Transcript about Light - K

More about Light
“Let there be Light”—Gen.1:3
SPH4U – Grade 12 Physics
Unit 2
Tribal Challenge!
In a ripple tank, a point on the third nodal line
from the center is 35.0cm from one source and
42.0cm from another. The sources are 11.2cm
apart and vibrate in phase at 10.5Hz. Calculate
the wavelength and the speed of the waves.
3 points – first team correct
1 point – every correct answer after that
Tribal Challenge!
In a ripple tank, a point on the third nodal line from the center is 35.0cm from one source and
42.0cm from another. The sources are 11.2cm apart and vibrate in phase at 10.5Hz. Calculate
the wavelength and the speed of the waves.
Solution:
Review – True or False
1) All the different colours of light have the same
wavelength, frequency, and speed.
Review – True or False
1) All the different colours of light have the same
wavelength, frequency, and speed.
FALSE!
As you saw in the
simulation, the different colours of
light have different wavelengths.
Since the speed of light is constant
for all light, the frequency for each
of these colours is also different.
Review – True or False
2) When light is shone on to atoms, sometimes
the atoms aborb the light. When this happens
their electrons gain energy.
Review – True or False
2) When light is shone on to atoms, sometimes
the atoms aborb the light. When this happens
their electrons gain energy.
True!
As you saw in the
simulation, when a photon pulse is
shot at the atom, the electrons are
able to move to a higher energy
level.
Review – True or False
3) The critical angle is the largest angle of incidence
before total internal reflection happens. It is only
possible to have when going from a less dense medium
to a more dense medium.
Review – True or False
3) The critical angle is the largest angle of incidence
before total internal reflection happens. It is only
possible to have when going from a less dense medium
to a more dense medium.
False!
As you saw in the
simulation, the critical angle is
possible going from a more dense
medium into a less dense medium.
Review – True or False
4) If you add green light to red light, you get yellow light.
Review – True or False
4) If you add green light to red light, you get yellow light.
True!
As you saw in the
simulation, addition of colours of
light does not work the same as the
addition of colour pigments in
paint. When you add all the
colours of light together you get
white light. (When you add all the
paint colours together you get
black/brown).
Review – True or False
5) Newton was a believer in the particle theory of light.
Huygens was a believer in the wave theory of light.
Review – True or False
5) Newton was a believer in the particle theory of light.
Huygens was a believer in the wave theory of light.
True!
More About Light

The history of the study of light is very
interesting. Originally people thought light was
made up of tiny particles that objects sent out.
Later, people thought that light was a kind of
wave.
More About Light

Scientists today use the quantum model to
explain light. This model says that light is an
electromagnetic wave, made up of microscopic
(very tiny) particles called photons. So light is
both a particle and a wave.
More About Light

We now define light as a form of radiant
energy that the human eye can detect. It
sometimes acts like waves and sometimes acts
like particles.
The Electromagnetic Spectrum
This diagram
shows different
wavelengths of
electromagnetic
waves. We can
only see the
waves when the
wavelengths are
around 10-7
meters. Other
types of
wavelengths
make radio
waves,
microwaves,
infrared, and Xrays.
The Electromagnetic Spectrum

Last semester, we learned that sound waves
cannot travel through a vacuum. But Light waves
can! This is because they are electromagnetic
waves. Electromagnetic waves are produced by
vibrating electric charges.
An electromagnetic wave
is a transverse wave
which has both an
electric and a magnetic
part.
The Speed of Light

The speed of light is such an important quantity
in physics that it is given its own symbol, c. It
takes light only one billionth of a second to travel
the length of a 30-cm ruler. It takes light 8.3
minutes to travel from the Sun to Earth. The
speed of light in a vacuum is defined as c = 299
792 458 m/s or c = 3.00 x 108 m/s (to three
significant digits)
The Speed of Light

From what we have already learned about
waves, we remember the universal wave
equation, v = fλ. Use v = c (the speed of light) in
the following problem…
Example 1

A laser gives off a light with a wavelength
of 1064 nm. Calculate the frequency of the
laser.
Example 1

A laser gives off a light with a wavelength
of 1064 nm. Calculate the frequency of the
laser.
Propagation of Light

Light carries energy outwards at very fast
speeds in all directions from its source.

Rectilinear propagation is the term used to
describe the fact that light appears to travel in
straight lines through a uniform medium. It will
travel in a straight line until it hits another
medium.
Propagation of Light

In physics a straight line is often used to show
the direction of the light energy, because it is
difficult to draw all the light. These straight lines
are called rays. A group of these lines is called
a beam.
beam
sun
ray
Sources of Light

There are two sources of light. Luminous
sources and Non-luminous sources.
Luminous Sources
Non-Luminous
Produce their own light
Do not produce their
own light. Reflect light
from a luminous source.
Eg. Candle
Eg. Moon
The Transmission of Light

When light hits an object, it may be transmitted,
absorbed, or reflected.

Some materials let certain frequencies of light to
move right through them. Materials that light
can travel through easily are called
Transparent. These are materials you can see
through very clearly, like glass and water.
The Transmission of Light

Translucent materials are materials that light
moves though, but the light is scattered so you
can not see through them very clearly, like tissue
paper.

Opaque materials do not let any light move
through them. All light hitting opaque materials
is either absorbed or reflected. For example, the
table absorbs light.
Polarization of Light

The Polarization of Light provides more evidence that
light behaves as a wave – specifically, as a transverse
wave.
Natural sunlight transmits light
waves whose electric field vectors
vibrate equally in all planes
perpendicular to the direction in
which the light is moving. When
their electric field vectors are
restricted to a single plane by
filtration, however, then the light is
polarized.
Homework

Begin your end of Unit 2 Review
Assignment