Light
So far when we have talked about waves we have
talked about sound waves. Light is a special type of
wave.
Differences Between Sound Waves and
Light Waves
- Sound waves need matter (gas, liquid, or
solid) to vibrate in order to move from one
place to the next. Light does not. It can travel
through a vacuum.
- A sound wave tends to spread out in all directions
from its source where light wave travel in straight
paths
Why do sound waves spread out while
light wave travel in straight paths?
-Scientists agree that light has a dual nature,
-it is part wave and part particle
- A light particle is called a photon
Photon – massless bundles of concentrated
electromagnetic energy
-This explains why light can pass though some
things and not others. The fact that it is part wave
allows it to travel through some things. The fact
that it is part particle means that it can not travel
through all things.
The Speed of Light
Light travels very fast (much faster than sound)
- this is why you may see a jet fly over head
going faster than the speed of sound but not
hear it until it is well past you. The light gets
to your eyes allowing you to see the jet before
the sound waves get to your ears allowing you
to hear the jet.
http://www.youtube.com/watch?v=7NJz_MOnu3A&feature=related
Speed of Sound Vs Speed of Light
Sound travels at 340 m/s where light is much
faster.
Light travels at 300,000 km/s
-Light from the sun takes 8 minutes to reach Earth.
How far is the Earth from the sun?
v = distance / time
8 minutes = 480s
300,000 km/s = distance / 480s
Distance = 144000000km
Electromagnetic Waves
Energy travels in a wave that is partly electric
and partly magnetic.
Electromagnetic Spectrum – the range of
electromagnetic waves
Types of Electromagnetic Waves
Infrared – electromagnetic waves of frequencies
lower than the red of visible light
-Infrared lamps are used to keep food
warm at fast food restaurants
Types of Electromagnetic Waves
Ultraviolet – electromagnetic waves of
frequencies higher than those of violet
- this is the light given off from the sun
that you can not see but burns you
Light can go through some
materials and not others
Transparent – materials that transmit light
- light passes through these materials
because their atoms absorb the energy
and immediately reemit it as light
Light can go through some
materials and not others
Opaque – materials that absorb light without
reemission and thus allow no light through
-Opaque materials give off a shadow when
light is shined on them
What happens when light hits an
object?
Three things can happen when light hits an
object;
1. It can get absorbed
2. It can pass through
3. It can get reflected off the object
When light gets absorbed by an object
When light gets absorbed by an object that
object heats up because it is absorbing the
energy from the light.
- The colors that object appear to us happen
because the object absorbs all the colors
but the one we see. The color we see is
reflected to our eyes.
White Vs Black
When we see something that is white all colors
are reflected back to our eyes.
-when we see all the colors of the visible light
spectrum at once we see white.
When we see something that is black all colors
are absorbed by the object and none are
reflected to our eyes.
Clothing for the summer
On a sunny day a white shirt feels much cooler
than black.
-black absorbs all of the visible light and its
energy so it is warmer
-white reflects all of the visible light and their
energy so it feels cooler
Why is the Sky Blue?
The sky is blue because particles in the
atmosphere scatter high frequency light
waves.
This scattering of light waves causes many high
frequency colors of light (purple and blue) to
be trapped in the atmosphere so that is what
we see.
Why are sunsets orange and red?
By the time light gets to the ground at sunset all
the high frequency light gets scattered.
All the low frequencies of light make it to the
ground and that is why we see orange or red.
Why are the oceans blue?
Water molecules absorb red light and reflect
blue. That is why we see large bodies of water
as blue.
When light passes through an object
When light passes through an object that light
was passing though air.
When it passes through another material such
as glass, that light bends or refracts.
Refraction: the bending of a wave as it
crosses the boundary between two media
at an angle
Refraction
The reason that light bends or refracts is
because when it passes into a new material
that material has a different density or
number of atoms per area.
Incident ray – the ray of light as it is entering a
new material
Normal – a line perpendicular to the surface of
the new material
Refraction
Angle of Incidence: the angel between the
incidence ray and the normal
Refraction Ray: The ray of light that is refracted
as it passes through a new substance
Angle of Refraction – the angles between the
normal and the refracted ray
Refraction
Emergent Ray – the ray of light as it exits or
emerges from a new substance
When light passes through a new substance it
gets refracted. This is why an object in
water is not where it may appear to be
when you are looking at it outside of the
water.
Special cases when you refract
light.
Some objects refract light just the right amount
as it passes through that it separates white
light into its individual colors.
A prism, or rain drops separate white light into
the individual colors so you can see all colors
when light passes through.
This is how a rainbow works.
On a rainy day when the sun is still shining light
passes from the air through the rain drops.
When light does this it get refracted enough
to see all the colors in white light making a
rainbow.
Will you ever see the sun in a picture of a
rainbow?
Reflection
Reflection – the return of a wave back to its
original medium
Angle of Reflection – the angle between the
incident ray and the normal
Reflection
Law of Reflection – the angle of incidence and
angle of reflection are equal to each other.
Mirrors
Mirrors reflect light so you can see the object in
from of the mirror.
Virtual Image – an image that appears to be in
a location where light does not really reach
Different Types of Mirrors
Plane Mirror – a mirror that produces only
virtual images
- what ever is in front of a plane mirror is what
you see.
- the object will appear as far away in a plane
mirror as it is standing from that mirror
Convex Mirrors
A convex mirror is bent away from the object in
front of it. It makes the object look smaller
than it really is.
Concave Mirrors
A concave mirror is curves out at the ends
toward the object looking at it. A concave
mirror makes objects look larger then the
really are.
Lenses
Lens – a piece of transparent material, such as
glass, that refracts light as it passes through
-Lenses are used in all types of optical
devices such as telescopes, cameras,
microscopes, scopes, binoculars,…
Different Types of Lenses
Lenses can be used to look at things that are far
away or at thing that are too small to see with
the naked eye.
Different Types of Lenses
Converging Lens or Convex Lens – a lens that is
thicker in the middle then at the ends, causing
rays of light that are initially parallel to meet
at a single point (called a focal point)
Different Types of Lenses
Divergent Lens or Concave Lens – a lens that is
thinner in the middle than on the sides,
causing the rays of light to appear to originate
from a single point.
Principal Axis – a line drawn through the centers
of curvature of a lens
Focal Point – for a converging lens is the point at
which a beam of light parallel to the principal
axis converges.
Focal Plane – a plane perpendicular to the
principal axis that passes through either focal
point of a lens.
Images formed by converging
lenses
Real Image – an image formed by a single
converging lens
-real images are formed through a single
convergent lens are always upside down or
inverted
-this is how movies are projected at the movie
theater
-the image after going through the convergent
lens is always larger.
Images formed by diverging lenses
When you look through a divergent lens the
image that you look at is always smaller.
-divergent lenses are use as view finders in a
camera