Light Waves: Seeing is Believing, CBS Talk 2016

Useful information
Kit List: 

At least two different colours of laser pointer.
Diffraction gratings/CD DVD segments.
Ripple tank.
Shive wave machine (made from duct tape, skewers and weights).
Bubble wand (piece of wire bent into a loop) + bubble liquid.
Glass/perspex prism.
Polarising filters.


I won't necessarily use all these demonstrations in the talk:

A) Polarisation: essentially a copy of the most basic part of the polarisation experiment, rotating two polarising filters.

B) Shive wave machine: two long duck tape strips stuck together with skewers sandwiched between at regular intervals. Weights are attached to the skewer ends. This is used as a wave model to demonstrate some basic characteristics.

C) Walking people model of refraction: use a line of volunteers walking at a regular pace to demonstrate what happens to a wavefront at an imaginary interface.

D) Refraction: shine a laser/white light through a prism to demonstrate refraction.

E) Use a slinky to demonstrate interference and standing waves.

F) Make bubbles/bubble films and relate colours being observed to standing waves in the film.

G) Ripple tank: used to demonstrate diffraction from a double slit or airy-type pattern formed if the tank is round.

H) Diffraction gratings: used to demonstrate that light diffraction is equivalent to what was observed in the ripple tank.

I) Get the audience to make a Mexican wave.

TW - This transcribes a set of talk notes which are believed to be from either this talk or a rerun of it.

Have any of you tried polarisation?
Here it is: turns sheet.
If chaos demo "did you know light is a wave?" - polarising filters
What does that mean?
I'll show you that it is and some cool effects.

Examples of wave?
Do mexican (hands)
Move energy, no one moves

To look at other wave things about waves use shive: present and demo - shive machine with plastacine
Note does same as mexican
Amplitude: how big, ppt, shive+mexican
wavelength: top to top ppt, shive
speed: shive+mexican

what happens when speed changes - shive machine w/o plasticine -> connect two shive machines -> 2 shives connected together
connect shives: some reflects some keeps going
something else happens: think water waves going into honey slow down
soldiers linked arms marching top of wave
1 step every second: click notice direction
when wave hits honey 1st slows down other catch up
click click see that direction changed
recap: some keeps going changes direction some reflected

try with light - laser smoke spray, glass tank w/ milky water, visualiser shield -> disconect two shives -> shive w/ Plasticine
laser beam invisible, only see where ends up: use cloudy water and smoke
see same as before
first evidence light is a wave

what happens when waves come together
will send two pulses through machine demo pulse
up and up what do you think will happen+do
up and down what will happen+do
when up and up come together to make big then keep going
up and down cancel out but also keep going

circular ripples like stone dropped in water
what happens when two come together
let's see: video veritasium

use bobs to make waves
in some places up and down cancel to give nothing
in other places add to give big wave
think about red line: here we have waves, here none: shout out

can make circular ripples with water in a different way: pass through slit
now try with light: grating has lots of slits - grating & green laser
make circular waves with light which add up cancel out just like water
see bright and dark patches just like along the red line

now see what happens change spacing of bobs and wavelength
to do this use these two slides which are just like ripples - transparencies big
when I overlap just like ripples coming together
see dark lines where we would see waves just like on video (slide!)
cover half of patterns!!!
move ripples together black lines move further: green dots move further apart
light: use different grating: dots move further apart: more evidence - grating & green laser

now what happens when we change wavelength/ring spacing - transparencies (small)
smaller wavelength closer together bigger further apart
red light: further, green closer blue closer - grating and 3 lasers
guess rgb different wavelengths with... : slide
In fact every colour in rainbow has wavelength
useful fact: white light made from all colours in rainbow
can split into colours, doesn't matter how -grating/prism and torch

use all this to explain bubble colours
blow bubbles/bubble film - bubble liquid, blower and black paper
cross section/side cut through bubble
wave in, some reflected some keeps going and reflected
if as here up and up when out big wave out: lots of red reflected
bubble changes thickness: what about this bit.
since red gets up and down:cancels out, not much red reflected
what about blue: adds up: lots of blue reflected see blue
all across bubble surface different thicknesses and at each one some colour is reflected more than others: lots of different colour

what about the experiment? re-demo!
use slinky as demo and explain - slinky + 2 wooden boxes
emphasise slits do same job as filter

final thing,, can make shive at home easily
thank you and thank friends
questionnaire on way out
out on bottom door
any questions/want to play come down at end

Risk Assessment
Date risk assesment last checked: 
Thu, 11/02/2016
Risk assesment checked by: 
Andrei Ruskuc
Date risk assesment double checked: 
Thu, 11/02/2016
Risk assesment double-checked by: 
Risk Assessment: 

Shining a torch in audience members' eyes.
Action to minimise:
Be aware of where torch is shining, don't aim at front few rows of audience.
If accident:
Might be dazzled, no permanent damage.

Sharp skewers hurting demonstrators/volunteers.
Weights falling off and falling on people.
Duck tape snapping under tension.
Wave machine hitting people whilst in motion.
Action to minimise:
Blunt skewer ends.
Ensure weights properly fixed on.
Test limit of duck tape strength prior to demonstration.
Control number and location of audience volunteers.
If accident:
Call first aider.

Volunteers tripping up.
Volunteers running/out of control.
Action to minimise:
Ensure area clear of trip hazards prior to talk starting.
Terminate demonstration if it gets out of control.
If accident:
Call first aider.

Prism breaking: sharp glass.
Eye injury from laser beam.
Action to minimise:
No volunteers and may want to place prism on a flat surface instead of holding it.
Use <= class II lasers: blink reflex should protect eyes, make sure to shine away from audience.
If accident:
Ensure shards of prism are collected/contained before continuing.
Call first aider in case of injury.

Fingers getting caught/cut by metal slinky.
Slinky hitting people.
Action to minimise:
Tape slinky ends and ensure volunteer knows where/how to hold.
Ensure area used for slinky demonstration is clear of people.
If accident:
Call first aider in event of injury.

Audience members allergic to washing up liquid.
Slip hazard is spilt.
Sharp edges on bubble wand.
Action to minimise:
Check with audience/blow bubbles so that they don't land on audience.
Ensure container is on stable, elevated surface.
Check and tape any sharp edges prior to use.
If accident:
Call first aider in event of injury.
Mop up spillages promptly.

Slip hazard if water split.
Allergic reaction to any dyes used.
Cutting on shards of glass/perspex if tank breaks.
Action to minimise:
Ensure container is on stable, elevated base.
Check dyes prior to use, don't let audience members do this part.
Ensure container is on a stable surface and don't move it during talk.
If accident:
Mop up spillages promptly.
Call first aider in event of injury.
Clear up/contain glass shards.

Eye injury from laser.
Action to minimise:
Use <= class II lasers: blink reflex should protect eyes, make sure to shine away from audience.
(test grating prior to talk to identify light paths)
If accident:
Call first aider in event of injury.

Audience members falling over.
Action to minimise:
Check space in front of seats in lecture theatre before talk.
If accident:
Call first aider in event of injury.