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Answers to Readers' Questions on Physical Science:

Optical Devices

The following 10 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• How do lenses refract light?
• Optical device to let us see our behind
• What is the exact equation for Young's slit experiment?
• Lenses used in glasses
• Is black a real color?
• confused about images
• Wants to detect gravity waves
• Materials for eyeglass lenses
• Light particles
• Indicator for lost glasses

Next 10

How do lenses refract light?

Question

March 16, 2009

Mr. Kurtus,

How do lenses refract light? Is it due to gravity? If it is, then light will not be refracted in a vacuum, Right?

If it is because speed changes, then why doesn't light get refracted when it falls perpendicular onto another medium?

Thanks in advance.
school for champions is a great site.

John - Sweden

17448

Answer

Light travels slower through glass than in air. So when it goes from one medium to the other at an angle, it appears to bend. See Refraction of Light at:
http://www.school-for-champions.com/science/light_refraction.htm.

Light moving from outer space into the atmosphere at an angle bends slightly.

The amount the light bends depends on the angle it enters the other material and its index of refraction, according to Snell's Law. See:
http://www.school-for-champions.com/science/light_refraction_snell.htm

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Optical device to let us see our behind

Question

December 1, 2007

Do we have an optical device which can let us see our behind? ( the only facility used is mirror)

- USA

14884

Answer

You could see your behind with one mirror, two mirrors, or with a camera. There is no specific device for that task.

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What is the exact equation for Young's slit experiment?

Question

November 19, 2007

In Thomas Young's experiment, this equation is given: ”y=»L/d

The proof of this equation depends on saying that sin¸=tan¸. Is their a more accurate equation? I reached this one:

”y=»L[(2/(d^2-4»^2))-(1/(d^2-»^2))]

Is it correct?

Thank you.

Mohammed - Egypt

14804

Answer

The reason they say that sin(b) = tan(b) is the requirement that b is a relatively small angle. The equation is accurate for small angles, but when the angle b becomes larger--like over 10 degrees--a more detailed and complex equation must be used. Unless there is some reason for exactness at larger angles, the approximation is usually sufficient. Unfortunately, I do not have the exact equation available.

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Lenses used in glasses

Question

February 10, 2005

what kinds lenses are used in specs

sakl - india

5929

Answer

Concave lenses make things small and are used for nearsighted people.

Convex lenses make things bigger and are used for farsighted people.

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Is black a real color?

Question

December 2, 2004

IS Black a real Color? please explain.

- USA

5226

Answer

Black is the absence of color. A pure black object absorbs all light coming in. A pure white object reflects all light.

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confused about images

Question

November 17, 2004

When using an optical bench, why does a double convex lense produced two upside down focused images...one large and one very small

Terri - USA

5062

Answer

If you are outside the focal point of the lense, you should only get one smaller upside down image on the other side of the lense. If you are inside the focal point, you will a larger right side up image on the same side as the object. This is a virtual image.

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Wants to detect gravity waves

Question

February 18, 2004

everyone knows that an accelerated mass produces gravity.also electron has mass.electromagnetic waves contains electrons.when ever lectromagnetic waves are produced electrons are accelerated.that means electromagnetic waves produce gravity when ever they are produced.though electrons are of negligible mass we cant feel the produced gravity.since the gravity waves cant be felt.but if we produce a bulk mass of electromagnetic waves..then we might be able to detect the gravity waves produced.
can u suggest me whether my theory is correct me or not? and morever plz give me some idea of how can i produce electromagnetic waves in lab and also give me some idea of a superconductor antenna which can detect the electromagnetic waves? also give me a suggestion if my theory is correct then it means we can produce gravity in microgravity area or space where there is no gravity..?

amiya -

2442

Answer

All matter has gravity. It does not need to be moving or accelerating.

Electromagnetic waves do not contain electrons. They are the result of electron movement. Light, radio waves, and x-rays are EM waves.

Since electrons are matter, they do exhibit a small amount of gravitational force.

There is a theory that gravity waves exist and there are some experiments being done to try to detect them. Since the waves are so small, it is difficult to detect them.

The further you are from matter, the less the force of gravity is from other sources. But you would create gravity yourself.

There are some tricky problems there. Keep on thinking and trying to figure out new ways of doing things.

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Materials for eyeglass lenses

Question

January 14, 2004

Hi. i am doing a project also on lenses and contacts. i emailed you about electromagnets. i was wondering if you could send me information about glass lenses perhaps about what they are made out of, what are more efficient materials for making the lenses, and how it is made. i would please like you to also send me information about contacts. what they are made out of, how to determine what is they right type of contact for you. thankyou very much.

evan -

1943

Answer

See http://horizoneye.com/lensmaterial.htm
Also http://www.allaboutvision.com/contacts/

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Light particles

Question

December 11, 2003

I read the static electricity pages and the light pages. To my understanding they both have some magnetic properties. What I'm wanting to know could light carry electrons? Because photons can hit a substance and cause it to heat up and on one program i saw light move an object so couldn't it hit/attract electrons to travel with it until it hit's something and give it a static - charge?

I know that the question may seam strange and far fetched but even if you don't have an answer please give an opinioun an the matter.

Andrew -

1557

Answer

Electrons have both electric and magnetic fields. When electrons move back and forth, they create electromagnetic (EM) waves. Visible light is a set of EM waves of certain wavelengths.

The duality theory says that EM waves like light can also be particles, called photons. Likewise, the theory says that electrons can be waves.

Being waves, they both have wavelengths. But also being particles, they both can move objects when they hit them.

A static charge is an electron or proton on the surface of a non-conducting material.

I hope that helps you understand the topic.

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Indicator for lost glasses

Question

August 18, 2003

I lose my glasses a lot. I gues I could apply that glow in the dark stuff to them and turn off the lights in each room. But, then I would glow in the dark at night and I don't want that kind of attention.

Is there some kind of glasses that I could buy that would see only wavelengths of the color of my frames?

Thanks

-

254

Answer

The problem with losing your glasses is that you usually put them in some unusual place, like on the top of the refrigerator or such. At least that is what I do when I lose my glasses. I have a second pair that I wear to find the first pair.

Glow in the dark is only good at night. What about losing them in the daytime?

If you had blue or green frames, wearing rose-tinted glasses would result in them standing out somewhat, if they were in a place you could see them.

A better method might be to have a strap that goes around your neck, so the glasses would hang there when not in use. Some people carry their sunglasses that way.

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