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© 2000 Microsoft Clip Gallery
LIGHT: What
Is It?
Light Energy
– Atoms
• As atoms absorb energy, electrons jump out to a
higher energy level.
• Electrons release light when falling down to the
lower energy level.
– Photons - bundles/packets of energy released
when the electrons fall.
Light: Stream of Photons
© 2000 Microsoft Clip Galler
Electromagnetic
Waves
Speed in Vacuum
– 300,000 km/sec
– 186,000 mi/sec
Speed in Other Materials
– Slower in Air, Water, Glass
© 2000 Microsoft Clip Gallery
Transverse
Waves
© 2000 Microsoft Clip Gallery
Energy is perpendicular to direction
of motion
Moving photon creates electric &
magnetic field
– Light has BOTH Electric &
Magnetic fields at right angles!
Part 1 – Properties of Light
Light travels in straight lines:
Laser
Light travels VERY FAST – around
300,000 kilometres per second.
3x10^8 m/s
At this speed it can
go around the world 8
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.
We see things because they
reflect light into our eyes:
Homework
Luminous and non-luminous objects
A luminous object is one that produces light.
A non-luminous object is one that reflects light.
Luminous objects
Reflectors
Shadows
Shadows are places where light is “blocked”:
Rays of light
Properties of Light summary
1) Light travels in straight lines
2) Light travels much faster than sound
3) We see things because they reflect light
into our eyes
4) Shadows are formed when light is blocked
by an object
Part 2 - Reflection
Reflection from a mirror:
Normal
Reflected ray
Incident ray
Angle of
incidence
Angle of
reflection
Mirror
The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at
____ _____ angle it hits it.
The
same !!!
Using mirrors
Two examples:
2) A car headlight
1) A periscope
Colour
White light is not a single colour; it is made
up of a mixture of the seven colours 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.
The colours of the rainbow:
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Seeing colour
The colour an object appears depends on the colours
of light it reflects.
For example, a red book only reflects red light:
White
light
Only red light
is reflected
A pair of purple trousers would reflect purple light
(and red and blue, as purple is made up of red and blue):
Purple light
A white hat would reflect all seven colours:
White
light
Using coloured light
If we look at a coloured object in coloured
light we see something different. For
example, consider a football kit:
Shirt looks red
White
light
Shorts look blue
In different colours of light this kit would look different:
Red
light
Shirt looks red
Shorts look black
Shirt looks black
Blue
light
Shorts look blue
Refraction
Refraction is when waves bend or slow down due to
travelling in a different medium. A medium is something
that waves will travel through. When a pen is placed in
water it looks like this:
In this case the light rays are slowed down by the water
and are bent, causing the pen to look odd. The two
mediums in this example are water and air.
Words – speed up, water, air, bent
Corpuscular Theory of Light
(1704)
Isaac Newton proposed that light consists of
a stream of small particles, because it
– travels in straight lines at great speeds
– is reflected from mirrors in a
predictable way
Newton observed that the reflection of light from a mirror resembles
the rebound of a steel ball from a steel plate
Wave Theory of Light
(1802)
Thomas Young showed that light is a wave,
because it
– undergoes diffraction and interference
(Young’s double-slit experiment)
Thomas Young (1773-1829)
Waves versus Particles
A particle is localized in space, and has
discrete physical properties such as mass
A wave is spread out over many wave-lengths
in space, and could have amplitudes in a
continuous range
Waves superpose and pass through each
other, while particles collide and bounce
off each other
When is light wave and when is
it particle?
Whether light displays wave or particle
nature depends on the object it is
interacting with, and also on the
experimental set-up to observe it
If an experiment is set-up to observe the
wave nature (such as in interference or
diffraction experiment), it displays wave
nature
If the experimental set-up has a scale that
is corresponding to the quantum nature of
radiation, then light will displays particle
behaviour, such as in Compton scatterings
26
Diffraction
Interference
applet