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Unit 8.11 LightOverview of the unit

In this unit, students learn about the properties oflight. The unit is introduced with reference to thespectrum of colors first met in Unit 8.8. Studentsthen consolidate their understanding of some of thefundamentals of light – its speed, the fact that ittravels in straight lines, and the difference betweenluminous and non-luminous objects. The formationof images in both a pinhole camera and the eye isinvestigated, and finally reflection, refraction anddispersion of light are treated in simple terms.

Concept strands

In completing this unit students will work towardsLearning outcome P3.

Learning

outcome

Emerging Developing Mastered

P3 Explainpropertiesand behaviorof light.

Identify properties oflight.

Describe propertiesandbehavior oflight.

Explain propertiesandbehavior oflight.

Investigating scientifically strands

The work in this unit will allow students to practiceand improve the following skills:

Investigation

skills

Emerging Developing Mastered

S5 Evaluatethe use of

selected

equipment.

Select equipment

for carrying

out the

investigation.

Selectsuitable

equipment

to collect

data.

Evaluate theuse of selected

equipment.

S11 Draw a

conclusion,

consistent

with the data,

and explain it

using scientific

knowledge and

understanding.

Draw a

conclusion.

Draw a

conclusion,

related

to the

scientific

question

or

prediction .

Draw a

conclusion,

consistent

with the data,

and explain it

using scientific

knowledge and

understanding .

Equipment and resources required

shoebox, wax paper or other translucent paper,needle and knitting needle, black paint, sticky tape,camera, scissors, tin foil, pinhole camera fromActivity 2.1, ray box, prism, white card

Key terms

absorb – when a ray of light isn’t reflected from theobject; it may be converted to heat energy

blurred image – an image that is not clear; wecannot see the shape accurately

cornea – the transparent layer covering the front ofthe eye

image – a picture of an object; when you see areflection of a tree in water, the reflection is animage; when you see an actor in a film, you arereally seeing an image of the actor

iris – the coloured part of the eye that surrounds thepupil

luminous – things that produce light are describedas luminous

non-luminous – things that don’t produce light are

described as non-luminousoptic nerve – this communicates information fromthe retina to the brain

prism – a triangular piece of glass with smooth sides

pupil – the part of the eye that lets light in; it has thesame function as the pinhole in a pinhole camera

ray – light moving in a straight line

reflect – when a ray of light ‘bounces’ off an objectand changes direction

refraction – the change of the speed of a rayof light when it moves between two transparentmaterials; the ray may also change direction

retina – the back surface of the eye; this is like thescreen in your pinhole camera

speed – speed in meters per second (mps) is distancetraveled (in meters) divided by time taken (inseconds):

speed = distancetime

screen – a flat surface that an image appears on sothat we can see it, for example at the cinema, or ona camera

sharp image – an image that is clear; we can seethe shape accurately

translucent – a material that light can travelthrough, but which you cannot see through clearly

transparent – a material that light can travelthrough; you can see through transparent objects

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white light – the light that comes from the Sun; itis made up of lots of colors, which you can see in thespectrum

Common misconceptions• White light is colorless.• Light rays travel out from the eye to other

objects.• Only shiny objects reflect light.• The pupil of the eye is black, not transparent.

Team, individual and class activities

Getting started

Remind students that white light is composed of aspectrum of colors, as seen in a rainbow. Differentpigments (such as chlorophyll in green plants) reflector absorb different colors. Students will look moreclosely at the spectrum later in this unit.

Does a green leaf absorb or reflect green light?(Answer: reflect)

Does a green leaf absorb or reflect blue light?(Answer: absorb)

What color of light does a banana reflect? (Answer:yellow)

If you take a photo of a white piece of paper in aforest, the paper will look green in the photo. Why?(Answer: The leaves have absorbed all colors exceptgreen, so only green light can be reflected from thepaper.)

1 Sources of light (P3)

This first section introduces students to somefundamental facts about light. Ask What do we knowabout light from everyday observation? Students maybe aware that light can travel through a vacuum(through space), travels at a very high speed, travelsin straight lines, and so on. Discuss the evidence forthese statements.

The most likely misconceptions concern luminousand non-luminous objects. Students may thinkthat light is emitted by the eye and is reflected by

an object, rather than the object reflecting ambientlight, which is then detected by the eye.

Which of these are luminous, and which are non-luminous? (Answer: The luminous objects are stars,light bulb, the Sun. The others are non-luminous.)

Ask Why do we see the Moon as a bright object in thesky, if it is not itself luminous? Elicit the answer that wesee the Moon because it reflects light from the Sun.

Discuss how the Moon would appear if it was itself aluminous body.

Worksheet 11.1 (page 161) a True or False exercise.

2 Pinhole camera (P3, S5, S11)

Tell students that they will make a simple camera,and carry out experiments to investigate the imageproduced by the camera.

