Waves Six lessons of fun with sound and light

1. Sound Waves

Wave Motion  Physicists think of light and sound as waves The idea of waves explains reflection and refraction The idea of waves explains reflection and refraction Both light and sound can be reflected and refracted Both light and sound can be reflected and refracted  When a wave passes through the medium, parts of the medium vibrate around equilibrium  For sound, the medium is a gas or liquid  Sound cannot travel in a vacuum

Wave Properties  Waves have an amplitude, wavelength, frequency and speed  Put together the worksheet on wave properties

Sound Wave Patterns  Amplitude = loudness (volume)  Frequency = pitch  Shape = timbre (quality of note) Transients

Starter Question  Find the frequency and amplitude of this sound  Volunteer – draw a quieter sound with twice the f

Ultrasound Sound with pitch above the hearing of humans is called ultrasound … Sound with pitch above the hearing of humans is called ultrasound … … just as ultraviolet is light with a higher frequency than we can see … just as ultraviolet is light with a higher frequency than we can see Humans can hear from 20Hz – 20kHz Humans can hear from 20Hz – 20kHz As we age we lose the ability to hear high frequency notes As we age we lose the ability to hear high frequency notes

Uses  The high-speed vibrations caused by ultrasound can be used to clean delicate objects (e.g. watch mechanisms)  Ultrasounds are reflected at the boundary between materials – so they are used for pre-natal scans and industrial quality control

Video clip

Reflections  Ultrasound echoes can be used to determine the position and type of materials present  Amplitude => type of interface  Time => distance to interface  Worksheet

3. Rays and Images

Drawing a ray diagram  Light sources emit rays in all directions

Drawing a ray diagram  We are usually only interested in rays in some particular directions  We might use a slit to cut out the others, or we might just ignore them

Drawing a ray diagram  When a ray hits a surface we draw a normal (‘normal’ = ‘at right-angles’) Mirrors are usually shown like this Normal shown dashed

Drawing a ray diagram  Angle of incidence = angle of reflection Incidence Reflection

Drawing a ray diagram  Finding the image in a mirror

Drawing a ray diagram  Head reflects in all directions – choose two

Drawing a ray diagram  Draw-in the normal(s) for each ray

Drawing a ray diagram  Draw-in the changed (reflected) rays

Drawing a ray diagram  Locate image (where changed rays meet) So top of head appears here

Drawing a ray diagram  Can repeat for another point, or just see … Image Same distance behind mirror

Images  You describe images by their: Size (magnified, diminished) Size (magnified, diminished) Orientation (upright, inverted) Orientation (upright, inverted) Nature (real, virtual) Nature (real, virtual)  A real image shows up if you put a screen in its place (real light rays pass through it)  Is the mirror image real or virtual?

Practical  Form images using the mirrors  Use the three terms to describe the image in each case  Does it make a difference if the object is at different distances from the mirror?

4. Spherical Mirrors

Starter  Complete the diagram showing the image and describe the nature of the image

More Normals  You can still draw a normal to a curved boundary

Concave Mirrors  Concave = bellies in  All reflected rays pass through the focus Optic Axis Focus F

Convex Mirrors  Convex = sticking out  All reflected rays extend back to the focus Optic Axis Focus F

Special Rays for Mirrors Object F Parallel ray Direct ray Image: diminished, upright, virtual Now your turn for a concave mirror (a) object within F, (b) object beyond F

5. Refraction and Lenses

Starter – construct the image Object

Refraction  Light waves travel slower in denser materials  When they change speed their direction changes  When they slow they move towards the normal Glass block slower faster

Ray Diagram 2 Appears shallower

Ray Diagram 3 ROYGBIVROYGBIV Different frequencies are refracted by different amounts “Blue bends best”

Practical Work  Observe refraction in the glass block What special case causes no change in direction? What special case causes no change in direction?  Observe refraction in water Does the penny appear deeper or shallower? Does the penny appear deeper or shallower?  Observe refraction in the prism What happens to light of different frequencies? What happens to light of different frequencies?  Observe refraction at a curved interface What happens to the three rays? What happens to the three rays?  Look through lenses and describe the images (as last time)

6. Lenses

Starter – complete the ray dig.

Convex Lenses  Convex = sticking out  All refracted rays pass through the focus Optic Axis Focus F

Concave Lenses  Concave = bellies in  All refracted rays extend back to the focus Optic Axis Focus F

Special rays for lenses Image: diminished, upright, virtual Optic Axis Object F Parallel ray Direct ray Now your turn for a convex lens (a) object within F, (b) object beyond F, (c) object beyond 2F

Process 1. Draw lens/mirror 2. Add focus (on both sides if necessary) 3. Draw direct ray Goes straight through lenses Goes straight through lenses Reflected like a plane mirror for mirrors Reflected like a plane mirror for mirrors 4. Draw parallel ray Goes through / extends back to focus Goes through / extends back to focus 5. Find image and describe it