Introducing the ray model

Other relevant Nodules: all subsequent light nodules

Can of worms (mainly linked to the ray model of light):

• We use a variety of models to elicit understanding of light. Many of these models are taken as fact. NB. Teachers don’t help by switching backwards and forwards between different models very quickly

• If light travels in a straight line, why do we draw it as a wave? Should we visualise it as a water/mechanical wave?

• Many non-specialists regard rays as a ‘real thing’ rather than an abstract model, and confuse beams and rays

• Language- medium, source-(luminous and non-luminous), detector; intensity and amplitude (in context of laser beams and projector beams)

Can opener Why SPT reference and other resources
Casting a shadow

Can teachers explain what they see?

All of the light continues to travel until it is absorbed, somewhere. So how do we see it? How does it travel to us?

Elicits teachers’ understanding of the subject matter.

The key thing here is to make a huge effort not to switch between the two models – introduce string model to explain
What are rays?

Light travelling in straight lines

A ray diagram for a shadow

Totally in the dark
Going further / follow onStart with the pathway – the ray and follow on with the mechanism of transfer later.


StrategyWhySPT reference and other resources
Waves / rays
Laser pens/rays and ‘smoke’/chalk dust - scatteringFundamental to understanding ray diagramsImagine the scene
Seeing with light
String line of sight demoA concrete model – clearly the string is not ‘light’
‘Seeing the duck’ modelled with spring then string ‘hedgehog’Shows the ray as the pathway – a simpler and sufficient model at this stage
Shadows and eclipsesLinks reality of what is seen and ray modelModelling light with ray diagrams
Pinhole cameras
Pinhole cameras – one hole, many holes, big hole, big hole + lensConcrete, purposeful, holistic, engaging
Leads to real images and cameras when a lens is used
A ray diagram for a pinhole camera
String diagram for pinhole cameraConcrete, Engaging, practical, helps explain the rules for construction. Choosing which rays to use (of the many)
Going further / follow onThese activities are all suitable for use in class

Use of ray model to explain and predict phenomena -

Revisit projector beam and laser beam with the hedgehog view – projector is many pathways with a general trend in one direction but diverging slightly. Laser is many parallel pathways.

Reflection and ray diagrams

Other relevant Nodules: Light and Images, Ray model

Can of worms (mainly linked to the ray model of light):

• Many non-specialists regard rays as a ‘real thing’ rather than an abstract model, and confuse beams and rays

• The majority of non-specialists can ‘do’ diagrams for mirrors, but it is an algorithm with no understanding behind it, rather than a way of modelling a physical situation and then interpreting it in terms of the physical reality e.g. position of object and image relative to mirror

• Real and virtual images

• Language- Diffuse scattering, specular reflection, incident/incidence

Can opener WhySPT reference and other resources
Pepper’s ghost - and can they model it as a ray diagram or with string?Uncovers misunderstanding of nature of rays, images and reflection.
Large scale introduction to ray diagrams, virtual images
Reflection from any surface

Mirror invention
Right lines/Wrong track card sortUncovers misunderstanding of nature of rays, images and reflection.
Things you'll need to decide on as you plan
Going further / follow onExplicit use of modelling with classes – string diagrams


StrategyWhySPT reference and other resources
Reflection of light by plane mirrors
String/wool/thread ray diagramsA concrete model – clearly these are not ‘light’ - purposeful, holistic, engaging – and a quick and easy way to check understanding
Light streaks and plane mirrorsCan be used to locate object and image – links reality of viewed image and ray model
Water tank and laser penShows reflection at a different interface – not just with mirrors/shiny solids/in air
Animated ray diagramsEasy to go through the basic principlesReflection of light

Reflection of light also animation in presenter extras (offline) SPT Li03 TA01

Drawing ray diagramsShows the power of ray diagrams as a predictive tool
Reflection from other surfaces
Light streaks with cylindrical mirrors (or 3 small plane mirrors) Make the point that it’s the same behaviour as with plane mirrors
Relevance – lots of applications
Why is the measurement made to the normal line?

VPLab10 – Mirrors (parabolic)
String/wool/thread ray diagramsQuick and easy way to check understandingHelen’s protractor
Animated ray diagramsRequested by teachers for GCSE curved mirrors
Diffuse reflectionStill the same law – not just with mirrorsNot just mirrors

Diffuse reflection of light
Laser reflectionQuick and easy, unpicks lots of ideasBrendan et al!
Going further / follow onFollow on – these activities are all suitable for use in class

Use of ray model to explain and predict phenomena

Pupil activity – periscopes and how to make one

KHET / Mirror maze challenge

One law for reflection


Other relevant Nodules: Light and images

Can of worms:

• Rays vs beams

• Questions: ‘Where does the speed go/come from’, and a related questions ‘Where does the energy go/appear from?’