Activity 2.1

Each team will need: a shoebox, wax paper or othertranslucent paper, needle and knitting needle, blackpaint, sticky tape, camera, scissors

Allow teams a few minutes to read instructionsand decide on roles. Check that they do not makethe hole with the knitting needle too large; adiameter of about 2 mm will be fine. Explain thatthis type of camera was used before lenses werereadily available. It does not need a lens, but it couldbe fitted with a photographic plate in place of thewax paper, in order to take a permanent photo.Instead of using a photographic plate, studentscan record the image with a smartphone or digitalcamera.

If students have difficulty locating an image on thescreen, you could place a piece of black card witha distinct shape over a bright window to act as anobject.

Activity 2.2

Each team will need: the pinhole camerafrom Activity 2.1, black paint, tin foil, sticky tape,needle

In this activity, students investigate modifications tothe camera to improve the image. This is followed upin Worksheet 11.2 (page 163).

3 The eye (P3)

Discuss the structure of the eye by comparing withthe pinhole camera. Although the eye uses both thecornea and lens to focus the light rays, whereas thepinhole camera does not have a lens, there are stillmany similarities.

The pupil appears black, but it is effectively a holeto let light in, like the hole in the pinhole camera.It looks black because the inside of the eye is dark.The iris changes shape to let more or less lightinto the eye. Ask students what this is similar to inthe pinhole camera (changing the size of the holeusing the tin foil). What is the retina similar to? (thetranslucent screen)

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Worksheet 11.3 (page 165) tests students’ recall andunderstanding of the function of the eye and camera.

4 Reflection

Activity 4.1

Each team will need: mirror

This short section allows students to see thatthe angle of reflection is equal to the angle ofincidence. Keep this simple – there is no need tointroduce the terminology of incident rays, normal,etc. at this stage.

5 Refraction (P3)

Introduce refraction by discussing the speed oflight. We saw earlier that light travels at 300 000 000 meters per second. But that figure isthe speed of light in empty space. The speed is almostthe same in air, but when light travels through othermaterials, such as water, glass or diamond, its speedis reduced. Show students a pencil in a beaker ofwater, or the photo of the polar bear in the StudentBook, and explain that this effect is caused by thechange in speed. If a light ray meets a boundary atright angles to it, it changes speed but not direction.However, if it meets the boundary at an angle, it alsochanges direction. The brain processes the light raysas if they have not changed direction, and this is whyobjects can appear bent.

Which of these sentences are true? (Answer:Refraction is most obvious when you are looking

across the surface of the water)Worksheet 11.5 (page 167) describes a thoughtexperiment to explain the change in direction whenlight crosses a boundary.

6 Splitting white light to see the spectrum

Activity 6.1

You will need: ray box, prism, white card

Set up the ray box so that it produces a single ray oflight along the horizontal white card. Place the prismin the path of the ray, and adjust its position so thatit produces a spectrum. Support a second piece ofcard vertically to act as a screen. The spectrum cannow be projected onto this screen for the students toobserve.

Traditionally the colors seen are red, orange, yellow,green, blue, indigo, violet. However, most peoplecannot distinguish the indigo.

Worksheet 11.6 (page 170) is a loop game that testsstudents’ comprehension of this unit. Cut out andshuffle the cards, then hand out a set to each team.

Their task is to put the chain of cards in order.Differentiation

Worksheets 11.2, 11.3 and 11.5 are more suitablefor Extension students. Worksheets 11.1 and 11.6(along with the additional activity on Newton’sdisk described below) are more suitable for Supportstudents.

7 Presentation task

Students produce a poster about the formation ofrainbows.

Assessment

Making use of peer assessment

Students within teams take it in turns to choose oneof the key terms from this unit, and check their team-mates can explain what the term means.

Collecting items for evidence of achievement

Completed worksheets and presentations

Success criteria for the learning outcomes

Students will be able to:

P3: Explain properties and behavior of light.• Emerging: Identify properties of light.• Developing: Describe properties and

behavior of light.• Mastered: Explain properties and behavior

of light.

Encouraging self-reflection and self-assessment instudents

Ask students to complete the following:

I was happy with my work in this unit because…

I was surprised by…

I would like more help with…

The most important thing I learned was…

I would like to find out more about…

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Further suggestions for additional activitiesand investigations

Students could try making a Newton’s disk. (Cut acircle from white card, about 12–15 cm diameter.Divide into six equal sections (i.e. like six cake slices).Color the sections red, orange, yellow, green, blue,

violet. Use a sharp pencil to make a hole in thecentre. Tape the pencil point onto the disk, so thatthe whole disk can be spun by twirling the pencil.Students should see that the colors almost disappearand that the spinning disk appears white.)

Links, references, URLs for the students to use

Two fun experiments using reflection:

http://www.physics.org/marvinandmilo.asp?id=78

http://www.physics.org/marvinandmilo.asp?id=71

A short explanation of why the sky is blue:

http://www.physics.org/article-questions.asp?id=108...and one on rainbows

http://www.physics.org/article-questions.asp?id=58

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