does light always travel at “c”, or 3.0 x 10^8 m/s?Light travels very close to c in air, so this is a good

approximationWhat happens when light hits an object?Objects can be Transparent, Translucent or OpaqueIf light passes into an object, it slows down (for example,

glass)When light slows down, we get refraction, but that’s getting

ahead of ourselves!

Propagation of lightUnlike sound, light does not require a

medium (“something”) to travel through. It used to be thought that, in order to travel

through space (a vacuum), light required the existence of an “ether”.

An “ether” was a medium that existed in outer space, through which light could travel.

We know now that there is no such thing as an “ether” – space really is a vacuum!

The Electromagnetic SpectrumYou may be familiar with X-rays and Ultra-

violet rays from dentist visits, broken bones or worries about sun burns.

Did you realize they were forms of light?

The ElectroMagnetic Spectrum

Notice how small the visible light section is! We only see a small portion of reality!

Notice energy increases to the right of the visible spectrum.

The EM SpectrumUsing instruments, we can “see” other areas of the EM

Spectrum and understand the universe better.

Chandra: looking at the X-rays of the Universe

Chandra, a space telescope, looks at the x-ray emissions of galaxies such as this one, to see features undetectable at the visible wavelengths.

In this case, this is a Pulsar in the Crab nebula, shooting out streams of matter and anti-matter.

Infra-redThe section of the EM spectrum given off as “heat”

Spitzer Space Telescope: InfraredSST examines

the universe through an infrared lens.

In this case, it is looking at the heat (IR) radiation given off by the Fireworks Galaxy

SST IR viewpoint reveals a large new ring of Saturn!

MicrowavesIn addition to being useful for cooking,

microwaves have allowed astronomers to examine the very beginning of the universe, which carries a residual microwave “glow”.

Fill in a chart for the EM Spectrum!

Include each area of the spectrum (Radio, Micro, IR, Visible, UV, X-ray, Gamma)

Also include where each type of light is found (examples)

Sources of lightCan you distinguish between “luminous” and

“non-luminous” objects? Which is the Moon? What sources of light can you think of?We can divide sources of light into artificial and

natural – what are examples of each?

Types of light sourcesIncandescentIs caused by heating up a material

Florescent Light Sources

Florescent lights are more efficient than incandescent

They are, however, more complex and do not render colour well.

They also create a “beat effect”

How florescent lights work

Florescent Light Sources

How Florescent Light Sources Work

Phosphorescent Light SourcesSimilar to florescence, but is slowerMany glow-in-the-dark items are examples –

they need to “charge up” or absorb light energy for a while before emitting again.

Chemiluminescent Light SourceArtificial example is glow sticksBioluminescence is a natural form

Electric Discharge

Light Emitting DiodesMuch more efficient than incandescent or

florescent lights!However, they are more expensive and

complicated, relying on semiconductors .

Lasers

How are lasers different from other light sources?

Word is an acronym/description: Light Amplification by Stimulated Emission of Radiation

They are a single frequency of lightMuch more powerfulHow can we use a laser to calculate the distance

to the Moon?If we know... From experiment.. That a laser

pulse takes 2.56 seconds to return to us, after bouncing off the moon.

Lasers Applied to make Holograms!

Calculations with lightc = 3.0 x 10^8 m/s ORc = 300 000 000 m/sVelocity = distance/timec = d/t for light in a vacuum

What is light?Particle Theory: light is made of individual

particles called photons. These photons carry energy.

Particle theory explains the photoelectric effect – release of electrons dependant on frequency of light, not intensity.

Wave Theory: light moves like a wave, and can bend around corners in some circumstances

Wave theory explained double-slit experimentThe Truth: it is both at once (particle-wave

duality)

Prisms & ColoursWhite sunlight can be separated by a prism into 7

colours. Can you name all 7 colours of the rainbow?What colour is found below?Why is it so difficult for people to recognize indigo?What colour do people perceive the best?Why might this be?

Prisms & Colours!How do prisms work?Notice which colour

is on top... (bend least while in the prism)

Ray DiagramsA “light ray” is a line and arrow representing

the direction and straight-line path of light.Combinations of light rays can be used to

construct a “ray diagram”.Such ray diagrams describe the behaviour of light

in a certain circumstance (e.g. Being emitted from a light bulb, reflected off a mirror, or refracted into water)

ReflectionReflection occurs when light bounces off an object.This follows two laws of reflection:

The angle of incidence is equal to the angle of reflection

The incident ray, the reflected ray, and the normal line all lie in the same plane.

Reflection: surfacesSmooth surfaces create a mirror-like effect called “Spectacular

reflection”:

Reflection off rough surfacesDiffuse reflection results from the reflection

of light off an irregular surface.

Reflection: surfacesReflection is how we see the world

Example of laser: can’t see it until use dust!Examine light ray diagrams of smooth and rough

surfaces:All individual incident rays will still follow the laws of

reflection

Images in MirrorsThe image you see in the mirror isn’t real.... It’s “VIRTUAL”How far away does the “virtual” you appear to

be compared to the real you and the mirror?Try creating a light ray diagram of yourself

looking in the mirror.