• Refraction only happens in glass blocks

• The maths of n = sin i/sin r, n = 1/sin c (when they don’t know what sine is)

• Density vs optical density

• Students may think objects bend

• Language- incident/incidence, refraction, refractive index, sine, optical density

Can opener WhySPT reference and other resources
Floating coin/Bent pencil/‘Cutting off’ a fingerElicits understanding of change of direction at a surface, and which way. Revises ‘seeing’.
Water bubbles / disappearing rod / test tube / cooking oil and beaker…there isn’t a change in direction , and the effect of that on what you ‘see’PNC LCI workshop
Going further / follow onOther everyday examples (bathtime)

Concept of being invisible


Strategy Why SPT reference and other resources
Qualitative to quantitative
Swimming pool problem, water tank with laser penAn introduction that will lead to a quantitative discussion re refractive indexSPT Li 03 TA 6 given – can’t find anywhere

Bent light - like bent rulers?

Real and apparent depth – lines and tankDo simple calculations, show n >1, revise construction of imageRefraction: not just glass blocks

Shallow pools

PNC LCI workshop
Ray tracing with glass blocks, rectangular and semicircularCould show n = sin i/sin r, could extend to TIRRefraction: a simple pattern

Why would you want to know about refraction?

Bent light - like bent rulers?

Refraction: not just glass blocks
Why refraction happens
Modelling- changing direction and speed

- people marching / trucks

- drowning person (least time)
Shows change in direction: bigger change in speed, bigger change in direction

Constant frequency, changing wavelength and speed (so the energy doesn’t go anywhere, or come back from somewhere)

Refractive index = speed in vacuum/speed in medium (rather than sini/sinr)
Going further / follow onApply to can openers TIR and critical angle using pouring light, jelly fibre optics (from Salters)

Lighting a home with water bottles

Optical density linked to speed

Least time and QM, photon exploring paths


Other relevant Nodules: EM spectrum, Refraction, Light and images

Can of worms:

• Real and virtual images and their formation

• ‘Choosing rays’, which ones and why?

• Ray diagrams for lenses – learning rules

• How to progress from ideas about rays in reflection and refraction to ray diagrams for lenses

• Concave lenses? Less obvious because lack of naturally occurring biconcave lenses, but important if you’re shortsighted

Can openerWhySPT reference and other resources
Right lines, wrong track activityElicits level of understanding about straight line propagation, refraction, rays etc.


StrategyWhySPT reference and other resources
Lenses focus light
Cylindrical lenses in 2D
- single ray at different positions
- multiple rays
- different powers
(diverging lenses KS4)
Shows what lenses actively do to lightPhet - Geometric optics

Virtual Physical Laboratory
Third world glassesA practical application – variable powerSelf-adjusting lenses
Breaking down a lens into prisms
fish eye view lenses
Shows that lens action is simply due to refraction
Lenses and images
Image from a distant object on a piece of paper, covering up the lens from the bottom Forming a real image
Concrete, purposeful, holistic, engaging

Choosing which rays to use (of the many)
Why would you want to know about refraction?

Refraction: not just glass blocks

How do lenses work?

Bent light - like bent rulers?

Shallow pools
Making a telescopeimage in mid-air formation of a real image
Magnifying glassFormation of a virtual image
Spherical lenses- image of filament on screen effect of moving the lensLink to formal ray diagrams
Needed for GCSE
Projector/camera/eyeApplications of lenses – on NC PoS and various specifications
Going further / follow onThey can predict what will happen to beams of light that travel through a simple convex lens using a ray model.

Teachers should be able to show pupils how to make a real image.

They can explain why we see using the idea of refraction by cornea and lens.

They can explain the use of different lenses to correct defects in vision.

Why is the prescription for your contacts different from the prescription for your glasses?


Other relevant nodules: EM spectrum, wave velocity, frequency and wavelength, refraction

Can of worms:

• Inconsistencies in the way colour is taught. Why is light made up of seven colours and then shown (via the spinning disc) that it can be put back together again and then there are only three primary colours etc. How many colours are there?

• Spectral frequencies/spectral colour vs. perceived colour

• White light going through a coloured filter… does the filter add colour to it? (Filters are unlikely to be pure and will probably allow a range of frequencies though.)

• Appearance of coloured objects in coloured light (NB. Many schools don’t have projectors capable of mixing coloured lights, LEDs are an alternative). Using pigments.

