Download Light and colour concepts - Christchurch Girls` High School

Objectives
1) Recall that the electromagnetic spectrum is a family of
waves and give 2 examples
2) Recognise that light travels in straight lines.
3) Explain, in terms of colour absorption and reflection, why
an object appears to be a particular colour and why its
appearance changes in different coloured light.
4) Explain the effect of coloured filters on light.
5) Demonstrate how white light can be spilt using a prism
and use the terms dispersion and spectrum.
6) Label the parts of the human eye and describe their
functions
7) Describe how we see ourselves in mirrors (plane and
curved).
8) Describe the effect of lenses on light
9) State some uses for concave and convex mirrors
10) State some uses for concave and convex lenses.
What is light?
• Light is a form of energy that travels in waves.
• It is part of the family of waves called the electromagnetic
spectrum (see powerpoint on electromagnetic spectrum)
• Light waves are too small to see but different colours of light
have different wavelengths.
• Red light has a longer wavelength than violet light.
Wavelength
• Wavelength is the distance between the
crests of two waves.
• The symbol for wavelength is the Greek
symbol, λ
Experiment: Can light bend?
• Collect ray box and a piece of rubber tubing for
your group.
• Plug in and switch on the ray box.
• Close one eye and look through the tubing at the
light with your other eye.
• What do you need to do to the tubing to be able
to see the light?
• What does this tell you about light?
• Draw a diagram in your Science book to explain
this experiment.
Light terminology
Light rays can be:
• Absorbed (taken in) by an object
• Reflected off an object
• Refracted (pass through an object but change
direction)
Objects can be:
• Transparent (let all light pass through)
• Translucent (let some light pass through)
• Opaque (let no light through)
Different Materials
Transparent
Opaque
Allows light to pass
through freely. Only a
small amount of light is
absorbed or reflected.
Prevents any light from
passing through it. It will
only absorb or reflect light.
Translucent
Most light rays pass
through, but are scattered
in all directions.
Behaviour of light
• Light travels at 300 000 kms-1 in space.
This is called the speed of light.
(the speed of sound is only 340 ms-1)
• Light travels in straight lines
• A stream of light travelling in the same
direction is called a ray.
Light travels in a
straight line.
Solid Object
Shadow
Light travels in straight lines
Rays and Reflection
Light rays which
are coming in
are called
incident rays
Light rays that
have bounced
off an object
are called
reflected rays
Incident ray
Object
surface
Reflected ray
Object
surface
The Normal
• The normal is an imaginary line at right
angles to the object surface at the point
where the incident ray hits the surface
normal
Incident ray
Object
surface
Drawing and labelling rays off of a plane mirror
-
Conclusion
The angle of incidence always equals the
angle of reflection.
Using plane mirrors
By positioning two plane
mirrors at 45° to each
other at either end of a
tube we can make a
___________.
periscope
Periscopes are used
in _____________.
submarines
How periscopes work
• Periscopes prove that
light travels in straight
lines
Plane Mirrors Reflected Images
The image formed in a plane mirror is always the
________ size as the object. It is the ________
way up, but the _________ are swapped over.
The image is always as far behind the mirror as
the object is in front.
We say the image is laterally inverted.
The image in a plane mirror is virtual because it
forms in a place where light does not pass.
(Answer)
The image formed in a plane mirror is always the
same size as the object. It is the right / same
way up, but the sides are swapped over.
Concave mirrors
(Shaped like a cave)
Concave mirrors cause
rays to CONVERGE
(come together) and meet
at a focal point called the
focus.
The focal length is the
distance from the focus to
the mirrors reflecting
surface
Concave Mirror
Solar cookers
A solar cooker is a device which uses the energy of direct sunlight to heat, cook
or pasteurize food or drink. Many solar cookers presently in use are relatively
inexpensive, low-tech devices, although some are as powerful or as expensive as
traditional stoves,[1] and advanced, large-scale solar cookers can cook for
hundreds of people.[2] Because they use no fuel and cost nothing to operate,
many not for profit organizations are promoting their use worldwide in order to
help reduce fuel costs and air pollution, and to slow down
the deforestation and desertification caused by gathering firewood for cooking.
Solar cooking is a form of outdoor cooking and is often used in situations where
minimal fuel consumption is important, or the danger of accidental fires is high,
and the health and environmental consequences of alternatives are severe.[3]
Cooking sausages on a solar cooker
Parabolic solar cookers can reach high
temperatures over 400°C (750°F) and can
be used to fry, grill , steam and
bake. SolSource solar stove is an example
of a high temperature parabolic cooker.
Optical (visible light) telescopes
Optical telescopes use a
concave mirror or a convex
lens to collect light from
distant objects.
