Core Physics Topic 9 - Wave Properties

Features of Waves

Any wave transfers energy from a point where there is a disturbance.  Drop a stone into water, and you will see the waves moving away from where the stone fell in (the disturbance).

All waves have a frequency.  This means the number of waves per second.  Frequency is measured in Hertz (Hz).

All waves have a wavelength.  This is the length of one complete wave, the distance between two successive peaks.  It is measured in metres (m).

Waves travel at different speeds as shown in the table:

 Wave Speed (m/s) EM waves 300 000 000 Sound in air 340 Sound in water 1500 Sound in steel 5000

The kind of wave that we are going to look at transfers energy from one point to another.  However only the disturbance travels.  The particles move up and down (or forwards and backwards).  But they do not travel.  We call these waves progressive waves.

Transverse and Longitudinal Waves

A mechanical wave needs a material (e.g. water) to travel in.  A water wave is like this:

A water wave is called a transverse wave.  The disturbance is perpendicular to the direction of travel.

However a sound wave is not like a water wave, but travels as a series of pulses of high and low pressure. This is shown in this picture.

Note the following:

• The regions of high pressure are called compressions.

• The regions of low pressure are called rarefaction.

Rarefaction is NOTHING to do with refraction.

A sound wave is a longitudinal wave.  The disturbance is parallel to the direction of travel.

 Question 1 What are the main similarities and differences between a longitudinal and transverse wave?

The Electromagnetic Spectrum

Electromagnetic waves are transverse waves.  They transfer energy from a source as waves.  They have an electrical component and a magnetic component, but you don't need to know the details of them at this stage.

All electromagnetic waves travel at the speed of light.

Speed of light (Physics code c) = 300 000 000 m/s = 3 × 108 m/s

(Many very big or very small numbers in physics are written in standard form or scientific notation.  Make sure you know what this means.  Ask your physics or maths teacher if you don't.  Make sure you know how to enter standard form on your calculator.)

Electromagnetic waves travel in straight lines.

Unlike other types of wave, electromagnetic materials do not need a material to travel through.  They travel in a vacuum, which is why we see light from the Sun, but don't hear its roar.

Light forms a small part of a large family of electromagnetic waves.  You will know how light splits into the colours of the rainbow.  The scientific term for this is a spectrum.

You can see that the colours run into each other.  There are no distinct boundaries.

The rest of the electromagnetic spectrum is like this as well.  Here is a picture that sums up the electromagnetic spectrum (em-spectrum).

Notice these features:

• The boundaries on the diagram are not distinct.  Waves of wavelength 0.01 nm may be called X-rays or gamma-rays.

• The shortest wavelengths are on the left and the longest wavelengths are on the right.

• The most energetic waves are on the left, while the least energetic are on the right.

• Therefore the shorter the wavelength, the more energetic the wave is.

Uses of Electromagnetic Waves

Note the following conversions:

1 cm (centimetre) = 1 × 10-2 m

1 mm (millimetre) = 1 × 10-3 m

1 mm (micrometre) = 1 × 10-6 m

1 nm (nanometre) = 1 × 10-9 m

The boundaries between various radio frequencies are agreed internationally.  This is to stop stations from interfering with each other.  For example FM radio broadcasts occupy the frequency band 88 to 108 MHz.  Above 108 MHz the band is occupied by the aviation industry for communication and navigation aids for aeroplanes.

Note these conversions:

1 kHz (kilohertz) = 1000 Hz

1 MHz (megahertz) = 1 × 106 Hz

 Question 2 Do the interactive matching exercise.

Wave behaviour

All waves can be:

• reflected;

• refracted;

• diffracted.

Although we usually show these effects with light, we can also show them with other wave types, for example, water waves in a ripple tank.

Reflection

When light strikes a plane (flat) mirror, it is reflected as shown in the diagram:

We should note the following:

• There is an incident and reflected ray;

• The angle of incidence and the angle of refraction are measured from the normal line;

• The normal line is a construction line that is at 90 degrees to the surface of the mirror.

• The angle of incidence = angle of reflection.

A common bear trap is to measure the angle of incidence or reflection from the surface of the mirror. You must remember to measure from the normal.

The picture below shows how the image in a mirror is formed.

We draw accurately two rays coming from the object and hitting the mirror at an angle. Since angle of reflection = angle of incidence, the two rays will be reflected as shown. We can then extend the rays back. Where the two rays meet, that is where the corresponding part of the image is found.

You can see where this is done at the top and bottom of the image.

There are two points to note about the image in a mirror:

• It laterally inverted. This means that, although the image is the right way up compared with the object, left is swapped with right.

• It is virtual. This means that if you look behind the mirror, you won't find the image there.

The picture below shows an example of lateral inversion:

In lateral inversion, left is right and right is left. However the image is the right way up.

 Complete the diagram to show how the rays are reflected down this simple periscope.  Is the image laterally inverted?  Explain your answer.

Refraction

In refraction, a ray passes the boundary between air and a transparent material like glass.

When light hits an air-glass boundary, there are three things that happen to it:

• Some light is reflected;

• Some light is absorbed;

• Most of the light is transmitted.

If the light strikes the boundary at 90 o, the ray carries on in a straight line.  No refraction occurs.  If we shine a ray of light at an angle, we find something a little strange. The ray does not carry on in a straight line as you might expect. Instead it bends inwards. This is called refraction.

