Transcript What is Light - edhs2dscience

Consider the Following ??
What is Light
Electric Field...
•Is the space around an
electric charge, it does
not have mass.
It is not matter
• It exerts a force on
other electrically charged
objects and is able to do
work.
Electric field is
…energy
Magnetic Field…
• surrounds magnetic
materials, it also does not
have mass and also is not
matter
•
it is detected by the force
it exerts on other magnetic
materials
Magnetic field also is
energy
Do electric and magnetic fields
interact?
• Static (not moving) Electric field
does not exert force on magnetic
materials
• Static (not moving) Magnetic field does
not exert force of electrically charged
objects.
»However….
Electric and Magnetic
Interaction
• Moving or alternating (changing in
strength) electric field will generate
magnetic field. ie an electro magnet
Similarly,
• Moving or alternating (changing in
strength) magnetic field will generate
electric field.
Electromagnetic
Wave
• Electric and magnetic fields may pass energy
one to another without any wire
• This energy moves ahead (propagates) in a
straight line at 300,000 km/s
• Electromagnetic wave is a transfer of energy by
constant interaction between electric and
magnetic fields.
Like in Transversal Water Waves,
amplitude
Direction of
propagation
• Particles of medium (a toy duck and water
itself) do not move ahead, but swinging up
and down
• Energy moves ahead
(Watch animation of surface wave now)
In Longitudinal Waves (like Sound),
• Medium (e.g. air) gets
more compressed. This
repeats again and again.
The compression moves
away from the source.
• Particles of medium (air molecules) do not
move ahead, they swing back and forth
• Energy moves ahead
(Watch animation of sound wave now)
Mechanical Waves vs Electromagnetic Wave
• water wave: energy moves
ahead by vibrating water
• Sound wave: energy moves
ahead by vibrating air
• Wave in a rope: rope twists
to send energy impulse
ahead
• Electromagnetic wave does
not need medium. It can
travel through vacuum, a
space free of matter.
•Watch animation now
Wavelength(λ): max to max distance
Speed for all wavelength in
vacuum is :
V = 300,000 km/s
Short or long, all the waves
will make same distance
in one second
Shorter waves will get
chance to oscillate more
times during that time
shorter waves have
higher frequency
Frequency(f): # of cycles per second
Shorter waves with higher
frequency are more
energetic
Lower frequencies (long waves)
Wavelength
measured in:
km (kilometres)
metres
cm (centimetres)
mm (milimetres)
Continues over!
Highest frequencies (short waves)
The other end of spectrum
Wavelength measured
in:
nm (nanometres)
1nm = 10-9m
size of protein
nm (nanometres)
or Å (Angstrom)
Even more
ouch! - more
energy
1
Å = 10-10m
size of water molecule
What’s in the Middle of the
Electromagnetic Spectrum?
Wavelength measured in:
mm (milimetres), 10-3m
µm (micrometres),10-6m,
Size of bacteria
nm (nanometres), 10-9m
Size of one cell
All the wavelength / frequency ranges:
Frequency
Energy
ROY G BIV
• Light is an electromagnetic radiation
• It is pure energy moving ahead, it does
not need medium to propagate
• It moves always in the straight lines
within same (while in the same) medium
• All wavelengths (colours) move at the
same speed in vacuum, c = 3.00*108m/s
Photons: how light is produced
• - Electrons within atoms can
move from one energy level to
another.
- Electrons move to higher
levels when gain energy; they
release energy when move to
lower levels.
- Light is the energy released
by electrons when they change
energy level in atoms.
- For that, energy has to be
input first.
• Which electromagnetic wave is shorter:
gamma ray or radio wave?
• Gamma ray
• Which wave is more frequent: gamma ray
or radio wave?
• Gamma wave
• Which one is more energetic?
• Gamma ray
Light does act as a wave
It can be cut by a narrow slot (polarization)
Light does act as a wave
Light diffracts: spreads around when exits
narrow gate
When waves overlap, intensities of two waves
add up (interferention)