Physics 3 - Waves

Question 1

3.1 List the units and unit symbols for: frequency

Answer 1

hertz (Hz)

Question 2

3.2 explain and sketch the difference between longitudinal and transverse waves

Answer 2

Longitudinal waves oscillate parallel to the direction of travel

Transverse waves oscillate perpendicular to the direction of travel

Question 3

3.3 Define amplitude, wavefront, frequency, wavelength and period of a wave and label them on appropriate diagram

Answer 3

Amplitude: maximum displacement of the wave (x axis to top of one wave)

Wavefront: A plane that joins points on adjacent waves together

Frequency: Number of complete waves that cross a point per second

Wavelength: Distance between adjacent peaks

Period of a wave: Time taken for one complete wave to pass a point

Question 4

3.4 State what waves transfer and what they definitely don’t

Answer 4

Transfer: Energy and information

Don’t transfer: matter

Question 5

3.5 Give the relationship between the speed, frequency and wavelength of a wave

Answer 5

Wave speed = Frequency x wavelength

v = f × λ

Question 6

3.6 Give the relationship between frequency and time period:

Answer 6

Frequency = 1/time period
F = 1/t

Question 7

3.7 Write down the approximate speed of sound in air and electromagnetic waves in a vacuum in m/s

Answer 7

Sound = 340m/s

EM waves = 3.0x10^8 m/s

Question 8

3.8 explain why there is a change in the observed frequency and wavelength of a wave when its source is moving relative to an observer, and give the name of this effect

Answer 8

Doppler effect

Wave speed is constant, so a moving source produces waves that are closer together in one direction and further apart in another. The wavelength is smaller and wave speed is the same. Since frequency = wave speed/wavelength, the frequency is greater

Question 9

3.9 Name the 4 things that can happen to a wave when they interact with matter

Answer 9

Reflected, refracted, diffracted or absorbed

Question 10

3.14 Are light waves transverse or longitudinal?

Answer 10

Transverse

Question 11

3.14 State the two pieces of evidence that suggest light is a wave

Answer 11

It can be reflected and refracted

Question 12

3.15 State the law of reflection

Answer 12

The angle of incidence equals the angle of reflection

Question 13

3.16 Sketch a ray diagram showing refraction. Label the incident and refracted ray, the incident and refracted angle and the normal

Answer 13

JFDI

Question 14

3.17 practical: Describe an experiment to investigate the refraction of light and explain the results. Do this using
rectangular blocks

Answer 14

Draw around the glass block, then direct a ray of light through it (at appx 40º)

Draw crosses to mark where the light enters and leaves the glass block. Join the crosses to create a line and attach these two lines.

Draw a normal at 90º and measure the angle of incidence and refracted angle using a protractor.

Calculate the RI using n = sin i/ sin r and repeat, taking averages.

Light refracts as different parts of it enter the glass and slow down.

Question 15

3.17 practical: Describe an experiment to investigate the refraction of light and explain the results. Do this using
semi-circular blocks

Answer 15

Aim the light at the centre of the curved edge so that it leaves the centre of the flat edge of the semi circle (it enters along the normal and doesn’t refract)

Draw around the semi circle and mark the mid point. Then continue to increase the angle of incidence, until all of the light leaves at the surface of the flat side.

Draw crosses to mark this ray, and then connect this to the centre of the flat surface. Draw in a normal and measure this angle - it is the critical angle.

Question 16

3.17 practical: Describe an experiment to investigate the refraction of light and explain the results. Do this using
triangular prisms

Answer 16

Shine white light through the prism until a spectrum of colours appears.

This happens as the different frequencies of light are refracted by different amounts (the higher it is, the more it is refracted)

Question 17

Pneumonic for refraction?

Answer 17

FAST

Faster - Away from t˙he normal
Slower - Towards the normal

Question 18

3.18 Write down the relationship between refractive index, angle of incidence and angle of refraction

Answer 18

n=sin⁡i/sin⁡r

Question 19

3.20 Describe what is needed for total internal reflection

Answer 19

Light enters at an incident angle higher than the critical angle

Light goes from a high refractive index to a low refractive index

Question 20

3.20 Describe and explain the use of total internal reflection in transmitting information along optical fibres

Answer 20

Optical fibres are glass or plastic that have an inner core that has a higher refractive index than the rest of it.

Therefore, once light enters at the critical angle, it is always reflected and never leaves

Question 21

3.20 Describe the role of total internal reflection in the use of triangular 90º prisms and where it can be used

Answer 21

If light enters one of the short sides at 90º it is TIR by the long side and leaves the other short side at 90º.

This can be used in a periscope or a camera

Question 22

3.21 explain the meaning of critical angle c

Answer 22

If the incident angle is the critical angle, the refracted ray comes out along the surface

Question 23

3.22 Write down the relationship between critical angle and refractive index

Answer 23

sin⁡c=1/n

Question 24

3.23 Is sound a longitudinal or transverse wave?

Answer 24

Longitudinal

Question 25

3.23 State the two pieces of evidence that suggest sound is a wave

Answer 25

It can be reflected and refracted

Question 26

3.24P State the frequency range for human hearing

Answer 26

20 - 20,000 Hz

Question 27

3.25P practical: investigate the speed of sound in air

Answer 27

A holds a drum 200 metres away from B (measure using a trundle wheel)

B measures the time between seeing A strike the drum and when he hears it (using a stopwatch)

Repeat several times and take an average.

Work out speed by s=d/t

Question 28

3.26P Describe how an oscilloscope displays a sound wave and sketch an example showing the time period and amplitude

Answer 28

The screen displaysa displacement time graph of the sound wave.

The amplitude shown represents the loudness of the sound

The horizontal axis can be used to work out the time period and then the frequency (f=1/t)

Question 29

3.27P practical: Describe an experiment to investigate the frequency of a sound wave using an oscilloscope

Answer 29

Connect a microphone
Adjust the time setting so that at least one division is showed on the screen and read the time period.
Work out time period - wave speed x number of divisions one wave takes up.
1/t = f

Question 30

3.28P Describe how the pitch of sound is represented by a wave

Answer 30

The pitch is represented by the frequency of vibration of the source

Question 31

3.29P Describe how the loudness of a sound is represented by a wave

Answer 31

Loudness relates to the amplitude of vibration of the source

Question 32

What is meant my critical angle?

Answer 32

The angle of incidence inside the glass (or whatever material) whereby TIR occurs

Question 33

Explain the Doppler Effect

Answer 33

Frequency in front of direction of motion is smaller than that in the opposite direction.

This is because the emitter produces waves closer together.

Therefore, the wavelength is smaller, but the wave speed is the same - f=v/λ - f is higher