GCSE Modular Science Course Outline

Waves Notes FT/HT

Waves can be produced in ropes, springs and on the surface of water.

When waves travel along ropes or springs or across the surface of water they set up regular patterns of disturbance. They are a way of transferring energy from one point to another without transferring matter.

. The maximum disturbance caused by a wave is called its amplitude

. The distance between a particular point on one disturbance and the same point on the next is called the wavelength

. The number of waves each second produced by a source (or passing a particular point) is called the frequency, and is measured in hertz (Hz).

We can find the speed of a wave from the formula;

wave speed = frequency x wavelength v  f  v = velocity in m/s or ms-1 f = frequency in Hz (s-1)  = wavelength in m.

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Example

This waveform has a wavelength of 3m. By inspection it takes 2 seconds for a complete cycle (1 up & 1 down)

To work out the frequency then wave speed we must find out how many cycles there are in one second; 1 f  v  f   time foracycle 1 v  0.5Hz 3m f  2s v  1.5m / s 1 f  0.5s v  1.5ms1 f  0.5Hz

The formula can be rearranged to work out any of the three variables. v  f   v   f v  f  Mr Powell HT & FT Notes Year 11 Physics 2003/4 Simple Wave Questions

1. Work out the frequency of a wave which travels at a speed of 666ms-1 and has a wavelength of 3m?

2. What is the speed of a wave which has a frequency of 125Hz, and wavelength of 0.03m?

3. What is the speed of a wave which has a frequency of 2000Hz, and wavelength of 73m?

4. What is the wavelength of a wave travelling at 10,000 ms-1 if its frequency is 25MHz?

5. What is the speed of a wave with wavelength of 0.000001m and frequency 1 MHz?

Answers

1. 666 ms-1/ 3 m = 222 s-1 = 222 Hz

2. 125 Hz x 0.03m = 3.75 m/s = 3.75 ms-1

3. 2000 Hz x 73m = 146,000 m/s = 146 kms-1

4. 10,000 ms-1 / 25 MHz = 10,000 ms-1 / 25,000,000 Hz = 0.0004 m = 0.4 mm

5. 0.000001m = 1 x 10-6 m and 1 MHz = 1,000,000s-1 Therefore the product of the two = 1; 0.000001m x 1,000,000s-1 = 1 ms-1

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Reflection

Waves travelling along a rope or spring, or across the surface of water, can be reflected. When this wave reaches a concrete harbour, some of the energy of the wave is absorbed and some is reflected.

If we look at this image we can see how light waves reflect off the surface of the water.

Normal Incident ray Reflected ray   i r  =  i r

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Questions

1. When a water wave reaches the shore what happens to the energy carried in the wave?

2. What is the law of reflection?

3. Draw a diagram to show this?

4. What happens when a light wave or ray is incident on a 100% mirror?

5. What would happen if a light wave or ray was incident on a 50% mirror?

Answers

1. Some energy is reflected back into the sea and some is absorbed by the land.

2. The angle of reflection is the same as the angle of incidence from the normal.

3. See notes above.

4. All of the light is reflected back away at the same angle from the normal as it arrives.

5. Half of the light would go through and half of the light would behave as in question 4.

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Refraction

Waves travelling from air through the surface of water can also be refracted. This is the technical word for the wave changing speed and course.

This happens when the “medium” or type of material changes. Mainly the refraction is to do with the change in density and structure of the medium.

In the case of this glass block we Glass can see that when the medium becomes denser (the glass) the light rays bend or refract towards Air the normal. Then when the medium becomes less dense (the air) the rays bend away or refract away Air from the normal.

We also find that the red light does not refract as much as the blue light. This is because the blue light has a shorter wavelength. So when light rays slow down or speed up they change direction

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Refraction & the Wave formula

This phenomemon can also be explained via mathematics and the wave formula; v  f  The frequency of a wave is fixed from the source. This means that if the wave slows down, for the relationship to stay true the wavelength must also decrease.

An example could be; v = f = 3Hz x 3m = 3 s-1x 3m = 9 ms-1

However, if v is reduced to 6 ms-1 so must the speed

v = f 6ms-1 = 3Hz x  6ms-1 = 3 s-1 x  6ms-1/ 3 s-1 = 2m

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Refraction & the Wave formula

The two rules are simple;

1. When waves slow down they are refracted towards the normal. (deep to shallow water or light to heavy medium)

2. When wave speed up they are refracted away from the normal (shallow to deep or heavy to light medium)

The reasons for the rules are;

 Heavy mediums i.e. glass slow down waves

 Shallow water i.e. the beach cause friction to the wave and slow them down

Mr Powell HT & FT Notes Year 11 Physics 2003/4 However, this picture is overly simplistic and we find that the effects of refection and reflection are usually combined.

This prism shows how part of the light is reflected and part is refracted.

A simpler diagram is the one below which shows a light ray entering a glass prism. The beam is split.

This diagram shows the combined ray diagrams which show both the effect of refraction and the effect of reflection.

The same effect is seen when water waves arrive at a shelf in the sea. When the water changes depth this makes a wave (except that travelling at 900 to the shelf) refract or change direction.

