Transcript Light

Light
Year nines it is time to light up and
understand some physics.
Mr Posthumus
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Terminology
Explain the meaning of the following
SE 9 page 170
• Luminous
• Transparent
• Translucent
• Opaque
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• Luminous- luminous objects produce
light. Example, sun, lights and fires.
• Transparent-materials like glass that
allow light to pass through.
• Translucent- a translucent material
allows light to pass through, but
scatters it so you can’t see through
clearly.
• Opaque -light can’t pass through.
Light is absorbed or reflected
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The sun ; Why is the sun Luminous
• Hydrogen- Helium
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Is the moon Luminous?
• Why do we see the moon?
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Answer
• it is non-luminous and reflects light
© 2003 Mike Maloney
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Light
• What is LIGHT?
• How does light travel
• WHERE DOES IT COME FROM?
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What is Light
• Light is a special type of wave called
an electromagnetic wave
• LIGHT is actually a type of something
called ELECTROMAGNETIC RADIATION.
• So, what is electromagnetic radiation
and electromagnetic waves?
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Electromagnetic Waves
• When something creates energy it
also emits radiation. Depending on
the amount of energy, the object
will emit different types of
electromagnetic radiation.
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Electromagnetic Waves
• Electromagnetic waves are special
in the fact that they do not need
a medium to propagate through.
• But what is creating the
disturbance? What is emitting
this energy?
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Electromagnetic Waves
• Electrons in materials are vibrated and
emit energy in the form of photons,
which propagate across the universe.
• Photons have no mass, but are pure
energy.
• Electromagnetic Waves are waves that
are made up of these “photons”.
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Electromagnetic Waves
• Electromagnetic waves are
everywhere.
• Light is only a small part of them
–
–
–
–
Radios
TVs
Microwaves
Light (Visible/UV/InfraRed)
– Radiation
– Lasers
– X-Rays
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Electromagnetic Spectrum
© 2003 Mike Maloney
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Visable spectrum
• The light we see is know as visible or
white light – although it is not that
simple.
• The light is not really white, the
white we see is a combination of all
the colors of the rainbow.
• Remember R-O-Y G. B-I-V from art
class. Or VIBGYOR
• When all of these light waves are
combined we see white light.
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Back to Light
• So, why can we only see a
small portion of these E-M
waves?
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Visible Light
• We now know what we see is part of
the electromagnetic spectrum. We
know that the light waves enter our
eye, and stimulate the retina.
Electrical impulses to be sent to the
brain which creates this visual
image.
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Our eyes
•
•
•
•
Lens : Focus of light (refraction)
Cornea: Transparent allows light to enter the eye
Iris: Controls amount of light entering the eye
Retina : Converts light into nerve impulses
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Why do we have two eyes?
Purpose of binocular vision
• the use of both eyes together to create a three dimensional
image (Have depth of vision)
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Eye defects and type of lens to correct the problem
• Short sightedness: Cannot focus on distant objects Concave
lens to correct
• Far sighted: Cannot focus on near objects Convex lens to
correct
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Colour
• Different objects may emit different
wavelengths of E-M radiation, so we
would see that light as different
colors.
• But why do we see colors in objects
that reflect light?
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Color Reflection
• So if we see something as WHITE,
that means …
– It reflected back all the wavelengths of
light to our eyes (White light is a
mixture of different colours)
• If we see something as RED or BLUE
– It reflected only the RED or only the
BLUE wavelengths
– The others were absorbed.
• And if we see something as black?
– It did not reflect back any of the light.
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Colour
• Why is the sky blue?
• Why are sunsets red?
• Why is water greenish-blue?
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Answer
• The sky is blue because light is scattered by dust particles.
Blue light is scattered more so it enters the eye from all
parts of the sky.
• Sunsets and sunrises are orange or red.
Red is less scattered which means more of it left to enter
the eye when looking at a sunset.
• the answer to ‘why is the sunset red?’ is: At sunset, light
must travel farther through the atmosphere before it gets
to you, so more of it is reflected and scattered and the sun
appears dimmer. The colour of the sun itself appears to
change, first to orange and then to red because even more
of the short wavelength blues and greens are now scattered
and only the longer wavelengths(reds, oranges) are left to
be seen.
