LIGHT - Shadows & Absorption:

OUTCOMES:

Students observe and describe how the absorption of light by materials and objects forms shadows, eg building shading

Students learn:

SCIENCE	Identify scientific principles underlying bhaviour of light
TECHNOLOGY	Record and analyse data
ART	Blended multimedia elements
ENGINEERING	Design, build and test experimental device(s)
MATHEMATICS	Use comparative data to differentiate materials

MATERIALS:

Example materials list:

Scissors
Card-board - Three pieces approx 75mm square for each student
One folding clip per student (to support the cardboard shapes to stand vertically)
One 300mm ruler per group/student
One or more tables pushed close to a wall in a well shaded location
A minimum of one torch (or similar light source) for each group of 4 - 5 students
A computer or similar device connected t Internet for access to on-line interactives
Google sheet or science journal to record results
Make a guess (your 'hypothesis') about what you will see on the wall when you hold the cards close to the wall and move the torch from side to side and away/towards the card (write down what you think you will see)?

PROCEDURE:

Each student should make one 'tall' shape, one 'wide' shape and one shape of their own choice. The card should be an outline cut - with no holes cut inside the shape. Students can draw and colour their shapes but only quick and rough - this is not a craft lesson.

Draw out one wide, one high and one optional (one shape per card)
Cut each of your pieces of cardboard to make three shapes.
Choose a shape and connect a clip to the bottom of it so that the card stands upright
Measure and record the height + width of the outline of the shaped piece of cardboard.
Place the upright shape within 100mm of the shaded wall
Place torch approximately 300mm from the wall so that it illuminates the card and throws a distinct shadow on the wall
Measure and record the distance from the torch to the card and from the card to the wall.
Measure and record the height + width of the shadow.
Write down what you guess will happen when you move the card towards the torch
Move the card closer to the torch.
Make notes about what has changed, Make and record new measurements about the size of the shadow and distance between the torch, card and wall.
The results can be recorded in science journal/spreadsheet, and an (optional) digital photograph for each test.

OBSERVATIONS:

Try moving the card and the light. Take measurements and and write down what you see.

Results Table #1 - How absorption of light by objects forms shadows

To make a shadow: Choose a location where you will measure the height and width of each card and measure the corresponding location where you will measure the height and width as those same locations appear on each shadow.

Make sure that you only change one thing when you compare and take a new set of measurements.

Shape	Card Height	Width	Torch to card	Card to wall	Shadow Height	Width
Tall	60mm	30mm	100mm	200mm	100mm	50mm
Wide	30mm	65mm	100mm	200mm	60mm	90mm
Other	50mm	40mm	100mm	200mm	100mm	50mm

Tall	60mm	30mm	50mm	250mm	150mm	75mm
Wide	30mm	65mm	50mm	250mm	90mm	135mm
Other	50mm	40mm	50mm	250mm	150mm	75mm

RESULTS:

Choose one example where the results show what happens to a shadow when you move an object towards and away from a light.

Draw and label two diagrams - Draw one for when the object is close to the torch and one other diagram when the same object is futher away from the torch and closer to the wall.

Q 1. Does the size of a shadow get bigger or smaller if you move a light away from an object?

Q 2. Do big things always have big shadows?

Q 3. Can you calculate the position of a light if you know how big an object is and how big its shadow is?

In your science journal, write down these questions and what you think the answers are.

DISCUSSION:

What do you think shadows are made of?

Some people believe that shadows are fixed to objects. How could you design an experiment to test whether this is true or not?

Write down what you think)?

More Science & Fun With Shadows:

Shadows - Opaque Objects (things that block light):

Video 2. Opaque Objects & Shadows

IF YOUTUBE FAILS, TRY VIEWPURE:Opaque Objects & Shadows

If you have time, cut out your own animal shapes using cardboard and find a dim room. Use a torch to make shadows of your cardboard cut-outs and see what happens to the shadows when you move things around. If you have a dark room and a lamp, you can even put on a shadow play using your hands, or make your own shadow puppets

Example Experiments: How Light & Shadows work

The purpose of this simple experiment is for students to determine the relationship between distance, shape and position of shadows when light falls on opaque objects.

Students will use a torch and cardboard cut-out's to discover more about light and shadows:

Video 1. |Light & Shadows

IF UNABLE TO ACCESS YOUTUBE VIDEO (requires Flash) TRY:|Light & Shadows

Light experiment - Shadows

Instructions	Results
Do shadows get larger or smaller when the light is closer to the object?
Do the edges of the shadow have sharp or blurry edges?
Can you see more than one shadow of the object?

