Measuring the Wavelength of a LASER
Big Idea
Demonstrate the concept of
Imagine water waves moving along the shore. What happens when
diffraction by calculating the
they suddenly met an obstacle, like a harbour wall, which contained
wavelength of red LASER light.
only a narrow gap for them to move through? How would the waves
What You’ll Need
behave once they passed through the gap in the wall?
Diffraction
•
1 Red LASER pointer
•
1 Diffraction Grating
MEASURING WAVELENGTHS
• Sheet of blank paper
Diffraction
refers to how waves bend when they move through a
BIG IDEAS
small opening, or around a narrow obstacle.
water wave
ruler What do you think
ImagineWhen
watera waves
moving along• theA shore.
• Demonstrate the concept
would.you
happen
they
a harbourgrating
wall which
moves through a small gap in in a barrier,
mightwhen
expect
thesuddenly•metDiffraction
holder
of diffraction by calculating
contained
only
a
narrow
entrance
gap
for
them
to
move
through?
waves
to continue
onwards in a uniform fashion, just slightly shorter.
(optional)
the
wavelength
of red
How would the waves behave once they passed through the gap?
But that’s
LASER
light.not what happens! Instead, the edges of the waves
bend towards the barrier as they pass
through (Figure 1).
DIFFRACTION
Diffraction refers to how
WHAT
waves
bend
when
they move
In theYOU’LL
same way, NEED
water waves bend or
diffract
as they
around
a
through a small opening, or
around a narrow obstacle.
• 1 Diffraction Grating
Consider Figure 1. When
Figure 1: Diffraction through a narrow slit
• Sheet
of
blank
paper
water
shows
a
wave
moving
Light is also a wave, and it also bends (or diffracts) when faced
through a small gap in slit in
• A with
rulereither a narrow slit or obstacle, similar
to how water waves
a
barrier,
you might expect
• Diffraction
behave! grating holder
the waves to continue in a
(optional)
uniform fashion, just slightly
shorter, but that’s not what
Diffraction Grating
happens! Instead, the edges Figure 2: Diffraction around a narrow obstacle
A diffraction
grating is a piece of plastic containing thousands of
RELATED
PRODUCTS
of the waves bend towards
Click the below to be taken right
thin,
vertical
running from the top of the plastic to the
to the
productscratches,
page.
the barrier as they pass through (Figure 1) .
bottom. These scratches act as very narrow obstacles, and the light diffracts as it passes through the
In the same way, water waves bend or diffract as they move past a
Red Laser Blox
diffraction grating, which is where it gets its name.
around a narrow obstacle . This is shown in Figure 2. - the waves
bend towards the obstacle as they pass.
As the
waves
diffract
Green
Laser
Blox and bend, however, they interfere with each other. When two crests meet, there is
Light is also a wave, and will also bend (or diffracts) when faced
constructive interference, and the wave
amplified,
or similar
made brighter.
When awaves
crest and
a trough meet,
withisan
obstacle,
to how water
behave!
destructive
interference
occurs, and the wave is dampened, or made dimmer.
Red & Green
Laser
obstacle (Figure 2).
• 1 narrow
Red LASER
Pointer
DIFFRACTION GRATING
A diffraction grating is a piece of plastic containing thousands of
Interference Pattern of LASER Light
thin, vertical scratches, running from the top to the bottom. These
scratches act as very narrow obstacles, and the light diffracts as it
passes through the grating, which is where it gets its name.
As the waves diffract and bend, however, they interfere with each
other. When two crests meet, there is constructive interference,
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Different wavelengths of light diffract at different angles. If you were to
pass white
light brighter.
through the
diffraction
grating,
observe
and the wave is amplified,
or made
When
two troughs
a you
crestwould
and a
troughits
meet, destructive
interference occurs, and
the wave
dampened,
or made
This
causes
the interference
spectrum,
or allisthe
colors of light
whichdimmer.
it consists
of, side
by side
and
pattern you see emerging
from
the
diffraction
grating.
smeared together. As a test, hold the diffraction grating up to one eye
take a look at a source of white light, like the overhead lights. You
INTERFERENCEand
PATTERN
OF LASER LIGHT
Different wavelengthsshould
of light
diffract
various angles.
If you
observe
itsatspectrum
as a rainbow!
Figure 3: Interference Pattern o
Different
light
diffractthe
at different
angles.
If you
were
to
were towavelengths
pass white of
light
through
diffraction
grating,
you
would
observe
all the
colorsthe
of light
whichgrating,
it consists
of or the
spectrum.
pass
white light
through
diffraction
you would
observe
its of only one wavelength of light. When all the light wav
But LASER
light is monochromatic:
it consists
As a test, hold the diffraction grating up to one eye and take a
spectrum, or all the colors
of light
it consists
of, side by
side and
diffract
at which
the
angle, because
the same wavelength, and we don’t observe a spectrum
look at a source of white
light,
likesame
the overhead
lights. they
You are
should
smeared
together.
As
a
test,
hold
the
diffraction
grating
up
to
one
eye
observe its spectrumobserves
as a rainbow!
a series of spots. This series of spots is an interference pattern that consists only of one c
and take a look at a source
of white
light,
like the
lights.
You
(Figure
3). And
we
useoverhead
this
pattern
But LASER light is monochromatic
- it can
consists
of only
oneto calculate the wavelength of the light!
wavelength
Whenasalla the
light waves diffract at the same
should
observeofitslight.
spectrum
rainbow!
Figure 3: Interference Pattern of LASER Light
angle, because they Calculating
are the same
a
thewavelength,
Wavelengthwe
of observe
Light
series of spots. This series of spots is an interference pattern that consists only of one color light, as
In order to calculate
wavelength
of the LASER
light,
you need
be able
to identify certain parts
But LASER light is monochromatic:
it consiststhe
of only
one wavelength
of light.
