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Unit 11 LIGHT

Fig. 1 Have you ever seen a rainbow?

Why can you see all the colors of thespectrum in a rainbow? What does

water have to do with it?

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Getting startedYou met the word spectrum in Unit 8. When you look at aspectrum you can see all the colors which make up the whitelight that comes from the Sun.

In previous grades, you learned that objects which reflect allof the light appear white. Colored objects reflect some colorsof light and absorb others. For example, objects that reflectjust blue light appear blue, and objects that reflect just red

light appear red.

1. Does a green leaf absorb or reflect green light?

2. Does a green leaf absorb or reflect blue light?

3. What color of light does a banana reflect?

4. If you take a photo of a white piece of paper in a forest,the paper will look green in the photo. Why?

White light

The light thatcomes fromthe Sun. It ismade up oflots of colors,which youcan see in thespectrum.

Fig. 2 This shows the

spectrum of ‘white’ light.

Fig. 3

A green leaf.

Absorb

When a rayof light isn’treflected fromthe object.It may beconverted toheat energy.

Reflect

When aray of light‘bounces’off an objectand changesdirection.

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ConceptsIn completing this unit you will learn to:

• Explain the properties and behavior

of light (P3)

Key TermsThe meanings of these terms can be foundin the glossary on pages 275–90.

absorb

blurred image

image

luminous

non-luminous

ray

reflect

refraction

screen

sharp image

translucent

transparent

white light

Investigating ScientificallyS5, S11

Success CriteriaLearning outcome P3

Here is what you might aim to achieve by the end of this unit:

• Emerging – identify properties

of light

• Developing – describe properties

and behavior of light

• Mastery – explain properties and

behavior of light

What level do you think you will be able

to achieve?

Learning Outcomes

I know what

these words

mean

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1 Sources of light

Tonight, if the sky is clear, find a dark place andlook at the stars. Think about what you can see.Where are the stars? How can you see them?

Seeing the stars can tell us quite a lot about the propertiesof light.

In Unit 10 you learned that sound can’t travel in space. This isbecause space is a vacuum. Sound can only travel through amaterial, for example air or water.

But we can see the stars – so this means that light can travelthrough a vacuum.

P3

Fig. 4 Here are

just some of the stars

you can see on any

clear night.

Fig. 5 These

astronauts can see

each other, but they

can’t hear each

other – unless they

use their radios.

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When you see stars you are looking at very distant objects.You can see a distant star because light travels from the starin a straight line. A light ray is the name we give to a lineof light.

l i g h t f r o m

s t a r

The speed of light

Alpha Centauri is the closest star to us. The light you mightsee tonight from Alpha Centauri was produced in the staralmost four and a half years ago. It has taken that long totravel the distance between the star and the Earth.

Alpha Centauri is about 41 000 000 000 000 kilometers awayfrom us!

We know the distance between Alpha Centauri and the Earth.We also know how long it takes light to reach us from thisstar. Using these facts, we can calculate the speed of light.

The speed of light is approximately 300 000 000 meters persecond. Light travels almost a million times faster than sound!

Luminous and non-luminous objects

Stars are luminous because they produce light. Most things

we see every day don’t produce light. These objects are non-luminous.

Ray

Light travelsin straight

lines. Theselines are rays.

Fig. 6 If you could

follow the ray of

light from a star in a

straight line you would

eventually get to the star.

Luminous

Things whichproduce lightare calledluminous.

Non-luminous

Things whichdon’t produce

light are callednon-luminous.

Fig. 7 You see a

luminous object when

the light it produces

reaches your eye.

Fig. 8 You see a

non-luminous object

when the light it reflects

reaches your eye.

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Which of these are luminous, and which are non-luminous?

Put a tick in the correct box.

luminous non-luminous

stars [ ] [ ]

light bulb [ ] [ ]

tree [ ] [ ]

eye [ ] [ ]

the Sun [ ] [ ]

water [ ] [ ]

the Moon [ ] [ ]

Think about luminous and non-luminous objects. Explainhow shadows show that light must travel in straight lines. Thediagram in Fig. 10 will help.

shadowshadow

starts here

shadow

ends here

Fig. 9

You can see

shadows on the

sand.

Fig. 10 How the shadow of

a wall forms.

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2 Pinhole camera

Activity 2.1

What to do:

1. Take the lid off the box so you can see inside.

2. Make a small hole in the middle of one end of the box. Usea needle first, and then a knitting needle to make the holelarger.

3. On the other end of the box, cut most of the end away.

4. Stick a sheet of translucent paper over the end that youhave cut away. This will be the screen.

5. Put the lid on the box. Your camera is ready to use!

6. Use your camera to look at some objects on the screen. Youshould be able to look at:

• objects outside the window

• luminous objects in the classroom.

7. Take a photo of the pinhole camera’s screen.

8. Describe the images you see on the screen.

P3 S5S11 Equipment:

A shoebox,

wax paper orother translucent

paper,needle andknitting needle,

black paint,

sticky tape,

camera,

scissors

Translucent

A materialthat lightcan travelthrough,but whichyou cannotsee throughclearly.

Screen

A flatsurface thatan imageappears onso that wecan see it, forexample on asmartphone.

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To understand why the images are upside down, rememberthat light moves in straight lines.

pinhole

screen

Activity 2.2

In this activity you will improve your pinhole camera.

What to do:

1. You can improve the camera by painting the inside of thebox black. Remove the screen while you paint the box.Don’t forget to paint inside the lid.

