The Physics of Light
MODULE 4 WEE 4
Why and how do we see
light?
What Is Light?
In this Powerpoint presentation:
1)
2)
3)
4)
Properties of light
Colors
Reflection - Mirrors
Refraction - Lenses
Properties of Light
Light travels in straight lines:
Laser
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.
Light travels VERY FAST – about
300,000 kilometers per second or
186,000 miles per second.
At this speed light
could travel the
equivalent of 8 times
around the world in
one second!
Light can create shadows
Shadows are places where light is “blocked”:
Rays of light
Light travels in waves
A wave is a disturbance that transfers
energy from place to place.
What characteristics do light
waves have?
Light waves are like other kinds of waves.
They have crests, troughs, wavelength,
frequency and speed. However, light waves
are electromagnetic! Light waves have a
component that is electric and magnetic!!
We see things because they
reflect light into our eyes! This
is literally the only way we get to
see things! ALL objects we see
either create light or reflect
light!!
Homework
Electromagnetic Radiation
•Light is a form of energy called
Electromagnetic Radiation Electromagnetic Radiation comes from the sun and
other sources
•Travels at the speed of light
•Is mostly invisible
•Carries energy
•Is VERY important to society and is something YOU
can’t be without for many reasons
Properties of Light summary
1) Light travels in straight lines and is a form of
energy
2) Light travels much faster (a million times 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
5) Visible light is only a small part of the
electromagnetic spectrum; the other parts are
invisible to the human eye but not necessarily the
eyes of other animals.
Part 2 - Reflection and
Mirrors
Regular vs. Diffuse Reflection
Smooth, shiny surfaces
have a regular
reflection:
Rough, dull surfaces have
a diffuse reflection.
Diffuse reflection is when
light is scattered in
different directions
Using mirrors
Two examples:
2) A car headlight
1) A periscope
Types of Mirrors: Plane, Concave, Convex
Plane mirrors are
flat and make
“virtual images”.
The images is
“virtual” because
the light rays do
not come from the
image, they only
appear to
 Concave Mirrors are curved inward - just
remember, you go “in” to a cave, and “in” to
a concave mirror!
 Convex mirrors are mirrors that curve
outward
 Convex mirrors are used for security
purposes, in cars etc. They allow us to see
a wide view. The images they make are
virtual
Review of Reflection
1. Two types of reflection, regular and diffuse
2. Mirrors come in 3 types: plane, convex and
concave. Each has a distinct shape and use.
3. Mirrors can make virtual images or real
images depending on their type
4. Virtual images - light rays do not meet and
the image is always upright or right-side-up
5. Real images - always upside down and are
formed when light rays actually meet
Refraction
•Refraction is when waves speed up or slow down due to
travelling in a different medium
•A medium is something that light waves will travel
through
•Light rays are slowed down by the water
•Causes the ruler to look bent at the surface
• The mediums in this example are water and air
The degree that light bends when it enters a
new medium is called the “index of refraction”
Light hitting at an
angle is bent
Light hitting
straight on is
not bent
Lenses work because of refraction! Light is
bent as it enters a new medium, glass!
1.
Thicker in middle, thin
on edges
2. Light rays meet to
form a focal point
3. Makes real images
since light rays come
together from a light
source, but can also
make virtual images
(center picture)
4. Used in cameras,
telescopes, human eye
Concave Lenses
1.
Concave lenses are
thin in the middle
and thicker on the
edges
2. Since light rays
never meet with
this lens, they make
virtual images only
3. Used in cameras &
telescopes to
correct spherical
aberation, and also
eyeglasses
Review of Refraction
1. Light rays slow down when they enter a new
medium
2. Refraction can cause light rays to change
their direction
3. All transparent materials have their own
“index of refraction”
4. Light is refracted when it passes through
lenses and this creates images
Color
Part 3 - Color In this section of the
Powerpoint you will learn about color and
how we can mix colors to create other
colors. We will also learn how light helps us
to see various colors around us.
Color
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.
The colors of the rainbow:
Just remember ROY G. BIV
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Adding colors of light
White 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 red
and green
makes yellow
Adding blue and
green makes cyan
(light blue)
Adding all
three makes
white again
Seeing color
The 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
In different colors of light this shirt would look different:
Red
light
Shirt looks red
Shorts look black
Shirt looks black
Blue
light
Shorts look blue
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 colors:
White
light
Using filters
Filters can be used to “block” out different colours of light:
Red
Filter
Magenta
Filter
Some further examples:
Object
Red socks
Blue teddy
Color of light
Color object
seems to be
Red
Red
Blue
Black
Green
Black
Red
Black
Blue
Green
Red
Green camel
Blue
Green
Red
Magenta book
Blue
Green
Investigating filters
Color of filter
Red
Green
Blue
Cyan
Magenta
Yellow
Colors that could be “seen”
Red
Blue
Green
Yellow
Cyan
Magenta
White
Part 2 - Reflection and
Mirrors
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 !!!