When
all thetolight
waves
shown in Figure 3. We can use this pattern to calculate the wavelength of the light!
interference
pattern.
Take
a close
look at Figure
3. The
of light, we
in the middle, is calle
diffract at the same angle,
because they
are the
same
wavelength,
and we
don’tbrightest
observespot
a spectrum,
central
maximum.
The smaller
dots
on either
sidethat
of the
centralonly
maximum
are called
CALCULATING
THE
WAVELENGTH
LIGHT
observes
a series of spots.
This
series of spots
is anOF
interference
pattern
consists
of one color
light the secondary
(maxima
is plural
maximum).
In white
the
central
of certain
each wavelength
overlaps wi
In order
to calculate
the
wavelength
the
LASER
light,
youlight,
need
tolight!
be ablemaximum
to identify
parts
(Figure
3). And
we can use
this
pattern
toofof
calculate
the wavelength
of the
of its interference pattern.
Take a close
at Figure
3 again.
Theabrightest
spot of of
light,
right of
in dots.
the But you can s
next wavelength
andlook
so the
spectrum
looks like
rainbow instead
a series
middle, is called the central maximum. The smaller dots on either side of the central maximum are
rainbow
pattern of white light repeat, the same way a monochromatic interference pattern repeats.
Calculating the Wavelength
of Light
referred to as secondary maxima. In white light, the central maximum of each wavelength overlaps
In with
orderthe
to calculate
the wavelength
thespectrum
LASER light,
youlike
need
to be able
to identify
parts
of its
next wavelength
and soof
the
looks
a rainbow
instead
of a certain
series of
dots.
You can
Thearepeat,
formula
forat
calculating
the
wavelength
𝜆𝜆 ofofinterference
light
asthe
follows:
see the rainbow
the
same
way
a monochromatic
pattern
interference
pattern.pattern
Take
close look
Figure
3. The
brightest spot
light,is in
middle,repeats.
is called the
(𝑋𝑋)(𝑑𝑑)
central
maximum.
smaller dots
on either side
of light
the central
maximum𝜆𝜆 are
The formula
for The
calculating
the wavelength
λ of
is as follows:
= called the secondary maxima
𝐿𝐿
(maxima is plural of maximum). In white light, the central maximum of each wavelength overlaps with the
where
𝑋𝑋 is the
between the
central
maximum
and a secondary
where X is the distance
between
thedistance
central maximum
and
one of
the secondary
maximaamaximum,
secondary𝑑𝑑 is the width o
next
wavelength
and
so
the
spectrum
looks
like
a
rainbow
instead
of
a
series
of
dots.
But
you
can
the
maximum, d is the width
of the
thediffraction
slits of thegrating,
diffraction
and L isfrom
the distance
from grating
thesee
diffraction
slits of
and 𝐿𝐿grating,
is the distance
the diffraction
to the screen where
rainbow
white showing
light repeat,
the same
way a monochromatic
gratingpattern
to the of
screen
thewhere
interference
pattern caninterference
be seen . pattern repeats.
interference pattern can be seen.
The formula for calculating the wavelength 𝜆𝜆 of light is as follows:
DIFFRACTION IN REAL
(𝑋𝑋)(𝑑𝑑) LIFE: HOLOGRAMS
𝜆𝜆 =
𝐿𝐿 capture perspective. This means that a
Holograms are 3D “photographs” of objects that
has depth,
and ifthe
youcentral
turn the
hologram,
picture changes
as 𝑑𝑑if is
you
wherehologram
𝑋𝑋 is the distance
between
maximum
andthe
a secondary
maximum,
thewere
widthholding
of the
the
real
object.
slits of the diffraction grating, and 𝐿𝐿 is the distance from the diffraction grating to the screen where
interference
can be seen.
To viewpattern
a transmission
hologram, you shine a LASER through a hologram plate. In some cases
this hologram plate is actually a glass square with opaque and transparent lines, which acts
as a diffraction grating. When you shine a LASER through the hologram plate, the LASER light
diffracts and an interference pattern is formed. This pattern becomes the hologram!
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ACTIVITY SHEET: MEASURING WAVELENGTHS
We’re now going to use what we know about diffraction patterns to measure the wavelength of a red
LASER.
1. Stick the blank paper to a wall above a flat surface on which you can rest the red LASER.
2. Place the diffraction grating upright in front of the paper. Measure the distance between the
grating and the wall, L, and record it in the table below.
3. Shine the laser pointer through the diffraction grating. If possible, place it on a small box or
similar to hold it steady.
4. Mark off the central maximum and one of the first-ordersecondary maxima on the sheet of
paper with a pencil.
5. Remove the sheet and measure the distance between maxima. Record the distance X.
6. Repeat twice more with different distances L.
Diffraction Grating
lines/mm
Slit Width (d)
cm/line
Distance from grating
to screen (L) in cm
Distance from maximum to
maximum (X)
1
2
3
7. Substitute the values for measurements 1 to 3 into the equations below to calculate the
wavelength of the LASER.
λ1 = (X)(d)
L
λ2 = (X)(d)
L
λ3 = (X)(d)
L
Average Wavelength
λ = λ1+λ2+λ3
3
Subsitutions of
Data in Formula
Final Value (cm)
Final Value (nm)
TIP: You need to convert your final value for λ from cm to nm in the table above: 1 cm = 1×107 nm = 10,000,000 nm
TIP: Red light usually has a wavelength of 620 nm -– 750 nm. Check that your average wavelength falls in this range!
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