Transcript LIGHT

Topic List
You will learn about the following:
spectrometer
Light and colour
Wave nature of light
Electromagnetic spectrum
Wavelength and colour
Greenhouse effect.
The diffraction Grating
Polarisation
Dispersion
The Spectrometer
Adjustments before use
Move eyepiece until
crossthreads are in focus.
Diffraction Collimator
grating
View a distant object
Telescope
moving eyepiece and
crossthreads until object is
in focus.
Put as source of light in
front of the slit and adjust
collimator until slit is in
focus
Adjust slit width for a
narrow beam of light
Level the turntable
Slit
With
adjustor
turntable
eyepiece
Objective lens
base
Levelling
screws
You must know the function of
each part of the spectroscope.
LIGHT: What Is It?
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Light Energy
Atoms
• As atoms absorb energy, electrons jump out to a
higher energy level.
• Electrons release light when falling down to the
lower energy level.
Photons - bundles/packets of energy released when the
electrons fall.
Light: Stream of Photons
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LIGHT: Particles or Waves?
Wave Model of Light
Explains most properties of light, but no the photoelectric
effect
Particle Theory of Light
Can't explain interference and diffraction but can explain
the photoelectric effect.
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The Wave Nature of Light
In the 1600's Christian
Huygens proposed that
light travelled as a wave.
Isaac Newton considered
light to be a stream of
very fast moving particles.
Both particles and waves
can be reflected and
refracted.
To prove light is a wave
we must show that it can
undergo interference and
diffraction.
This was first done by
Thomas Young in 1805.
Finally it was established
beyond doubt that light is
a wave, or is it?
Young's Slits Experiment
1) A monochromatic light was
allowed to spread out by
passing through a slit.
2) This light was then passed
through two slits
3) Diffraction occurs at each slit
and then interferenence where
they overlap.
4) An interference pattern of
bright and dark fringes results
on the screen.
Show Young's slits Video
As the fringes are so close together it is difficult to measure
the wavelength of light using Young's slits.
Diffraction of light
Diffraction – Bending of waves around the edge of a
barrier. New waves are formed from the original. breaks
images into bands of light & dark and colors.
Refraction – Bending of waves due to a change in speed
through an object.
Diffraction Grating – a piece of material often glass with
finely ruled parallel lines whicn act as slits
Diffraction Grating
Remember
diffraction will
be to left as well
as to the right
this is why there
are bright fringes
either side of the
central bright
image.
A diffraction grating. Each space between the ruled
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grooves acts as a slit. The light bends around the edges
and gets refracted.
Diffraction Grating Formula
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The path difference ‫׀‬AC‫׀‬is 
results in constructive
interference.
When the path difference
‫׀‬AC‫׀‬is 2 results in
constructive interference also.
The same is true for n and
results in constructive
interference
n = 1,2,3, ... always a whole
number as constructive
interference only occurs if the
path difference is a whole
number or wavelengths.
Diffraction Grating Formula
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●
●
●
|AC|= d sinɵ
The path difference n is
= |AC|
Therefore n = d sinɵ
The max value of n is got
by taking sinɵ =1 (why?)
nmax = d / 
Ө
Diffraction Grating
For constructive interference to occur after diffraction the path
difference has to be a whole number of wavelengths out of phase,
1, 2, 3 etc wavelength path difference.
There is no diffraction for the central bright image.
Since the light can be out of phase on both sides of the normal by
1,2, 3, etc wavelengths there are always two bright images formed
one either side of the normal for the same angle. For all other
angles destructive interference occurs giving dark spots.
n=1 is called the first 1st.order bright image either side of the
central image. n=2 is called the 2nd. Order image either side of the
central image and so on.
Diffraction Grating
The formula to calculate the angle for which there are bright
images formed is:
n λ = d Sin ɵ
The maximum no. of images either side is got by taking Sin θ = 1
when the angle is 90o .
nmax = d/λ
The grating constant d = the distance between two slits beside
each other (see diagram). This is calculated by
d= 1/n (mm)
Where n in this case is the number of lines ruled on the grating.
Interference pattern due to a diffraction grating
Bright images are formed due constructive interference due to
diffraction at the slit. Destructive interference occurs in between.
Top Photo:
Bright images due to laser
light passed through a
diffraction grating.
Bottom Photo:
Bright images due to
white light passing
through a diffraction
grating.
Measuring the wavelength of
monochromatic light using a laser
ELECTROMAGNETIC SPECTRUM
Brief review:
Water and sound waves transfer
energy from one place to anotherthey require a medium through which to
travel. They are mechanical waves.
Electric field-region in which charged
particles can be pushed or pulled.
Nature of Electromagnetic Waves
• They are Transverse waves without a medium.
(They can travel through empty space)
• They travel as vibrations in electrical and
magnetic fields.
• Have some magnetic and some electrical
properties to them.