Light & Shadows:

2. Light & Shadows - Drag different things into the path of the light:

Follow the instructions:	Enter your answer below:
1. What happens when you drag the pink handle closer towards the object?
2. Can you move the light to make all of the objects fit between the lines on the screen?
3. What happens when you use the blue handle to tilt the mirror up?
4. What happens when you use the blue handle to tilt the mirror down?
5. What happens to the shadow when you make the light dimmer?

How Shadows Are Formed:

Video 2. How shadows are formed (umbra & penumbra)

IF UNABLE TO ACCESS YOUTUBE VIDEO (requires Flash) TRY:How shadows are formed)

Why do you think that the shadow has an 'umbra' and a 'penumbra'
Why was the shadow sharper when the screen was close to the light?

3. Think of examples of things that depend on light from the sun

	Enter your own examples here:
1	Why does a shadow have an 'umbra' and a 'penumbra'
2	Why is the shadow sharp when the screen is close to the light
3	Could you change one thing in the experiment to get a sharp shadow

ABSORPTION:

Video 1. Eureka - Light Radiation & Spectrum

IF YOUTUBE FAILS, TRY VIEWPURE:Eureka - Light Radiation & Spectrum

Video 1. How Light Is Absorbed (Radiation & Colour)

Demonstrate your understanding by explaining this scientific explanation about how light is absorbed:

The selective absorption of light by a particular material occurs because the selected frequency of the light wave matches the frequency at which electrons in the atoms of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light. ¹⁾

Colour & Shadows

Two on-line colour mixing interactives are shown below:

In one interactive, you mix coloured lights, and on the other, mix coloured shadows.

	Is there any difference between mixing coloured lights and coloured shadows?
1

Fig 1. Mixing Red, Green and Blue (RGB) Light . . . . . . . . . . . . . . . . . Fig 1. Mixing Coloured Shadows

When you turn different combinations of lights on and off, what changes?

	What happens to the shadow(s) when:
1	Only the red light is on
2	Only the green light is on
3	Only the blue light is on
4	What colour lights make yellow

In the colour shadow interactive, learners explore how coloured lights become blocked and result in the formation of a “color shadow” on the screen located behind the person.

Three different lights can be turned on and off, resulting in various coloured shadows on the screen behind the person.This demonstrates both colour subtraction (the person blocks or takes away some of the incident light) and colour addition (the incident light that reaches the screen combines together to produce a different color).

Furthermore, the interaction of the incident light with the person's clothes reveals the colour of the clothes under different lighting conditions.

If you are able, we recommend preceding the use of this Interactive with a demonstration of the same phenomenon. The demonstration will require that you have three coloured spotlights available to project upon you and a screen behind you upon which the coloured shadows can be cast. Begin with one light at a time and get students to understand the “geometry of shadow formation.” A light placed to the left side of you will cast a shadow on the screen on the right side of you. One your understand the geometry of shadow formation, begin experimenting with various combinations of two coloured lights.

Once you've done the demonstration, allow students to explore the physics of shadow formation with this Interactive.

Related resources:

Appendix

Visible light - Absortion

Atoms and molecules contain electrons. It is often useful to think of these electrons as being attached to the atoms by springs. The electrons and their attached springs have a tendency to vibrate at specific frequencies. Similar to a tuning fork or even a musical instrument, the electrons of atoms have a natural frequency at which they tend to vibrate. When a light wave with that same natural frequency impinges upon an atom, then the electrons of that atom will be set into vibrational motion. (This is merely another example of the resonance principle introduced in Unit 11 of The Physics Classroom Tutorial.) If a light wave of a given frequency strikes a material with electrons having the same vibrational frequencies, then those electrons will absorb the energy of the light wave and transform it into vibrational motion. During its vibration, the electrons interact with neighboring atoms in such a manner as to convert its vibrational energy into thermal energy. Subsequently, the light wave with that given frequency is absorbed by the object, never again to be released in the form of light. So the selective absorption of light by a particular material occurs because the selected frequency of the light wave matches the frequency at which electrons in the atoms of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light.

So exactly why does the yellow shirt of that actor change red when the stage lights change color? And how could one use rules of color subtraction to predict the color an object would observed when illuminating with a specific color of light? And perhaps even more practical, how can one decide what color spotlight should be used to make an object appear a desired color? That's enough questions. It's time to get some answers so launch the interactive and start learning.

Stage lighting interactive Stage Lighting Work sheet

Reflection off of smooth surfaces such as mirrors or a calm body of water leads to a type of reflection known as specular reflection. Reflection off of rough surfaces such as clothing, paper, and the asphalt roadway leads to a type of reflection known as diffuse reflection. Whether the surface is microscopically rough or smooth has a tremendous impact upon the subsequent reflection of a beam of light. The diagram below depicts two beams of light incident upon a rough and a smooth surface.

¹⁾

http://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission

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