phy exp

8/10/2019 Phy exp

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Quantum Mechanics. Something that scientists in movies and prestigious universities

investigate, right? Not necessarily! Do this experiment and you’ll be able to see

quantum mechanics in action by exploring the wave-particle duality of light.

Problem: How can we see quantum interference?Materials:

Laser (be careful not to shine this in anyone’s eyes)

Needle

Tape

Table

White printer paper (cardstock works best)

Dark room

Flat wall

Procedure:

1. Fold and unfold your sheet of printer paper once so that it can stand upright.

2. Poke a tiny hole in your paper with your needle.

3. Stand your printer paper upright on a table that is at least ten feet away from the wall you will project

your laser onto.

4. Use your tape to mount your laser pointer to a stable object, like a heavy book. Place the mounted laser

on the table.

5. Turn your laser on. Adjust the angle of your laser so that it passes through the hole in your paper and

onto the wall. What did you see? Is it what you expected to see?

6. Poke another hole in your paper right next to the first one so that they’re as close together as possible

without creating one larger hole.

7. Adjust your laser so that it now passes through both holes. Observe the shapes created on the wall. What

do you see? Was it what you expected to see?

8. Cover one of the holes with a small piece of paper, leaving the other open. How does the projected

image on the wall change?

Results:

You should have seen a blob of light from the laser when it was passing through one

hole, and a striped blob of light when it was passing through both holes. You should

have noticed that the stripes disappeared when you covered one of the holes.

Why?

8/10/2019 Phy exp


What is happening is that the photons that are passing through the two holes

are interfering with each other in much the same way that waves in water do: making

patterns of light like the ripples in a pond. Two different things can happen to waves

during interference: waves that are in-phase with each other (happening at the same

time) add together to become stronger and waves that are out of phase cancel each

other out and become weaker. The reason you saw a striped blob of light when the

laser passed through two holes is because of constructive interference—spots where the

waves added together to become stronger.

This is weird because there is ample evidence that points to light being made up of

small particles. It doesn’t make sense for particles to interfere with each other—this

would be like baseballs caring about whether they pass through holes that are near

each other! The only logical conclusion is that light is both a particle and a wave. The

individual photons (light particles) coming from the laser act as waves as they travel

through the two holes—that is, they interfere with each other before finally hitting the

wall.

Another experiment was designed to test this theory. Scientists created a source of

light that only let out one photon at a time. They aimed this at a sensitive detector and

left it on for a few days. The detector only saw individual photons strike it: one photon

here, another there. But after they stopped the experiment and looked at the data,

they saw the same interference that you’re seeing with the laser! Individual photons

were able to travel throughboth holes at once. When the scientists repeated this

with electrons, tiny particles that make up parts of atoms, they saw the same thing.

From this, they concluded that all matter is simultaneously a particle and a wave, but

when it comes to relatively heavy things like electrons, the particle behavior is a lot

easier to observe than the wave behavior.

Bernoulli's Principle Experiment

If you’ve ever been pushed by the wind, you know that the wind might be invisible, but it’s strong. In this

experiment, you’ll see how a can reacts when you blow on it with a hair dryer and how it reacts when you blow just

beside it.

Problem: How does moving air change pressure?

Materials

2 empty cans

Hair dryer

String

Carpenter’s level

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Long, thin rectangle of wood

Two identical chairs

Ping pong ball

Three books

Safety goggles

Nail

Hammer

Procedure

1. First, create your mobile. Get two identical cans and ask an adult to help you punch a hole in the middle

of the bottom of each can using a hammer and a nail. Make sure that you wear eye protection!

2. Run a piece of string through the hole in the can and tie a knot at the bottom. Tie the other end of the

string around a long, thin rectangle of wood. Do this with both cans, making sure that they’re about 6

inches apart.

3. Support the wood on either side on two chairs of identical height. Place your carpenter’s level on top of

the wood once you’ve balanced it. If the air bubble is in the middle of the level, this means that your

mobile is level.

4. Now, turn on the hair dryer. Point it at a can. What happens?

5. Wait until everything has stopped moving, then point the hair dryer directly in the middle of the

two cans. What happens to the cans now?

Results

When you blow on a can, it moves. When you blow air between the two cans, they

move together.

Why?

When you pointed your hairdryer at a can directly, the can moved due to the invisible

push of the wind. When a force like the wind pushes on a specific area,

it's called pressure. It’s as if you moved the can with the pressure of your hand.

But why did the cans move together when you blew the air between them? Doesn’t that

cause pressure as well?

This is due to Bernoulli’s principle. This principle of physics says that as the speed of the

air increases, its pressure decreases. Even when air is just sitting around, it still has pressure. That’s

called static pressure, and it’s due to the weight of the air pressing down. Even though it’s not

moving, air still puts pressure on the sides of the cans.

When air is moving, it has velocity pressure. Bernoulli’s principle says that the pressure of

a fluid when it’s moving is lower than when it’s static, or resting. When the air is no

8/10/2019 Phy exp


longer still, the pressure on its edges decreases. When the pressure around the

cans decreases, they move into the center.

How does this work? Let’s try the experiment with something you can see. Place your

three books on end, and press in very gently on the outside of the two outer books.

This is like the static pressure exerted on your cans. The book in the middle is the

pressure of the static air in the middle of the cans.

What happens when the “pressure” in the middle of your books decreases? If you take

out the book in the middle but leave the other two in the same place, then press gently

on the two books, the books will move inward. Imagine that the air between the cans is like that

middle book. When you blow it quickly out of the middle, the pressure in the center of the cans decreases and

the cans move inward.

Going Further

Place a ping-pong ball on top of a hair dryer that’s balanced with the air vent pointingdirectly up. Turn the hair dryer on at low or no heat. The ball will rise into the air and

stay there as long as the dryer is turned on. You can’t see the column of air, but it is

there. This experiment shows off the invisible pressure that air creates.

You can also experiment with other aspects of fluid dynamics. Bernoulli’s principle is

often applied to the movement of water. If you have the same amount of water in a

small tube or a large one, does the water flow at the same velocity through each one?

Turn on a garden hose and let it run, and then place your finger over half the hole.

When is the water faster—in the beginning or when you make the hole in the hose

smaller? When the same amount of water has to move through a smaller space, it

needs to go faster.

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