• Speed of electromagnetic waves = 300,000,000
meters/second (Takes light 8 minutes to move from
the sun to earth {150 million miles} at this speed.)
• When an electric field changes, so does the magnetic field.
The changing magnetic field causes the electric field to change.
When one field vibrates—so does the other.
• RESULT-An electromagnetic wave.
• Click here Animation: Interaction of vibrating charges
Waves or Particles
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•
•
Electromagnetic radiation has properties of waves but
also can be thought of as a stream of particles.
Example: Light
Light as a wave: Light behaves as a transverse wave
which we can filter using polarized lenses.
•
Light as particles (photons)
•
When directed at a substance light can knock electrons off
of a substance (Photoelectric effect)
• Electromagnetic Spectrum—name for the range of
electromagnetic waves when placed in order of increasing
frequency
• Click here (Animation—Size of EMwaves)
RADIO
WAVES
INFRARED
RAYS
MICROWAVES
ULTRAVIOLET
RAYS
VISIBLE LIGHT
GAMMA
RAYS
X-RAYS
RADIO WAVES
(1)Have the longest wavelengths and lowest frequencies of
all the electromagnetic waves.
(1)A radio picks up radio waves through an antenna and
converts it to sound waves.
(1)Each radio station in an area broadcasts at a different
frequency. # on radio dial tells frequency.
(1)MRI (MAGNETIC RESONACE IMAGING)
Uses Short wave radio waves with a magnet to create an
image
RADIO WAVES
MRI of the Brain
RADIO WAVES
AM=Amplitude modulation—waves bounce off ionosphere can
pick up stations from different cities.
(535kHz-1605kHz= vibrate at 535 to 1605 thousand times/second)
+
RADIO WAVES
FM=Frequency modulation—waves travel in a straight line &
through the ionosphere--lose reception when you travel out of range.
(88MHz-108MHz = vibrate at 88million to 108million times/second)
+
RADIO WAVES
Bands of Radio/TV/Microwaves
MICROWAVES
• Microwaves—have the shortest wavelengths and the highest
frequency of the radio waves.
– Used in microwave ovens.
• Waves transfer energy to the water in the food causing
them to vibrate which in turn transfers energy in the
form of heat to the food.
– Used by Mobile phones
– RADAR (Radio Detection and Ranging)
• Used to find the speed of an object by sending out radio
waves and measuring the time it takes them to return.
INFRARED RAYS
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•
•
•
•
Infrared= below red
Shorter wavelength and higher frequency than microwaves.
You can feel the longest ones as warmth on your skin
Heat lamps give off infrared waves.
Warm objects give off more heat energy than cool objects.
• Thermogram—a picture that shows regions of different
temperatures in the body. Temperatures are calculated by
the amount of infrared radiation given off. Therefore people
give off infrared rays.
INFRARED RAYS
VISIBLE LIGHT
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•
•
•
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Shorter wavelength and higher frequency than infrared rays.
Electromagnetic waves we can see.
Longest wavelength= red light
Shortest wavelength= violet (purple) light
When light enters a new medium it bends (refracts). Each
wavelength bends a different amount allowing white light to
separate into it’s various colors ROYGBIV.
Dispersion of Light
Refraction – Bending of light due to a change in
speed as it changes from one medium to another.
Dispersion – separation of light into its different
wavelengths (colours) How does it happen?
Prisms – Glass that bends light. Different
wavelengths are bent different amounts & light is
broken out into different colors.
VISIBLE LIGHT
Formation of spectrum by splitting white light into it's colours
using a prism.
ULTRAVIOLET RAYS
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•
•
•
Shorter wavelength and higher frequency than visible light
Carry more energy than visible light
Used to kill bacteria. (Sterilization of equipment)
Causes your skin to produce vitamin D (good for teeth and
bones)
• Used to treat jaundice ( in some new born babies.
• Too much can cause skin cancer.
• Use sun block to protect against (UV rays)
X- RAYS
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Shorter wavelength and higher frequency than UV-rays
Carry a great amount of energy
Can penetrate most matter.
Bones and teeth absorb x-rays. (The light part of an x-ray
image indicates a place where the x-ray was absorbed)
• Too much exposure can cause cancer
– (lead vest at dentist protects organs from unnecessary
exposure)
• Used by engineers to check for tiny cracks in structures.
– The rays pass through the cracks and the cracks appear
dark on film.
X- RAYS
GAMMA RAYS
• Shorter wavelength and higher frequency than X-rays
• Carry the greatest amount of energy and penetrate the
most.
• Used in radiation treatment to kill cancer cells.
• Can be very harmful if not used correctly.
Using the EM waves to view the Sun
Animation—View a Galaxy at different
wavelengths
• Brief SUMMARY
• A. All electromagnetic waves travel at the same speed.
(300,000,000 meters/second in a vacuum.