The greater the size of the
mirror or lens:
- the more light is collected
- the more the image can be
magnified
- the brighter the image seen
- the more detail seen in the
image produced.
The Mount Palomar telescope in
California, with an objective mirror of
5m (200 inches), was for many years
the world’s largest telescope
Convex mirrors
Convex mirrors
cause rays to
DIVERGE.
The focal point is
behind the mirror
and virtual.
Images in a convex mirror
Convex mirror
Small/large
Upside down/right
way up
Close up
Far away
Light rays onto a covex mirror
Uses of Convex Mirrors
Lenses
There are two different types of lenses.
• Converging/biconvex
• Diverging/ Biconcave
Below you can see the effect these lenses have
on light.
-
Convex lens
Concave lens
Light bends as it goes through the lenses
Measuring the focal length of a lens
-
-
Focal length is the distance between the middle of the lens and
the focal point
Uses of lenses
•
•
•
•
•
•
Binoculars
Glasses
Camera
Telescope
Magnifying glass
Projectors
How do glasses correct vision.
Contact lenses work
in same way as
glasses. However
the lens is not glass,
but a type of
breathable plastic
and it sits on your
cornea.
The Eye
Parts of the eye and their functions
• Cornea – thin clear covering over the front of the
eye.
• Pupil – hole in the centre of the iris, it changes
size in bright light and dull light
• Iris – coloured part of the eye, makes the pupil
bigger or smaller
• Lens – focuses light onto the retina
• Retina – cells at the back of the eye that can
detect light
• Optic nerve – carries message from the retina
cells to the brain.
Parts of the eye and their functions
• Cornea – thin clear covering over the front of the
eye.
• Pupil – hole in the centre of the iris, it changes
size in bright light and dull light
• Iris – coloured part of the eye, makes the pupil
bigger or smaller
• Lens – focuses light onto the retina
• Retina – cells at the back of the eye that can
detect light
• Optic nerve – carries message from the retina
cells to the brain.
The lens in the eye is a convex lens
.
Convex lenses are thicker at the middle. Rays of light that pass
through the lens are brought closer together (they converge). A
convex lens is a converging lens.
When parallel rays of light pass through a convex lens the
refracted rays converge at one point called the principal focus.
The distance between the principal focus and the centre of the
lens is called the focal length.
In the eye the convex lens turns the image upside down. The brain
turns it back up the right way, so we do not trip over things.
An experiment was done where a special pair of glasses was designed
that turned everything a person saw upside down. After a few days of
wearing these glasses the brain automatically corrects what it sees.
Remarkably when you take the glasses off everything you then see is
upside down. It then takes the brain a few days to turn it back up the
right way!
How we see things!
Splitting white light
-
When white light passes through a
prism it is DISPERSED – this means
it splits into separate colours.
We see the 7 colours of the
SPECTRUM when this happens.
Red light bends the least (it has the
longest wavelength).
Violet light bends the most (it has
the shortest wavelength).
The colours of the spectrum - ROYGBIV
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Newton’s
disc
Colour in a paper or card circle with the colours of the spectrum.
Using string or a pencil spin your
disc around.
What did you observe?
What do you predict you will see?
What does this tell you?
Newton’s disc
animation
Seeing
colours
How do you see non-luminous objects
such as a book?
You see a non-luminous object
when light hits the object and is then
reflected into your eyes.
So how do we see different colours?
Why does a red dress look red?
Why does a green apple look green?
Primary colours animation
Primary and secondary
colours
Colours are made by mixing other colours of light.
There are three primary colours of light used to make all other colours.
What are these colours?
The three primary
colours of light are
red, green and blue.
green
red
magenta
blue
The colours made by mixing two primary colours are called
the secondary colours – magenta, yellow and cyan.
Which primary colours?
Seeing red
Why does a red snooker ball look red in white light?
White light is made up of
a spectrum of colours.
The snooker ball absorbs all the
colours of the spectrum except
red.
Only red light is reflected into your
eye, so the snooker ball appears
red.
Seeing green
Why does a green snooker ball look green in white light?
The snooker ball absorbs all the
colours of the spectrum except
green.
Only green light is reflected into your
eye, so the snooker ball appears
green.
Seeing black
Why does a black snooker ball look black in white light?
The snooker ball absorbs all the
colours of the spectrum.
No light is reflected into your eye,
so the snooker ball appears black.
Seeing white
The snooker ball does not absorb
any of the colours of the
spectrum.
The whole spectrum of light is
reflected into your eye, so the
snooker ball appears white.
Why does a White snooker ball look
in white light?
Seeing magenta
Why does a magenta ball look magenta in white light?
The snooker ball absorbs all the
colours of the spectrum except red
and blue.
Red and blue light are reflected into
your eye, so the snooker ball appears
magenta.