• Interesting fact
• It is said that outback skies(in desert regions) are more
vividly blue than European winter skies because the
outback has more dust= more scattering
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In the beginning……there were 3 colours
• Most people will be familiar with the ideas of basic colour mixing
ie
• Yellow + Blue makes Green
• Red + Yellow makes Orange
• Blue + Red makes Purple (some texts refer to the mix as being
“Violet”…..but you get the general idea)
• So….we understand that we can mix colours and arrive at another
colour. BUT there are some colours that cannot be mixed and they
are called “Primary Colours”….they are Red, Yellow & Blue. We
cannot create these colours from other colours but if you start
with these 3 colours you can mix an almost infinite number of
other colours eg Green, Orange & Purple.
• The 3 colours that were the result of mixing two Primary colours
are called Secondary colours.
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Reflection
• SE 9 pp.170-172
• Images are formed by mirrors
because they reflect light.
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Reflection. Plane mirror
When light is reflected the angle of incidence is equal to the
angle of reflection.
A= incident ray B= angle of incidence
C=Normal D=Angle of reflection E= Reflected ray
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Image reflected by a plane mirror
• The image reflected is the same size
and the right way up
© 2003 Mike Maloney
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Curved Mirrors •
Concave mirror: concave means to bend inwards(caved in)
• Concave mirrors can give you an enlarged image if you
stand close
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Convex mirror
• Convex mirrors curve outwards. Some examples of convex
mirrors are a car's side mirrors, spoons and security
mirrors.
• When beams of light come into contact with convex mirrors
the light rays diverge (spread out)
• Convex mirrors improve the field of view (make it possible
to see more the area behind
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Optical fibres
• Fibre optic technology has virtually replaced copper wire in longdistance telephone lines, and it is used to link computers within
local area networks. Fibre optics is also the basis of the fibrescopes
used in examining internal parts of the body (endoscopy) or
inspecting the interiors of manufactured structural products.
• Through a process known as total internal reflection, light rays
beamed into the fibre can propagate within the core for great
distances with remarkably little attenuation
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Practical Time: Reflection
• Inquiry 1,2 and 3 (can be done with
Hodgson light boxes to show the ray
paths and then the image formation
can be done with concave and
convex lenses) p. 170-171
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Speed of light
• It has been found that the speed of
light is --– 3 x 108 or 300,000,000 m/s
– 300 000 km/s. Speed of light is
constant throughout the universe,
as long as light is in a vacuum.
– The density of the medium through
which light passes effects the
speed of light
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Refraction – bending of light
• When light changes direction as it enters a new
material this is known as refraction. It is due to a
change in the speed of the light.
• Convex lenses are thicker in the middle .They
cause parallel rays of light to converge. Convex
lenses are used in magnifying glasses (make
objects bigger)
• Optical devices like a projector use a convex lens
to create an enlarged image on the screen
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Concave lenses
Concave lenses are thinner at the middle. Rays of light
that pass through the lens are spread out (they diverge).
A concave lens is a diverging lens. They make objects
smaller
©
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Mirages : Refraction of light
• Mirages commonly form when the air near the ground is much
warmer than that above it. Warm air has a different density than
cold air. When light waves travel through one medium and into
another, they are bent or refracted from their straight-line path.
• What causes the mirage illusion is that our mind initially interprets
the light rays reaching our eyes as having come along a straight
path, which in this example must have originated on the ground.
Thus, we see that patch of sky and cloud "on the ground" and
interpret the image as a surface "pool of water.
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Refraction and Images
Clickview; Refraction and Images.
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• Refraction – bending of light, total
internal reflection. Inquiry 5 p. 174
SE9 Investigation 1 p. 175 SE9 (Skill
p. 176 top class only and if there is
time) Eyes and Seeing Inquiry 7 p.
177
• Investigation 2 p.178 (Eye dissection)
(Omit refractive index p. 173)
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How far is the sun?
• I takes about 8 minutes for light to reach the
earth from the sun. So how far away is the sun?
• Remember speed of light = 300 000 Km per
second.
• If you calculate the number of seconds in 8
minutes you can work out the distance of the sun
https://www.childcarejobs.com.au/
• 8 minutes
= 480 seconds
• 480 times 300 000= 144000000km
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