• B. They all have different wavelength and different
frequencies.
– Long wavelength-lowest frequency
– Short wavelength highest frequency
– The higher the frequency the higher the energy.
Electromagnetic Waves
Speed in Vacuum
300,000 km/sec
Speed in Other Materials
Slower in Air, Water, Glass
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Light is a tranverse wave
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Energy is perpendicular to direction of motion
Moving photon creates electric & magnetic field
Light has BOTH Electric & Magnetic fields at right
angles!
Colour of Light
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Transparent Objects:
Light transmitted because of no scattering
Colour transmitted is color you see. All other colours are
absorbed.
Translucent:
Light is scattered and some transmitted.
Opaque:
Light is either reflected or absorbed.
Colour of opaque objects is colour it reflects.
Colour of Light (Cont.)
Colour of Objects
White light is the presence of ALL
the colours of the visible spectrum.
Black objects absorb ALL the colours
and no light is reflected back.
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Colour of Light (Cont.)
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Primary Colours of Light
Three colors that can be mixed to produce any other colored
light
Red + blue + green = white light
Secondary Colours of light
Two primary colours of light mixed give a secondary colour e.g
red and green = yellow
Complimentary Colours of Light
Two complimentary colors combine to make white lightMagenta and green, Cyan and red ,Yellow and blue all combine
to give white light.
LIGHT & ITS USES
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Sources of Light
Incandescent light –
light produced by
heating an object until
it glows.
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LIGHT & ITS USES
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Fluorescent Light –
Light produced by electron bombardment of gas
molecules
Phosphors absorb photons that are created when
mercury gas gets zapped with electrons.
The phosphors glow & produce light.
LIGHT & ITS USES - Neon
Neon light – neon
inside glass
tubes makes red
light. Other
gases make
other colours.
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The Greenhouse Effect
Infrared radiation is given out by all bodies – even cold ones.
However, essentially, the hotter the body is, the shorter the
wavelength of the infrared radiation.
Shortwave infrared
radiation
Longwave infrared
radiation
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The Greenhouse Effect
Shortwave infrared radiation, which is predominantly emitted by hotter objects is very
penetrating.
Longwave infrared radiation, which is predominantly emitted by cooler objects is more easily
absorbed.
Shortwave infrared radiation
Longwave infrared radiation
Strongly penetrating
More easily absorbed
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The Greenhouse Effect
The surface of the Sun has a temperature of the region of 6,000 K.
As a complete
aside that has
nothing to do
with the
greenhouse
effect, this is
Venus!
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This means that the Sun gives out very shortwave infrared radiation.
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The Greenhouse Effect
This shortwave infrared radiation from the Sun is very penetrating.
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The Greenhouse Effect
This infrared radiation from the Sun is very penetrating. It can
certainly penetrate all the way through the Earth’s atmosphere, right
down to the ground.
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The Greenhouse Effect
This infrared radiation from the Sun is very penetrating. It can
certainly penetrate all the way through the Earth’s atmosphere, right
down to the ground.
In fact, it will even penetrate
glass which includes, of course,
the glass in a greenhouse.
However, the infrared radiation
from the Sun will then be
absorbed by the objects in the
greenhouse and warm them up.
But temperatures rarely rise much
above the low 300 Ks mark.
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The Greenhouse Effect
The warmed objects in the greenhouse then also emit infrared
radiation.
However, they are at a much
lower temperature than the
surface of the Sun.
This means that the infrared
radiation that they emit will be
of a much longer wavelength.
This in turn means that the infrared
radiation from the plants and things
in the greenhouse will be much less
penetrating.
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The Greenhouse Effect
This means that the heat is trapped in the greenhouse.
You will have noticed how
much warmer it is in a
greenhouse on a sunny day than
it is outside.
The reason is the greenhouse
effect.
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The Greenhouse Effect
The greenhouse effect keeps the Earth much warmer than it would be
without it.
In fact, life would be unlikely
to exist if the greenhouse effect
did not retain a lot of the Sun’s
heat.
It is the Earth’s atmosphere that traps the heat. A lot of the
longwave radiation, given off by the objects on Earth which are
clearly not as hot as the surface of the Sun, is absorbed by the
Earth’s atmosphere.
Clouds play an important role too, reflecting some of the radiation
back to the Earth – that is why it can get so cold on a clear night with
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no cloud cover.
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The Greenhouse Effect
So why are we worried about the greenhouse effect and, so called,
greenhouse gasses?
At the moment, much of the Sun’s heat that we receive
on Earth does escape the atmosphere.
However, some gases such as carbon dioxide, are particularly
good at absorbing longwave infrared radiation. These gases are
called greenhouse gases.
If the amount of greenhouse gas
in the atmosphere increases, there
will be more absorption of
longwave infrared radiation and
so the atmosphere will get
warmer: much too warm!
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