Note the following:

• All angles are measured from the normal.

• The angle of incidence is greater than the angle of refraction. The ray therefore bends towards the normal.

• When the ray emerges from the glass, it bends away from the normal. The angle of refraction in this case is bigger than the angle of incidence.

Refraction occurs because the speed of light in air is greater than the speed of light in glass.

For a prism, the ray diagram is like this, using a ray of monochromatic (single colour) red light.  White light is a mixture of colours.

If we use a ray of white light, we see that the light ray gets split into the colours of the rainbow (a spectrum).  This is because different wavelengths refract by different amounts.

This is called dispersion.

 Explain how a prism splits white light into the colours of the rainbow

Diffraction

If we pass waves through a single slit, we observe that the waves spread out due to diffraction.

Notice:

• If the slit is narrow, the diffraction is more marked.

• The wavelength remains the same.

• Diffraction does not need a slit.  Waves can bend round a barrier by diffraction.  Radio signals can be picked up behind hills for this reason.

• The longer the wavelength, the more the waves will diffract.

• All waves diffract.

If the slit is less than one wavelength, no diffraction will occur.

In this picture, we can see that the radio waves diffract around a hill.

A radio can pick up signals even though it's not in direct line of sight of the transmitter.  Immediately behind the hill there is a radio shadow where no signals can be picked up (or they are of very poor quality).  We see the same behind the tall building.  If we go far enough away from the tall building, we come out of the radio shadow, and we can pick up diffracted waves.  The longer the wavelength, the more the waves diffract.

 In a ripple tank, waves pass through a gap to make a diffraction pattern.  The gap is made narrower.  State and explain what you see happen

How Electromagnetic Waves Behave

When an electromagnetic wave (radiation) hits a material, it can be:

• reflected, like light in a mirror;

• absorbed, like heat absorbed by a black surface;

• transmitted, or passed on, like light passing through glass.

Sometimes two, or even all three processes can occur.  Substances behave differently depending on what kind of radiation is falling on them, as the following examples show.

When energy is absorbed by a surface, it heats up.  For example microwaves are absorbed by water molecules and warm up.  This is how a microwave oven works.  Light waves simply pass through water.

A dark painted metal surface absorbs radio and light waves.  However X-rays and gamma rays can pass straight through.

Wax can transmit microwaves, but absorbs light waves.

 Wave Behaviour with materials Radio waves Radio waves can pass through the atmosphere, but longer wave radio waves are reflected by the ionosphere. They pass through walls and our bodies. Microwaves These pass through the atmosphere and the ionosphere. Water molecules absorb them and gain energy. In early experiments with powerful radars, birds flying across the beams dropped out of the sky partially cooked. Infra red Absorbed to a limited depth by human skin. Light Strongly absorbed by our bodies, but any heating effect is very slight. Ultra violet UV light is absorbed. Some ionisation can occur, leading to damage to cells (sunburn) and DNA. X-rays Soft tissues are transparent to X-rays, but they are absorbed by bones. Ionisation can occur, leading to tissue damage. Gamma rays Pass through virtually anything. They can ionise atoms in our tissues, leading to damage.

The Wave Equation

There are three measured quantities in electromagnetic waves:

• The speed;

• The wavelength;

• The frequency.

They are linked by the following simple equation:

 Learn this for the exam: wave speed (m/s) = frequency (Hz) × wavelength (m) In Physics Code: c = fl

In triangle form:

The strange looking symbol that looks like an upside-down "y" is lambda, a Greek letter "l".  It is the physics code for wavelength.  The other codes are:

• c - wave speed (for electromagnetic waves c = 3 × 108 m/s)

• f - frequency.

 Worked Example What is the frequency of Radio 4 long wave that broadcasts at a wavelength of 1500 m? Formula first: c = fl We want the frequency so we must rearrange: f = c/l Now put in the numbers: f = 3 × 108 m/s ÷ 1500 m = 200 000 Hz = 200 kHz

 Question 6 What is the wavelength of a station that broadcasts on 95 MHz (95 000 000 Hz)?

Hazards of Electromagnetic Waves

We have seen that the shorter the wavelength, the more energetic the wave.  This can produce hazards, and we must take precautions to prevent these hazards from causing us harm.

 Wave Wavelength Hazard Prevention Long Wave Radio 1500 m No hazard Medium Wave Radio 300 m No hazard Short Wave Radio 25 m No hazard FM Radio 3 m No hazard UHF Radio 30 cm No hazard Microwaves 3 cm Heating of water in the body Metal grid Infra red 3 mm Heating effect Reflective surface Light 200 - 600 nm No hazard Ultra violet 100 nm Can cause cancer Sun cream (or cover up) X-ray 5 nm Causes cell damage Lead screens Gamma rays <0.01 nm Causes cell damage Thick lead screens or concrete

 Question 7 Answer the interactive matching exercise.

 Summary Electromagnetic waves form a large spectrum of which light is a small part; Electromagnetic waves travel at 3 × 108 m/s in a vacuum in straight lines; EM waves can be absorbed, reflected, or transmitted; Their behaviour depends on the material; Radiation energy increases as the wavelength gets shorter; Energetic radiation has hazards to life; Low energy EM radiation can be used for broadcasting and communication; c = fl.