[NakedScientist (OP)]

ANSWER TO LAST WEEK'S QOTW:

"WHAT IS A HOLOGRAM, AND HOW ARE THEY MADE?"

Holograms are made using lasers. You need a laser because the light that is produced is "coherent". That is, all of the light waves in the beam are synchronised, and of the same amplitude (size), so they diffract (bend) identically. You can demonstrate this by shining a laser at a prism. Compared with white light, which splits up into its composite wavelengths because each bends by a different amount upon entering the prism, when a laser is fired at a prism just a single band of light is produced.

To make a hologram a laser beam is fired at a beam splitter. This sends part of the light beam, termed the reference beam, to a piece of photographic film. The other part of the light beam, let's call it the image beam, is directed at, and subsequently reflected off, the object which you intend to turn into a hologram. It, too, then shines onto the same piece of photographic film as the reference beam mentioned above.

When the reference beam and the image beam meet at the photographic plate they have travelled different distances and their light waves no longer line-up with each other precisely. As a result they "interfere" with each other with some waves adding together constructively to make brighter patches and other parts of the wave behaving destructively and cancelling each other out to make darker areas. This is called an intereference pattern, and it's what is subsequently recorded onto the photographic film. This is the hologram, the 3D representation of the original object.

TNS

[NakedScientist (OP)]

Ok, here's this week's QOTW:

"HOW DO STINGING NETTLES WORK ?"

TNS

[wim]

Stinging Nettles ,each of it’s leaves are about 10 cm long, roughly heart-shaped and have large teeth around the leaf edge. They also have tiny hollow hairs on the main stem, leaf stems and on veins on both upper and lower sides of the leaves.
When a human brushes by the plant and it touches their skin, the tiny hollow hairs break off and release formic acid, histamine, acetylcholine, serotonin (5-hydroxytryptamine), plus some unknown compounds. These irritate the skin and cause white itchy spots to appear.

grtz

[daveshorts]

A hologram is a pattern that when light is reflected (or transmitted depending on the hologram) from it, the light reflecting from the parts of the pattern interfere with each other to produce light leaving as if it came from a 3D object.

Doing this by drawing the pattern from the start is immensely difficult ( although a company in Cambridge is developing a projector that works on a similar idea http://www.lightblueoptics.com/ ) however if you shine light reflected from an object and light coming directly from a laser the two light beams will interfere producing a pattern. Luckly this is the pattern you need to make a hologram.

If you used non-coherent light (light where you can't predict whether it is going to be a peak or a trough from one moment to the next) it would produce an interference pattern that was not stable for long enough to take the photo, so you would actually get an averaging of lots of different interference patterns which won't produce a hologram.

[chris]

Here's this week's QOTW:

"HOW DOES A SOLAR CELL TURN SUNLIGHT INTO ELECTRICITY?"

Have a go, below...

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[Hadrian]

The Structure of a Solar Cell
Over 95% of all the solar cells produced worldwide are composed of the semiconductor material, silicon. As the second most common element in the earth’s crust, silicon has the advantage of being available in large quantities. Furthermore while the material is been processed, it does not have an effect on the environment. [http://www.solarserver.de/wissen/photovoltaik-e.html] Another reason for the use of silicon for solar cells is that the energy needed to ionize silicon electrons matches well with the energy of photons coming from the sun. If the photons had less energy (if the solar spectrum were more red), there would not be enough energy to free the electrons, and if the photons had more energy (if the solar spectrum were more blue or ultraviolet), then all the energy above what is needed to break the electrons free would be lost as heat. [http://www.astropower.com/how_solar_cells_work.htm]

To produce a solar cell, the semiconductor (silicon) is “doped” or contaminated. “Doping” is the intended introduction of chemical elements, which can obtain excess positive charge carriers (p-conducting semiconductor layer) or negative charge carriers (n-conducting semiconductor layer) from the semiconductor material. If two differently contaminated semiconductor layers are combined, a p-n-junction results.

At this junction, an interior electric field is built up, which leads to the separation of the charge carriers that are released by light. Through metal contact, an electric charge can be tapped. If the outer circuit is closed, meaning a user is connected, then direct current flows. A transparent anti-reflection film protects the cell and decreases the reflective loss on the cell’s surface. [http://www.solarserver.de/wissen/photovoltaik-e.html]
 


What you do speaks so loudly that I cannot hear what you say.

[razorbill]

I have'nt progressed that far yet...I still get Duckbumps!

[Cut Chemist]

How fast is warp speed??

