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Waffa
science forum beginner
Joined: 27 Jun 2006
Posts: 2
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 Posted: Tue Jun 27, 2006 12:39 pm Post subject:
How find focal length of lens at home?
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How can I find the focal length of lens using items I have at home?
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses. |
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PD
science forum Guru
Joined: 03 May 2005
Posts: 4363
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| Posted: Tue Jun 27, 2006 12:42 pm Post subject:
Re: How find focal length of lens at home?
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Waffa wrote:
| Quote: | How can I find the focal length of lens using items I have at home?
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I assume this is a converging lens. Diverging lenses have focal points
too.
Take it outside where there are ants. Hold lens between sun and ant.
Move lens until ant gets real busy and expires with a sizzling pop.
Measure distance between lens and scorched spot where ant was.
PD
| Quote: |
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses. |
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Sam Wormley
science forum Guru
Joined: 30 Apr 2005
Posts: 1491
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| Posted: Tue Jun 27, 2006 1:34 pm Post subject:
Re: How find focal length of lens at home?
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Waffa wrote:
| Quote: | How can I find the focal length of lens using items I have at home?
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses.
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Sunlight and smoke will do it.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html |
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donstockbauer@hotmail.com
science forum Guru
Joined: 11 Sep 2005
Posts: 733
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| Posted: Tue Jun 27, 2006 4:20 pm Post subject:
Re: How find focal length of lens at home?
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PD wrote:
| Quote: | Waffa wrote:
How can I find the focal length of lens using items I have at home?
I assume this is a converging lens. Diverging lenses have focal points
too.
Take it outside where there are ants. Hold lens between sun and ant.
Move lens until ant gets real busy and expires with a sizzling pop.
Measure distance between lens and scorched spot where ant was.
PD
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses.
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Oh, oh. You'll have PETA on your case. They also try to protect
mosquitoes and ticks. |
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John C. Polasek
science forum Guru
Joined: 30 Apr 2005
Posts: 321
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| Posted: Tue Jun 27, 2006 6:23 pm Post subject:
Re: How find focal length of lens at home?
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On 27 Jun 2006 09:20:58 -0700, donstockbauer@hotmail.com wrote:
| Quote: |
PD wrote:
Waffa wrote:
How can I find the focal length of lens using items I have at home?
I assume this is a converging lens. Diverging lenses have focal points
too.
Take it outside where there are ants. Hold lens between sun and ant.
Move lens until ant gets real busy and expires with a sizzling pop.
Measure distance between lens and scorched spot where ant was.
PD
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses.
Oh, oh. You'll have PETA on your case. They also try to protect
mosquitoes and ticks.
|
Just form a sharp image of a lamp that is fairly far away, or the sun
if want exact value and get F inches. The diopters =
40"(Meter)/Finches. Obviously you can use metric Di = 1/Fmeters.
An image forming lens is no more than an enlarged pinhole, so you can
draw a diagram as if it were a pinhole to get image size ratio.
John P |
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Waffa
science forum beginner
Joined: 27 Jun 2006
Posts: 2
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| Posted: Tue Jun 27, 2006 6:29 pm Post subject:
Re: How find focal length of lens at home?
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On 27 Jun 2006, Sam Wormley<swormley1@mchsi.com> wrote:
| Quote: | Waffa wrote:
How can I find the focal length of lens using items I have at
home?
The web pages I have seen either use an optical bench or show an
arrow as a starting object but I can't project an image of an
arrow into a lens.
Could I somehow use a bright light bulb and focus the light from
that onto a sheet of paper until the filament is clearly defined?
Or will this give incomplete info because I am not able to compare
the size of the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with
one another for power and also to compare with the dioptre
strength in lenses found in off-the-shelf reading glasses.
Sunlight and smoke will do it.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
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Do you mean this page?
http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/foclen.html
It refers to a can of artificial smoke. Even if I make my own smoke
then how do I get the focal length from this? Do I actually need any
smoke?
Can I not just focus the sun's rays to create an image of the sun or
is that in some way too inaccurate?
A I am in the UK, then what alternative to the sun can I use for
overcast days or for when the sun is behind some obstruction? |
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Dirk Van de moortel
science forum Guru
Joined: 01 May 2005
Posts: 3019
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| Posted: Tue Jun 27, 2006 6:40 pm Post subject:
Re: How find focal length of lens at home?
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"PD" <TheDraperFamily@gmail.com> wrote in message news:1151412145.787236.166180@b68g2000cwa.googlegroups.com...
| Quote: |
Waffa wrote:
How can I find the focal length of lens using items I have at home?
I assume this is a converging lens. Diverging lenses have focal points
too.
Take it outside where there are ants. Hold lens between sun and ant.
Move lens until ant gets real busy and expires with a sizzling pop.
Measure distance between lens and scorched spot where ant was.
|
and finally, imagine what it's like to be an ant :-(
Dirk Vdm |
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Photo Guy
science forum beginner
Joined: 27 Jun 2006
Posts: 1
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| Posted: Tue Jun 27, 2006 7:10 pm Post subject:
Re: How find focal length of lens at home?