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Questions

1. When a water wave reaches the shore what can happen to the direction of the wave?

2. Why do waves refract?

3. For a glass block draw an accurate diagram of the wave in which an incident red light ray will behave?

4. For a triangular glass prism attempt to draw the same diagram, also add in the reflected ray?

Answers

1. The wave will refract and slow down if travelling at an angle to the normal.

2. If the density of the medium in which they are travelling changes the wave will either speed up or slow down. This causes the change in direction.

3. As per this diagram;

Mr Powell HT & FT Notes Year 11 Physics 2003/4 4. As per this diagram;

Normal Normal

  i   i r r

The Critical Angle

The critical angle is quite simply the angle at which a refracted ray becomes a reflected ray off the internal surface of a prism. As the angle of reflection moves towards 42.2 the refracted ray moves towards the surface of the prism. At 42.2 the refracted and reflected rays meet;

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Prisms

This picture shows how we can use a glass prism and the effect of refraction to separate light rays of different wavelengths. As the beam of white light passes through several prisms it diverges or splits more and more.

The blue light (shorter wavelength) is refracted more than the red light (longer wavelength).

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Transverse & Longitudinal

There are two types of disturbances which fall under the main categories of waves.

Transverse - where the motion of the particles or energy contained in the wave moves at 900 to the direction of the wave. We think of this type of wave as a side to side or up and down motion. Examples of this type of wave are any EM waves from the EM Spectrum. The can travel through vacuums. The wave has Peaks and Troughs

Longitudinal – where the motion of the particles or energy contained in the waves moves in the direction of the wave. We think of this type of wave as a push pull type of wave as the particles are pushed and pulled forwards and backwards. An example of this type of wave is sound. This type of wave needs a medium and cannot travel through a vacuum. The wave has Compressions and Rarefactions just like a transverse wave. The diagram helps compare the two;

R C R C R

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Transverse & Longitudinal Questions

1. Why can’t longitudinal waves such as sound travel through a vacuum?

2. What do we call a peak on a longitudinal wave?

3. Draw an example of a longitudinal waveform.

4. Draw an example of a transverse waveform.

5. Where is the energy directed in a transverse wave?

Answers

1. Longitudinal waves like sound depend on the push and pull of matter. In space there is no matter to push and pull hence the waveform cannot progress.

2. A Compression.

3. See notes.

4. See notes.

5. At 900 to the direction of wave motion.

Mr Powell HT & FT Notes Year 11 Physics 2003/4 6. EM Spectrum FT & HT

This picture shows the EM Spectrum. In the middle is visible light. Between 400nm and 700nm (nano metres or 1x10-9m = 0.000000001m). The eye is sensitive to this light and by monitoring the reflection of light from the objects around us we are able to distinguish the distance of the objects and the type of materials from which they are made. Our eyes are very complex organs which have specialised cells on the back of the eye to absorb the light. This is what we call the retina.

If the wavelength gets shorter or longer you get a different type of wave which has different properties. However, all EM waves travel at the same speed in a vacuum. 3 x 108 m/s or 300,000,000 m/s or you may see it as 186,000 miles per second (in old units)

This is because the frequency of an EM wave increases as the wavelength decreases. If we increase the wavelength by x2 the frequency will half, and the effect is cancelled out.

v  f   v  f   1 v  const v  f  2 2 v  f

Mr Powell HT & FT Notes Year 11 Physics 2003/4 This is the EM Spectrum. The shorter the wavelength the more dangerous the waves are. We only see the visible light ROY-G- BIV in the centre.

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Seismic Waves FT and HT

Our knowledge of the structure of the Earth comes mainly from studying how the shockwaves from earthquakes (seismic waves) travel through it. These waves are detected using seismographs. By examining how the different waves travel through the core of the Earth we can tell which parts are liquid and which parts are solid. This is because the transverse or S- waves will only travel through a solid and not a liquid. By this technique we have found that the Earth has a structure like this;

Crust Core

Inner core Mantle

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Seismic Waves HT Only

Earthquakes produce surface waves that can cause earthquake damage and two types of waves that can travel through the Earth.

. faster travelling P waves, which are longitudinal and travel through liquids as well as solids

. slower travelling S waves which are transverse and travel only through solids.

The speed of both types of waves increases with depth through the mantle. The waves travel in curved paths as their speed changes gradually through a material. When the state of the transmitting medium changes abruptly e.g. when moving from solid to liquid, the wave direction also changes abruptly.

It is by observing the paths of these waves that scientists have been able to work out details of the Earth’s layered structure.

Candidates should be able to interpret diagrams of the paths of seismic waves inside the Earth in terms of:

. the liquid nature of the Crust Earth’s outer core; . refraction at the boundaries between layers; . refraction due to Inner changes in speed within core a particular layer. Core

Mantle S-wave shadow p region s

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Seismometers

It is easy to find the Focus of an Earthquake. If three seismometers are recording an earthquake, they can each work out how far they are from the epicentre. Combining the three measurements finds where the epicentre is.

70km C A 40km Focus

Mr Powell HT & FT Notes Year 11 Physics 2003/4 Structure of the Earth

. From the outside in, there is: . The Crust: The hard surface we live on. . The Mantle: This is a very thick liquid which goes down half way to the inner core. . The core: Very hot liquid . The inner core: Very hot, but under so much pressure this part of the Earth is a solid.

Crust Core

Inner core Mantle

Mr Powell HT & FT Notes Year 11 Physics 2003/4