Can openerWhySPT reference and other resources
How do YOU teach colour?What do you say to a pupil who asks why are we going from white splitting into seven colours to white light being made from only three?Mixing light of different colours

Mixing light... mixing paint


StrategyWhySPT reference and other resources
Spectral colour and perception
Dispersion with prism and recombination

The Big Spectrum
Prism shows white light is made up of ROYGBIV and separates when passed through the prism. The eye has colour detectors which can only detect R, G and B. Stimulating them to different degrees allows the full range of colours to be detected.
The spectrum of white light is a continuous range of frequencies that streams through space regardless of whether our eyes are there or not.
A spectrum of colour: the dispersion of light

Demonstrating the production of a spectrum

Working with two prisms

Striking potential
Introduces link between spectral colours, and frequency.
Links colour to voltage needed to make light and hence to energy. Can be extended to UV and IR
Mixing lights of different colours
Mix R, G and B light (if apparatus not available, 3 LEDs could be used.Ask what this might be used for?
When R,G B are mixed together in equal proportions, white is produced. A discussion can take place about why this jump is made from one idea to the other.
Adding beams of light

Colours in a television

Mixing light of different colours

Seeing coloured objects

Visible light and the eye's response
Big Spectrum through filters Establishes that filters work by colour subtraction
Look at coloured objects through filtersWhat we perceive is caused by the light stimulating the cones
Coloured surfaces
Shine coloured light onto different coloured surfacesEstablish that it is the frequencies reflected into our eyes that determine the colour perceived. Other frequencies are absorbed by the surfaceMixing paint
Going further / follow onTeachers should be able to teach pupils to distinguish between light and what we see; that the eye is a detector

Predict what will be seen (perceived colour) when lights of different spectral frequencies enter the eye at the same time.

Predict effect of combining filters/food colours

Explain how mixing of primary pigments causes perception of secondary pigment colours

EM Waves

Other relevant Nodules: all

Can of worms:

• There must be something, maybe small amounts of air, that are vibrating to make up the light wave

• Why are they called electromagnetic waves?

• Questions: ‘‘why do we say light travels in a straight line but also say the wave vibrates at right angles?”

Can opener WhySPT reference and other resources
Draw and label a waveElicits understanding of what a wave is, and what we are drawing when we draw the usual sine curve to represent a wave (picture; graph - if graph what are the axes and why do we not label them?)What is the electromagnetic spectrum?

What is light, em waves, properties, v=flambda – includes Interactives
Draw and label a light wave. How is it different from above?Elicits if teachers have an understanding of what em wave is
Going further / follow on

PhET radio waves

PhET EM fields


StrategyWhySPT reference and other resources
The nature of em waves
Show polarisation of
(a) microwaves
(b) light
Show em waves have no amplitude measure in metres – it’s not a case of “fitting between the rods”Polarisation

Polarisation explored

SEP booklet - Waves and radiation sample pack 9. Polarisation

SEP booklet - Light and matter The effect of small objects (A4)
Make explicit the link between em waves and em
Deconstruct a transformer until you have a source and receiver of em waves
Make radio waves with simple short circuit
Show unifying concept
Sources and detectors across the spectrum
Investigate UV beads, SEP microwave detector, mobile phones etc.
Remote control cars (for radio and IR), infra-red thermometer
Emphasis on source-medium-detector
Examples of interference
Ripple tank
Interference with light
Emphasise ’common’ wave properties


Being in and out of step, stationary patterns


Coherence, how interference comes about - Interactives

Vibrations account for interference
Show example of diffraction
IOP demonstration video
Use camera on phone and gap between fingers to show diffraction
Diffraction glasses (learning about
Emphasise ’common’ wave propertiesDiffraction – from beams to point sources - Interactive
Online zip:

Photons and interference

Diagnostic questions on superposition

Going further / follow onVPLab optics

SEP booklets are in the National STEM elibrary

TIR and critical angle using pouring light, jelly fibre optics (from Salters)

SEP booklet:

Radiation and communication - Creating radio waves (A8)

SEP booklet: Waves and radiation sample pack

The visible spectrum
Using SEP Spectroscope:
1. Frequencies in the visible spectrum
2. Combining colours
3. Signalling using fibre optics
Beyond the visible
Using Infrared radiation detector/transmitter
1. Detecting infrared radiation
2. Signalling with infrared radiation
3. Microwave radiation
4. Ultraviolet radiation
VPLab waves
PhET wave interference
SEP booklet: Waves and radiation sample pack:

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