Which
colour
is
reflected?
Which colours of light are reflected by these clothes?
Seeing different colours
What colours are absorbed by this frog’s skin? What colours are
reflected into your eyes?
This part of the skin absorbs all
the colours of the spectrum except
red, and so reflects red light.
This part of the skin absorbs
all the colours of the spectrum
and none are reflected.
Seeing
different
colours
What colours are absorbed by this flower?
What colours are reflected into your eyes?
This part of the flower absorbs
all colours except red and green.
It reflects red and green light,
and so appears yellow.
This part of the flower
absorbs no colours.
It reflects them all
and so appears white.
Using coloured filters
Filters let certain colours of light pass through, but absorb all other colours.
Using different coloured filters placed in front of
your eye, look around the classroom and see
what effect they have on your vision.
object
filter
Red, blue and green filters
A red filter absorbs all
colours…
…apart from red light.
A blue filter absorbs all
colours…
…apart from blue light.
A green filter absorbs all
colours...
…apart from green light.
Magenta,
cyan
and
yellow
A magenta filter absorbs all
colours…
filters
…apart from red and blue.
A cyan filter absorbs all
colours…
…apart from green and blue.
A yellow filter absorbs all
colours...
…apart from red and green.
Using colour filters
Seeing colours in coloured
light
Why do colours look different in different coloured light?
Consider a red ball in red light.
The red light
shines on the
ball.
The red ball reflects the red
light and so appears red.
Seeing colours in coloured
light
What colour does a red ball appear in green light?
The green light
shines on the
ball.
The red ball only reflects red light
and so it absorbs the green light.
So in green light, this ball does not
reflect any light and so appears
black.
Seeing colours in coloured
light
What colour does a green ball appear in blue light?
The blue light
shines on the
ball.
The green ball only reflects green
light and so it absorbs the blue
light.
So in blue light, this ball does not
reflect any light and so appears
black.
Seeing colours in coloured light
What happens when using a coloured filter which lets through more than one
type of light?
What will a red ball look like in magenta light?
The magenta
light, which is a
mixture of red and
blue light, shines
on the ball.
The red ball only reflects red light
and so absorbs the blue light.
So in magenta light, this ball
reflects the red light and appears
red.
Coloured light activity –
instructions
The next two slides include a girl wearing a t-shirt and trousers.
The girl is standing in a different coloured light each time. The colour of this light
is written at the top of the slide.
The aim of each activity is to decide what colours the girls’ clothes would
appear in each type of coloured light.
Coloured light activity 1
Coloured light activity 2
Flag colours in different
coloured light
How would the colours in this flag appear under these lighting conditions?
a)
red light
b)
green light
c)
blue light
What colour does it appear?
What colour does each object appear under the given lighting conditions?
Object (Colour) Colour Filter
red ball
red
red ball
blue
blue book
green
blue book
magenta
green apple
cyan
green apple
magenta
red and blue tie
red
Appearance
red
black
black
blue
green
black
red and black
Rainbows
Sunlight is refracted as it enters a raindrop, which
causes the different wavelengths of visible light to
separate. Longer wavelengths of light (red) are bent
the least while shorter wavelengths (violet) are bent
the most.
Page 137 SW9
Anagrams
Multiple-choice quiz
18. Explain, in terms of colour absorption, why an object appears to be a particular
colour and why its appearance changes in different coloured light.
How and why we see colour
• http://www.amonline.net.au/colour/colour14.swf
To demonstrate why objects are certain
colours click on the hyperlink below and
go to:
Seeing colour
Reflection and absorption
http://www.amonline.net.au/colour/colour14.
swf
Telescopes
• A telescope is an instrument designed for the
observation of remote objects. The earliest known
telescopes are credited to three individuals, Hans
Lippershey and Zacharias Janssen, spectacle-makers in
Middelburg, and Jacob Metius of Alkmaar "Telescope"
(from the Greek tele = 'far' and skopein = 'to look or see';
teleskopos = 'far-seeing') was a name invented in 1611
by Galileo Galilei.
There are three types of telescopes
• Click here to see the telescopes in action
Refractor
• Refracting telescopes gather light with a lens,
directing it to the eyepiece.
Reflecting telescopes
Reflecting telescopes gather light with a mirror,
reflecting it before directing it to the eyepiece.
The simplest type of reflecting telescope is
called a "Newtonian," after Sir Isaac Newton
who invented them.
Compound
• Another major class is the Catadioptrics
(compound) these systems combine both
mirrors and lens elements. One of their main
features is a "folding" of the optical system, to fit
a long effective focal length into a shorter tube.
Pictures from telescopes
Spiral Galaxy
Mars
Eagle Nebula
Sun