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Photograph an extended measuring tape but don't include the ends. The
tape must be at right angles to the camera. Try hanging it vertically
and make sure camera is level.
Measure the distance from the camera to where you hung the tape.
Divide this distance by the actual amount of tape you can see on film.
Multiply this figure by the actual vertical size of the film and you
will get your focal length.
example:
You hang the tape 10 feet from the camera
On the picture you can see 4 feet of the tape.
Divide and get 2.5
If you used 35mm film which has a vertical dimension of 1 inch
you can now multiply 1 inch by 2.5 and you have a lens with a focal
length of 2.5 inches or 63.5mm
Correct me if I'm wrong
Robert
On Tue, 27 Jun 2006 19:29:29 +0100, Waffa <waff@nonono.com> wrote:
| Quote: | On 27 Jun 2006, Sam Wormley<swormley1@mchsi.com> wrote:
Waffa wrote:
How can I find the focal length of lens using items I have at
home?
The web pages I have seen either use an optical bench or show an
arrow as a starting object but I can't project an image of an
arrow into a lens.
Could I somehow use a bright light bulb and focus the light from
that onto a sheet of paper until the filament is clearly defined?
Or will this give incomplete info because I am not able to compare
the size of the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with
one another for power and also to compare with the dioptre
strength in lenses found in off-the-shelf reading glasses.
Sunlight and smoke will do it.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Do you mean this page?
http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/foclen.html
It refers to a can of artificial smoke. Even if I make my own smoke
then how do I get the focal length from this? Do I actually need any
smoke?
Can I not just focus the sun's rays to create an image of the sun or
is that in some way too inaccurate?
A I am in the UK, then what alternative to the sun can I use for
overcast days or for when the sun is behind some obstruction? |
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Sam Wormley
science forum Guru
Joined: 30 Apr 2005
Posts: 1491
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| Posted: Tue Jun 27, 2006 7:54 pm Post subject:
Re: How find focal length of lens at home?
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Waffa wrote:
That will do it! |
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jimz
science forum beginner
Joined: 24 May 2005
Posts: 13
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| Posted: Tue Jun 27, 2006 8:07 pm Post subject:
Re: How find focal length of lens at home?
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Sorry, but if the lens is large,
the ant will pop long before the lens is
at the focal point.
You are better off taking the lens out at night when
the moon is out. focus the moon on a sheet of paper.
Then measure the distance. ( or use a far distant street light if it is
coludy out. )
"PD" <TheDraperFamily@gmail.com> wrote in message
news:1151412145.787236.166180@b68g2000cwa.googlegroups.com...
| Quote: |
Waffa wrote:
How can I find the focal length of lens using items I have at home?
I assume this is a converging lens. Diverging lenses have focal points
too.
Take it outside where there are ants. Hold lens between sun and ant.
Move lens until ant gets real busy and expires with a sizzling pop.
Measure distance between lens and scorched spot where ant was.
PD
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses.
|
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| Back to top |
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Llanzlan Klazmon
science forum addict
Joined: 29 Apr 2005
Posts: 70
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| Posted: Tue Jun 27, 2006 10:45 pm Post subject:
Re: How find focal length of lens at home?
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"PD" <TheDraperFamily@gmail.com> wrote in news:1151412145.787236.166180
@b68g2000cwa.googlegroups.com:
| Quote: |
Waffa wrote:
How can I find the focal length of lens using items I have at home?
I assume this is a converging lens. Diverging lenses have focal points
too.
Take it outside where there are ants. Hold lens between sun and ant.
Move lens until ant gets real busy and expires with a sizzling pop.
Measure distance between lens and scorched spot where ant was.
PD
|
I see you are not a Jainist  .
Klazmon.
| Quote: |
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses.
|
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|
| Back to top |
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PD
science forum Guru
Joined: 03 May 2005
Posts: 4363
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| Posted: Tue Jun 27, 2006 10:52 pm Post subject:
Re: How find focal length of lens at home?
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Llanzlan Klazmon wrote:
| Quote: | "PD" <TheDraperFamily@gmail.com> wrote in news:1151412145.787236.166180
@b68g2000cwa.googlegroups.com:
Waffa wrote:
How can I find the focal length of lens using items I have at home?
I assume this is a converging lens. Diverging lenses have focal points
too.
Take it outside where there are ants. Hold lens between sun and ant.
Move lens until ant gets real busy and expires with a sizzling pop.
Measure distance between lens and scorched spot where ant was.
PD
I see you are not a Jainist .
|
My karma is so worn that if it weren't for grace, I wouldn't even have
to die to be reincarnated as a cockroach.
PD
| Quote: |
Klazmon.
The web pages I have seen either use an optical bench or show an arrow
as a starting object but I can't project an image of an arrow into a
lens.
Could I somehow use a bright light bulb and focus the light from that
onto a sheet of paper until the filament is clearly defined? Or will
this give incomplete info because I am not able to compare the size of
the projected image of a filament with the original?
I have several handheld magnifying lenses I want to compare with one
another for power and also to compare with the dioptre strength in
lenses found in off-the-shelf reading glasses.
|
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