[JimBob]

Only Scotty and Sulu (and Spock) know. It isn't scientific, it is scince fiction, believed to be fasteer than the speed of light.


The mind is like a parachute. It works best when open.  -- A. Einstein

[chris]

Now here's a question that I think you'll have fun with:

"Why does a mirror reverse things in the horizontal, but not the vertical axis?"

Answers below please...

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[another_someone]

quote:Originally posted by chris
"Why does a mirror reverse things in the horizontal, but not the vertical axis?"

Answers below please...

This is a question that comes up so often that I feel I should have a standard template with which to answer it.

A mirror does not reverse things.  What a mirror shows is a true image.  It is what we compare a mirror image to that is reversed.

You expect a mirror image to look like a person facing you.  The reason why a mirror image does not look like a person facing you is because the person facing you has turned around to face to.  It is the person who is facing you who is reversed.  The person who is facing you is reversed in the horizontal plane because (s)he has revolved around a vertical axis when turning around to face you.  If that person, rather than turning on their heals to face you, turned around by doing a handstand, then they would be reversed in the vertical plane and not in the horizontal plane.

George

[chris]

But in terms of the physics of the light waves hitting the mirror, what's happening?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[ROBERT]

quote:Originally posted by chris

But in terms of the physics of the light waves hitting the mirror, what's happening?

I found these sites on this subject:-
http://www.mathpages.com/home/kmath142.htm
http://www.mathpages.com/home/kmath354.htm
http://www.mathpages.com/home/kmath441.htm

[ROBERT]

quote:Originally posted by chris

But in terms of the physics of the light waves hitting the mirror, what's happening?

I found these sites on this subject:-
http://www.mathpages.com/home/kmath142.htm
http://www.mathpages.com/home/kmath354.htm
http://www.mathpages.com/home/kmath441.htm

[rochelle]

I have a question for you scientists! Lets see if you can answer this one. Tell me why.......tell me why I've never gotten goose bumps? I do get cold, and I do get the shivers, but goose bumps have never followed after experiencing those feelings.
 Hopefully someone will have an answer!
My bf's answer is that I have no soul, but thats obviously not a logical answer.

Rochelle Eloranta

[Mjhavok]

Goose bumps are a vestige from the days when humans were covered with hair.

When it's hot and you need to cool down, little muscles at the base of each hair relax. Your hair becomes relaxed. Your sweat glands pump out body heat in sweat. Your blood vessels get big to take more heat to the skin to get rid of it. When it's cold, the arrector muscle pulls the hair up. The duct to the sweat glands gets small to conserve heat. Our blood vessels also get small to save heat.

Hair standing up doesn't make very good insulation - we don't have enough fur for that. Humans don't have very much hair on their bodies anymore. Millions of years ago, humans probably did. And that hair standing on end helped keep people warmer. Those little muscles we have on the end of each hair still work. They still make goose bumps.

Cold is not the only thing that can cause our hair to stand on end. Fear or anger can cause the same reflex. The same is true for other mammals. You'll notice that on a cat or dog. Their fur gets bigger when they're angry or afraid.


Perhaps you have some kind of genetic abnormailty that stops you from getting goosebumps. I have heard of someone who couldn't produce tears before but never the goosebumpless disease.

Sorry
Steven

[Mjhavok]

"Why does a mirror reverse things in the horizontal, but not the vertical axis?"

I think the entry in wikipedia below explains this quite well.

http://en.wikipedia.org/wiki/Reflection_(physics)

Steven

[narasimeena]

quote:Originally posted by NakedScientist

THIS WEEK'S QUESTION : - Have a go - brownie points to the person who gets closest to the correct answer, which we'll post in a week's time.

"WHY IS THE SKY BLUE ?"

This is mainly due an effect called as Tindall Effect a property of collides. "If you shine a beam of light through a solution, the light is not effected and passes through. If you shine the beam through a colloid the small aggregates scatter the light and the material looks cloudy or milky" This causes the sky to apprea blue.
Do I get a brownie for this!


Narasi Ramachandran

[narasimeena]

"WHY IS THE SKY BLUE ?"

This is mainly due an effect called as Tindall Effect a property of collides. "If you shine a beam of light through a solution, the light is not effected and passes through. If you shine the beam through a colloid the small aggregates scatter the light and the material looks cloudy or milky" This causes the sky to apprea blue.
Do I get a brownie for this!



Narasi Ramachandran

[Mjhavok]

Yes, this question was answered a while back. :-D