Do now!

Can you discuss with your partner all the things you can remember

about the WAVES topic

(topic 3)

Topic 3 Waves

Waves

Waves can transfer energy and information without a net motion of the medium through which they travel.

They involve vibrations (oscillations) of some sort.

Rays

Rays highlight the direction of energy transfer.

Transverse waves

The oscillations are perpendicular to the direction of energy transfer.

Direction of energy transfer

oscillation

Transverse waves

peak

trough

Transverse waves

• Water ripples

• Light

• On a rope/slinky

• Earthquake

Longitudinal waves

The oscillations are parallel to the direction of energy transfer.

Direction of energy transfer

oscillation

Longitudinal waves

compression

rarefraction

Longitudinal waves

• Sound

• Slinky

• Earthquake

Wave measurements

Amplitude - A

The maximum displacement from the mean position.

amplitude

Period - T

The time taken (in seconds) for one complete oscillation. It is also the time taken for a complete wave to pass a given point.

One complete wave

Frequency - f

The number of oscillations in one second. Measured in Hertz.

50 Hz = 50 vibrations/waves/oscillations in one second.

Period and frequency

Period and frequency are reciprocals of each other

f = 1/T T = 1/f

Wavelength - λ

The length of one complete wave.

wavelength

Wave speed - v

The speed at which the wave fronts pass a stationary observer.

330 m.s-1

The Wave Equation

The time taken for one complete oscillation is the period T. In this time, the wave will have moved one wavelength λ.

The speed of the wave therefore is distance/time

v = fλv

λf x

1) A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving?

2) A water wave travels through a pond with a speed of 1m/s and a frequency of 5Hz. What is the wavelength of the waves?

3) The speed of sound is 330m/s (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?

4) Purple light has a wavelength of around 6x10-7m and a frequency of 5x1014Hz. What is the speed of purple light?

Some example wave equation questions

0.2m

0.5m

0.6m/s

3x108m/s

Electromagnetic spectrum

Long Wavelength Short

Wavelength

Low Frequency High Frequency

What do they all have in common?

• They can travel in a vacuum• They travel at 3 x 108m.s-1 in a vacuum

(the speed of light)• They are transverse• They are electromagnetic waves (electric

and magnetic fields at right angles to each oscillating perpendicularly to the direction of energy transfer)

Light travels faster than sound

Speed of light = 300 000 000 m/s

Speed of sound (in air) = 330 m/s

Can you copy this please?

Law of Reflectionnormal

Angle of incidence

Angle of reflection

mirror

angle of incidence = angle of reflection

Can you copy please?

Image in a mirror

• Upright

• Same size as object

• Distance from mirror to object = “distance” from image to mirror

• Laterally inverted

Why?

Refraction

Glass block

Beam of lightLight going from air to glass bends towards the normal

Light going from glass to air bends away the normal

Can you copy this please?

Refraction

When a wave changes speed (normally when entering another medium) it may refract (change direction)

Snell’s law

i

r

Ray, NOT wavefronts

speed in substance 1 = sin(i)speed in substance 2 sin(r)

Snell’s law

i

r

Ray, NOT wavefronts

In the case of light only, this quantity is called the refractive index (η)

η = sin(i)

sin(r)

Critical Angle

Critical angle (when angle of refraction is 90º)

Copy!

90º

Total Internal

Reflection

Critical angle

sin c = 1/η

sin(critical angle) = 1/(refractive index)

Examples of Total Internal Reflection

Examples of Total Internal Reflection

Diffraction

Waves spread as they pass an obstacle or through an opening

Diffraction

Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

Diffraction

Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

Analogue and digital

Let’s read pages 164 and 165.

104 103 102 101 100 10-1 10-2 10-3

1 5

25 24 23 22 21 20 2-1 2-2 2-3

1 1 1 1

01111001101111001101111011101111

7 0 1 1 1

9 1 0 0 1

11 1 0 1 1

12 1 1 0 0

13 1 1 0 1

14 1 1 1 0

14 1 1 1 0

15 1 1 1 1

15 1 1 1 1

15 1 1 1 1

Advantages of digital signals

• Less affected by interference

• Interference is not increased when the signal is amplified

• Uses weaker signals than analogue.

Sound Longitudinal waves

The oscillations are parallel to the direction of energy transfer.

Direction of energy transfer

oscillation

The Cathode Ray Oscilloscope

Can you stick the sheet in?

Amplitude = volume

Can you copy this

CAREFULLY please?

Pitch = frequencyCan you copy

this CAREFULLY too please?

Range of hearing

Humans can hear up to a frequency of around 20 000 Hz (20 kHz)

Can you copy this too please?

Your own mind-map