1. Light, Sound, Waves
GCSE Introduction

2. Questions
Why do we put sound and light together? They are so different.
Why do we call them “waves”?
What do we know about them?
What are they made of?

3. How does light behave? It radiates out from a source In straight lines
unless it gets reflected or there is a change of material
It seems to move so fast it arrives instantaneously and measuring its speed is difficult (but we can: 3 x 108 m/s)
A source is often so hot it glows and emits light in all directions (there are cold sources like LEDs, glow-worms, fireflies, etc.)

4. How do we draw it? rays beam With beams or with rays Barrier Barrier
Source
Barrier
Source
Barrier
rays
beam

5. Filament Bulbs and point sources
Are too spread out

6. Spread out sources

7. Compact sources
From above a vertical filament looks like a point source of light and gives clear rays.

8. Ray boxes
From above the filament gives a compact source of light and clear rays
The lamp slides forwards and backwards
Different shapes of glass/plastic can be put in front to investigate the effect on rays.

9. Using ray boxes Connect the bulb to 12 V dc or ac and check it works.
Use a triple slit in the slot at the front, put the ray box on a sheet of white paper, slide the bulb forwards and backwards.
Stand a cylindrical lens on the paper and note the effect the lens has on the rays.
What makes you think light travels in straight lines?

10. Things to try

11. Pin hole Cameras
We need to think in terms of rays to see what is happening.
The image is opposite the pinhole on the screen
It is upside down. Why?
Because light travels in a straight line from top of object to top of image, etc.

12. Images
Each image is opposite one pinhole so light travels straight there.
A pinhole camera shows what a lens does to light

13. Rays forming images
A lens brings all the rays from one point on the source to a single point in the image
Converging rays form a real image
A real image is one that can be formed on a screen

14. Reflection

15. Reflection and images
A plane mirror reflects regularly
A rough surface scatters the rays and produces diffuse reflection – no image

16. It’s the Law
Incident ray, normal and reflected ray all lie in the same plane.
The angle of incidence equals the angle of reflection

17. Does it always work elsewhere?

18. Rays and images Converging rays form an image with lenses
What about rays after reflection?
What sort of image do we see? Is it real?
Follow the instructions carefully .... an accurate drawing of rays will be obvious in the result!

19. Real Rays Reflected
Fold the A3 sheet in half and set up the mirror on the fold. Mark its position.
Set up a ray box (don’t use a lens) and triple slit so that the rays diverge within the area of the paper.
Make sure the lamp is over the sheet of paper and direct three rays onto the mirror.
Mark on the paper along the centre of each incident and reflected ray.
Draw in a pencil line for each ray. Where do they cross?
Look back along the rays; where does it look like the raybox is?

20. Finding the Image behind the mirror
Look back along the reflected rays to see where they apparently come from. Is each one straight or curved or crooked?
Draw behind the mirror with a dotted line where each ray appears to come from.
Where do the dotted lines cross?
Draw a line from Object position to Image position.
What is the angle between this line and the mirror?
What is the mirror to object distance and the mirror to image distance?
How does the image compare to the object?

22. Virtual images - an illusion
Rays seeming to come from an object behind a mirror are an illusion: a virtual image forms from diverging rays.
The image is the same distance directly behind the mirror as the object is in front

23. Hotel pool with glass side = endless fun.
Allegedly.
Distorts !

24. Refraction
Set up a single ray going through a parallel sided glass block.
Look back along the emergent ray, through the block.
What is strange about it?
Try different angles of incidence, including at 90⁰ to the side of the block i.e. 0⁰ to the normal. C B A
90°

25. Try these!

26. Refraction
The pattern is that a ray entering an optically more dense material is bent towards the normal, and vice versa.
Special case: a parallel sided block causes the emergent ray to be parallel to the incident ray.

27. Why Refraction? Water waves slow down in shallower water
Light waves slow down in denser materials

28. Refraction: Why?
Because waves slow down in denser materials (like glass), just as sea waves slow down in shallow water

29. The part of a wavefront entering the glass first slows down first and the rest of the wave follows.

30. Refraction and images
The bending of the light at the water air surface means the image you see under water is an illusion!
How?

31. Refraction and image formation
Looking back along the rays, they seem straight
so they seem to come from one place, whereas in fact they come from another creating an illusion, a virtual image.
Real images can be formed on a screen, virtual images cannot.

32. Good Vibrations
A Sound Study

33. What is “sound” Something our ears respond to!
Seems to spread out from anything that oscillates fast enough
Seems to be mechanical
Travels through the air
Travels under water
Travels through solid objects

34. Surround Sound
It spreads out and travels behind obstacles (unlike light and shadows) – diffracts
It is reflected by large flat surfaces - the Abbey, a canyon wall, the seabed for sonar
It changes direction in denser air – refracts – so sometime bends upwards and cannot be heard on the ground.
Is it like light or not?

