Lab: finding out about waves

Students will be working in groups of 4. Each rotation will have

Rotation 1

Quiz

Rotation 2

Ripple tank demonstration (teacher)

• Reflection

• Refraction

• Diffraction

Rotation 3

The wave equation

• Problem solving worksheet

Rotation 4

Slinkies demonstrating transverse and longitudinal waves

Rotation 5

Waves & patterns worksheet

Rotation 1: Quiz

Read the following paragraphs together. Take turns to read out loud while the rest follow along. Then, quiz each

other with the questions on the card.

What is a wave? A wave transfers energy from one place to another without transferring matter.

What is a transverse wave? Give an example. The particles in a transfer wave oscillate at right angles to the direction of energy transfer. Example: ocean waves, light waves (electromagnetic spectrum)

What is a compression wave? Give an example. The particles in a compression wave oscillate in the same direction to the direction of energy transfer. Example: sound waves

What is amplitude? Maximum distance that a particle moves away from its undisturbed position.

What is frequency? The number of wavelengths that pass a certain point per second. Its units of measure are Hertz.

What is wavelength? From one crest to the next (or one trough to the next) in a transverse wave. From one area of compression to the next (or one area of rarefaction to the next) in a compression wave.

Why do we need scientific models? To explain and describe phenomena that are not easily observable so that we can understand and increase our knowledge.

What is physics? The study of the nature and properties of matter and energy.

What are the two types of waves we will be studying this term?

• Electromagnetic waves

• Mechanical waves

How is a wave affected when it travels from one medium to another? Give an example. Its speed is affected – it either slows down or speeds up. Example: sound waves travel slower through water than steel; light travels fastest in a vacuum.

What is the wave equation? Speed = wavelength x frequency

What are some of the electromagnetic waves that you have used and/or experienced? Give examples of specific situations for each.

Examples of answers:

• Microwaves: Used the microwave to heat food.

• Infrared waves: Warmed up in the sun

• X-rays: Had an x-ray at the dentist

• Ultraviolet rays: When I got sun burnt

• Radio waves: When I listened to the radio this morning.

Rotation 2: Ripple tank

You will be looking at the behaviour of waves using a ripple tank. In particular, you will be able to describe:

• Reflection of a wave front

• Diffraction of a wave front

• Wave interference

Reflection

Diffraction

Interference

Rotation 3: The wave equation

This rotation is about becoming familiar with the wave equation and practising the calculations. For each question, write

out the wave equation as you would use it to answer the question. Show all of your working out so that you get into the

habit. Give your answer to 2 decimal places. Convert centimetres and minutes to metres and seconds before you do your

calculations.

Wave equation Speed = frequency x wavelength

v = fʎ

Where:

v = speed or velocity (metres /second – m/s)

f = frequency (wavelengths/second – Hertz)*

ʎ = wavelength (metres – m)

1. Stan and Anna are conducting a slinky experiment. They are studying the possible effect of several variables upon the speed of a wave in a slinky. Their data table is shown below. Fill in the blanks in the table, analyze the data, and answer the following questions.

Medium Wavelength Frequency Speed Zinc slinky 1.75 m 2.0 Hz ______ Zinc slinky 0.90 m 3.9 Hz ______

Copper slinky 1.19 m 2.1 Hz ______ Copper slinky 0.60 m 4.2 Hz ______

Zinc slinky 0.95 m 2.2 Hz ______ Zinc slinky 1.82 m 1.2 Hz ______

2. A wave had a wavelength of 2m and frequency of 4 Hertz. What is its speed?

3. A wave had a frequency of 20 Hertz and a wavelength of 5cm. What is its speed?

4. A wave travelled at 40m/s and had a frequency of 240 Hertz. What is its wavelength?

5. A wave travelled at 2500cm/s and had a frequency of 500 Hertz. What is its wavelength?

6. A wave travelled at 30cm/s and had a wavelength of 6.5m. What is its frequency?

7. A wave had a frequency of 234 Hertz and wavelength of 21cm. What is its speed?

8. A wave had a velocity of 83m/s and frequency of 670 Hertz. What is it wavelength?

Rotation 4: Making waves with slinkies

Rotation 5: Waves and patterns

Read this together, taking turns to read aloud while the rest follow along.

There is never just one wave that travels through air, water or space. We are surrounded by waves. These 3D waves

interact with one another. Sometimes it means that this interaction increases their amplitude, sometimes it means it

decreases their amplitude and at other times it means they cancel each other out.

If the wave pattern of two or more waves is in sync (that is, their troughs and crests or areas of compression and

rarefaction are synchronised), then the result is additive, and one wave with an increased amplitude is produced.

This is called constructive interference and it produces antinodes or antinodal lines (high crests and deep troughs)

If the wave pattern of two or more waves is out of sync (that is, their troughs and crests or areas of compression and

rarefaction are not synchronised), then the result is negative, and one wave with a decreased amplitude is produced.

If it happens that the waves are exactly opposite of each other, then they cancel each other out and no wave is

produced (eg. there is no light, no rippling effect, no sound). This is called destructive interference and it produces

nodes or nodal lines (areas where where waves flatline).

The diagrams below are two simple examples of wave interference:

Do the following activities together, helping each other with understand about interference patterns.

1. Draw the resulting wave pattern for the following waves. Identify whether it is constructive or destructive

2. Measure the amplitude and wavelength of each wave.

A picture of water waves that have been produced with a two-point source is pinned onto the board at the

front of the lab. To find out what kind of 2D pattern you would have if you were to ‘cut’ through this two-

point source interference pattern, follow the instructions below:

• Pin nodal lines and antinodal lines. The first have been done for you in the same picture below (nodal line is

black and antinodal line is white)

• If this was light, what would the resulting 2D pattern be? Draw that pattern on the ‘board’ located at the end

of the picture, showing the bands of light at the antinodal lines and the bands of dark at the nodal lines.

‘Board’