The Nature of The Nature of LightLight

Part 1 – Properties of Part 1 – Properties of LightLight

Light travels in straight lines:

Laser

Light travels VERY FAST! Approx. 299,792,458 m / s

At this speed, a traveler could go around the world 7.5 times in one second.

Light travels much faster than sound. For example:

1) Thunder and lightning start at the same time, but we will see the lightning first.

2) When a starting pistol is fired we see the smoke first and then hear the bang.

It’s tricky…It’s tricky…

Light can be described as a wave OR as a stream of particles! Wave-Particle Duality!

http://www.youtube.com/watch?v=DfPeprQ7oGc

Light produces interference Light produces interference patterns (interacts) like patterns (interacts) like

waves do! waves do!

Light: -is a transverse wave-is an electromagnetic wave-can be described by amplitude,

wavelength, and frequency

But the wave model But the wave model doesn’t explain doesn’t explain

everything!everything!Example:When light strikes a piece of metal,

electrons may fly off the metal’s surface. Like they are separate particles?

So… So… Light can also be modeled Light can also be modeled as a stream of particles.as a stream of particles.In this model, units of light are called

photons.

* Photons are not ordinary matter!• Have no mass• Little bundles of energy• Energy carried by a photon is

proportional to frequency.

IntensityIntensity

• Amount of light illuminating a surface.• Depends upon the number of photons

per second that pass through an area of space.

• Intensity of light decreases as distance from light increases. (Light spreads out!)

We see things because they reflect light into our eyes:

Homework

Part 2 - ReflectionPart 2 - ReflectionReflection from a mirror:

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

The Law of ReflectionThe Law of Reflection

Angle of incidence = Angle of Angle of incidence = Angle of reflectionreflection

In other words, light gets reflected from a surface at ____ _____ angle it hits it.

The same !

!!

Clear vs. Diffuse ReflectionClear vs. Diffuse Reflection

Smooth, shiny surfaces have a clear reflection:

Rough, dull surfaces have a diffuse reflection.

Diffuse reflection is when light is scattered in different directions

Using mirrorsUsing mirrorsTwo examples:

1) A periscope

2) A car headlight

RefractionRefraction

Light waves BEND (because they change speed) when passing from one medium to another!

Electromagnetic Electromagnetic SpectrumSpectrum

Human eyes can detect light waves that have wavelengths of 400nm (violet) to 700nm (red) Visible Spectrum

Visible Spectrum is only a small part of the EM Spectrum!

ColorColor

White light is not a single color; it is made up of a mixture of the seven colors of the rainbow.

We can demonstrate this by splitting white light with a prism:

This is how rainbows are formed: sunlight is “split up” by raindrops.

Adding colorsAdding colorsWhite light can be split up to make separate

colors. These colors can be added together again.

The primary colors of light are red, blue and green:Adding blue and

red makes magenta (purple)

Adding blue and green makes cyan

(light blue)

Adding all three makes white again

Adding red and green makes yellow

Seeing colorSeeing colorThe color an object appears depends on the

colors of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is

reflected

A white hat would reflect all seven colours:

A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and

blue):

Purple light

White

light

Using filtersUsing filtersFilters can be used to “block” out different colours of

light:

Red Filte

r

Magenta

Filter

http://www.youtube.com/watch?v=R3unPcJDbCc

EM SpectrumEM Spectrum

• Consists of waves at all possible energies, frequencies, and wavelengths.

• Each part has unique properties!• Radio waves – communications/radar• Microwaves – cooking/communication• Infrared Light – warmth• UV Light – part of sunlight• X rays & gamma rays – medicine