35. So what is sound?
Pumping the air out of a bell jar shows that sound cannot exist in a vacuum
We can see the bell vibrating, but cannot hear it.
We can see it. So is sound like light or not?
By the way, don’t buy comics on the moon ...

36. Er, no air on the moon, so no speech in the bubbles?

37. I said, what is it?
If you watch a loud speaker, the paper cone vibrates back and forth
It pushes the air forward, then pulls it back.
This pressure wave travels out through the ear and is still strong enough to make eardrums vibrate
What about tuning forks?

40. So it is waves of pressure travelling out ?
Yes. They are longitudinal waves
Longitudinal waves are when the displacement is parallel to the direction of the wave…

41. You need to be able to sketch and label something like this diagram!
Animated version!

42. How do we hear?

43. From there to ear

44. How does sound sound?
Frequency (f ) – this is a count of how many waves pass by every second and is measured in Hertz (Hz)
The time for just ONE wave to pass is the Time Period of the wave; T = 1/f.
The sensation of Pitch increases as the frequency increases – high notes, high frequency.
More singers!
Proper wine glass video

45. Sounds interesting ...
Sounds in the range 20– Hz (approx.) can be detected by the human ear.
< 20 Hz is infrasound (elephants, pigeons, ...)
> 20 kHz is ultrasound (dogs, bats, ...)

46. How else does sound sound?
The amplitude of a wave is the maximum movement of a vibration from the midpoint
Loudness increases as the amplitude of the wave increases.
Wavelength is the distance between two compressions or peaks

48. Wave Speed, frequency and wavelength
If a wave starts out at f=2Hz, it completes 2 wavelengths per second
If f= 4Hz, it completes 4 wavelengths per second.
If we know the wavelength = 3m, then
A travels at 2 x 3 m/s
B travels at 4 x 3 m/s
Speed = wavelength x frequency

49. Some example wave equation questions
A water wave has a frequency of 2 Hz and a wavelength of 0.3 m. How fast is it moving?
A water wave travels through a pond with a speed of 1m/s and a frequency of 5 Hz. What is the wavelength of the waves?
The speed of sound is 330 m/s (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
Purple light has a wavelength of around 6x10-7 m and a frequency of 5x1014 Hz. What is the speed of purple light?
0.6 m/s
0.2 m
Note: Speed of sound questions included – although not transverse waves!
0.5 m
3x108 m/s

50. Homework
Research one group of instruments of the orchestra , and explain how their sounds are different in pitch, loudness and quality ( i.e. what they sound like and why they sound different)
Choose one from strings, or woodwind, or percussion (inc piano: why?)

51. That’s not all about how sound sounds
Sounds can seem as “pure” as a choir boy’s top note
Or as screechy as a learner on the violin.
Listening to someone on the phone, you know who it is as soon as they say “it’s me”
Mmmmm.

52. Waveform or Quality (timbre)
We need to have a way of watching the pressure waves as they go past
An Oscilloscope can do this for us, drawing a real time graph of pressure against time

53. The horizontal scale (or time base) is marked in the time each division represents, hence “time/div”
Usually there is a grid of 1 cm squares, so a time/div of 0.5 ms means each square represents s

54. NB
You must be able to:
compare amplitudes and frequencies of sounds from diagrams of oscilloscope traces.

55. Compare these oscilloscope traces ..
time A C B
If the time/div is 0.2 ms what frequencies are these traces?
time D F E

56. The Speed of Sound

58. Speed is Distance travelled time taken The trouble with sound is
It goes so fast!
It gets more spread out and weaker as it travels
Wonderful students

59. How can we measure it?
Crash some cymbals 100m away and time how long after we see the clash until we hear it because light travels so much faster than sound.
Problem: It is still a very short time and hard for mere humans to measure!
Solution: send the cymbals further away
Problem: The sound gets fainter as it spreads out
Solution: make a louder sound – blow something up!
Problem: get arrested.
Solution: use electronic timing ...

61. If we know how fast sound travels we can work out how far away a thunder storm is.
How exactly?

62. Echoes Sound reflects best from large flat surfaces
Can use this to measure speed of sound

63. Stand in front of a large building
Measure distance to building
Clap rapidly but in time with the echo
Time a number, say 20 echoes,
Calculate (20 x distance there and back)/time

64. Echoes and Geology
Echoes and medicine

65. Echoes and Sonar

66. Ultrasound scans