How do haloes, sundogs, primary rainbows, secondary
rainbows, and supernumerary bows occur?
Haloes
Solar halo, which is also called
gloriole, icebow or nimbus, is a light
phenomenon that happens when light
shines through clouds that are
composed of ice crystals. Light
refracts upon passing through the ice
crystals and also reflects upon hitting
the crystal’s faces; these events
cause the formation of the bright
ring around the Sun or Moon. Halo is
usually bright white ring but may also
have colors due to the dispersion of
light upon striking the ice crystals.
Below is a picture of a halo produced
around the Sun.
How do haloes, sundogs, primary rainbows, secondary
rainbows, and supernumerary bows occur?
Sundogs
Sundogs, or parhelion (with the sun),
happen due to the refraction of light
upon hitting the small crystals that
make up cirrus or cirrostratus clouds.
These crystals are hexagonal in shape
and with faces almost horizontal upon
drifting; these cause the formation
of spots of light (sundog) on either
side of the Sun, or the Moon, when
light strikes them at a minimum angle
of 22 degrees as shown in the image
below. Since red light is the least
refracted compared to blue this
makes the inner edge of a sundog to
be red hued.
The image below shows an actual sundog which has
a red-hued inner edge.
How do haloes, sundogs, primary rainbows, secondary
rainbows, and supernumerary bows occur?
Rainbows
A rainbow is a light phenomenon
formed from the combination of
several light properties like refraction,
reflection, and dispersion. Rainbows
are usually seen after rainfall because
they are formed when light strikes the
scattered raindrops in the atmosphere.
As shown in the image below, light is
refracted upon hitting a raindrop that
serves as the prism. Due to the
differences in the frequency of the
colors of visible light, they are
refracted at different amounts and
are dispersed. The dispersed colors
reflect upon hitting the other side of
the raindrop and then refracted again
as they go out. The colors go out
dispersed and reach the observer’s
eyes.
Why clouds are usually white and
rainclouds dark
Rain Cloud
White is how our eyes perceive all
wavelengths of sunlight mixed
together. When it's about to rain,
clouds darken because the water
vapor is clumping together into
raindrops, leaving larger spaces
between drops of water. Less light is
reflected. The rain cloud appears
black or gray.
Why is the sky blue? Why are
sunsets red?
Blue and Red
Take a look at light through a prism and
notice all the different colors that you can
see. Light that looks white to our eyes
actually is made up of many different
colors. Each color can be thought of as a
light wave with a different wavelength (or
size). Within the small range of
wavelengths (or colors) that we can see
with our eyes, the shorter waves are blue
and the longer ones are red. Colors such as
green, yellow, and orange lie in between
the blue and red ends of the visible
spectrum.
Why is the sky blue? Why are
sunsets red?
Blue and Red
When light comes from the sun, all
these light waves of different
wavelengths travel through empty
space. When they reach Earth’s
atmosphere, the light waves can
interact with particles in the air like
dust, water droplets, and ice crystals.
Because of the extremely small size of
visible light waves (less than one
millionth of a meter), these light waves
also interact the tiny gas molecules that
make up the air itself. The light waves
bounce off these particles just like you
might bounce and get jostled in a busy
hallway. As the light waves bounce in
lots of different directions, we say they
have been scattered.
Why is the sky blue? Why are
sunsets red?
Blue Sky
How light waves get scattered
depends strongly on the size of the
particle
compared
with
the
wavelength of the light. Particles
that are small compared with the
light wavelength scatter blue light
more strongly than red light.
Because of this, the tiny gas
molecules that make up our Earth’s
atmosphere (mostly oxygen and
nitrogen) scatter the blue portion
of sunlight in all directions,
creating an effect that we see as a
blue sky.
Why is the sky blue? Why are
sunsets red?
Blue Sky
Within the visible range of light,
red light waves are scattered the
least by atmospheric gas molecules.
So at sunrise and sunset, when the
sunlight travels a long path through
the atmosphere to reach our eyes,
the blue light has been mostly
removed, leaving mostly red and
yellow light remaining. The result is
that the sunlight takes on an
orange or red cast, which we can
see reflected from clouds or other
objects as a colorful sunset (or
sunrise).
Why is the sky blue? Why are
sunsets red?
Small particles of dust and pollution
in the air can contribute to (and
sometimes even enhance) these
colors, but the primary cause of a
blue sky and orange/red sunsets or
sunrises is scattering by the gas
molecules that make up our
atmosphere. Large particles of
pollution or dust scatter light in a
way that changes much less for
different colors. The result is that
a dusty or polluted sky is usually
more grayish white than blue.
Similarly, cloud droplets (typically
10 millionths to 100 millionths of a
meter) are much larger than visible
light waves, so they scatter light
without much color variation. This is
why light scattered by clouds takes
on the same color as the incoming
light. For example, clouds will appear
white or gray at midday and orange
or red at sunrise or sunset. In this
way, clouds act as a screen on which
nature’s colors are painted. This is
why sunsets or sunrises are so much
prettier when some clouds are
available to show us the colors.