2. Use your camera to look at the same objects. Again, youcan take photos. Are the camera’s images better? If theyare, explain why.

3. Take the lid off the box again to make the next step easier.

4. Make a small hole in the tinfoil with a needle. Stick thetinfoil in front of the pinhole on the camera, so that thehole in the tinfoil lines up with the hole in the box.

Image

This is apicture of anobject. Whenyou see areflection of a

tree in water,the reflectionis an image.When you seean actor in afilm, you arereally seeingan image ofthe actor.

Fig. 11 Here is

a pinhole camera.

You can see the rays

of light which pass

through the pinhole.

Equipment:Your pinholecamera,

black paint,

tinfoil,

sticky tape,

needle

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5. Put the lid back on and try the camera again. Whatdifference has this change made?

Images through the pinhole camera

Some images are sharp and some are blurred.

Here are two images of an object seen through a pinholecamera.

object imageobject image

A large hole lets more light through. The light reflected fromone point on the object travels in a straight line to many partsof the image. This makes the image blurred.

A small hole lets less light through. The light reflected fromone point on the object can travel in a straight line to just asmall part of the image. This makes the image sharp.

Sharp image

An imagewhich is clear.We can see

the shapeaccurately.

Blurredimage

An imagewhich is notclear. Wecannot seethe shape

accurately.

Fig. 12 We cansee a sharp image

of the apple on the

left; the apple on the

right is blurred.

Fig. 13 A pinholecamera with a large

hole (on the right)

makes a blurred

image; a pinhole

camera with a small

hole (on the left)

makes a sharp

image.

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3 The eye

Your eye works in a similar way to the pinholecamera. The pupil lets light into your eye, just like the pinholein the camera. You can see the parts of the eye in Fig. 14.

pupil

iris

cornea

optic nerve

PupilThis is the part of the eyethat lets light in. It has thesame function as the pin-hole in a pinhole camera.

IrisThe coloured part of theeye that surrounds thepupil.

CorneaThe transparent cover in

front of the eye.

RetinaThe back surface of theeye. This is like the screenin your pinhole camera.

Optic nerveThis communicatesinformation from the retinato the brain.

retina

The image in your eye, just like the image in the pinholecamera, is upside down! Your brain does the work so that yousee things the right way up.

4 Reflection

We are going to look at what happens when light

hits a mirror.

Activity 4.1

What to do:

1. Arrange your team like this:

A

B C

P3

Fig. 14 How an

image is formed in

your eye.

P3

Equipment:

Mirror

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2. Who does student B see in the mirror?

3. Who does student A see in the mirror?

4. Who does student C see in the mirror?

A mirror always reflects light in the same way. Here areseveral rays of light hitting a mirror:

mirror

Light reflects away from the mirror at an angle which equalsthe angle at which it hits the mirror.

mirror x

x

Fig. 15 Rays of light

hit a mirror and are

reflected from it.

Fig. 16 The angle of

the ray hitting the mirror

and the reflected ray are

the same.

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object

apparentobject

Which of these sentences are true?

Refraction is most obvious when you look directlydown through the water [ ]

Refraction is most obvious when you are lookingacross the surface of the water. [ ]

Refraction is the same at whatever angle you lookat the surface of the water [ ]

6 Splitting white light

You know that sunlight is made up of light of lots ofdifferent colors. But it would be nice to check!

In the previous section you learned that light is refractedbecause it changes speed when it moves from one transparentmaterial to another.

In fact, for many materials, the different colors of light arerefracted by different amounts. The angle of refraction isslightly bigger for blue light than it is for red light.

Fig. 19

Refraction makes an

object seem to becloser to the surface,

and further away

horizontally.

P3

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This means you can use a glass prism to split sunlight into itsdifferent colors.

Fig. 21 White light is split into many colors by a glass prism.

yellow

green

blue

indigo

violetglass prism

white lightorange

red

Fig. 20 This is a glass prism.

Activity 6.1

What to do:

1. Your teacher will show you how a prism splits light from

the ray box into a spectrum.2. What colors can you see? Discuss them with the class.

7 Presentation Task

The unit began by asking about rainbows. You already knowthat rainbows show the colors in a spectrum, but why are theycaused by water, and why are they curved?

Use the knowledge from this unit to research why a rainbow isformed. Present your explanation on a poster.

• Include photos of rainbows.

• Can you make your own rainbow? Include photos andan explanation.

• Use a diagram to show how light rays are split to causea rainbow.

Equipment:

Ray box,

prism,

white card

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8 Feedback Medals and Missions

Self Assessment

Shade in the level you have achieved for each outcome in this unit.

Concept Learning

Outcome

Emerging Developing Mastery

P 3Identify the properties

of light.

Describe properties

and behavior of light.

Explain properties and

behavior of light.

Skill Learning

Outcome

Emerging Developing Mastery

S5Select equipmentfor carrying out the

investigation.

Select suitableequipment to collect

data.

Evaluate the use ofselected equipment.

S11Draw 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.

MedalsWhat have been your greatest achievements during this unit? For example,mastering a concept outcome, improving a skill or feeling proud of yourorganizational abilities, team work or presentation.

What did you do well? How did you do it?

1.

2.

3.

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Missions

What are your targets for improvement? Select two Learning Outcomes to focuson and set yourself a target. For example, if you have reached ‘developing’, whatdo you need to do next time to achieve ‘mastery’?

Learning Outcomes Target

1.

2.

9 Science I have learned in this unit

● Light travels in straight lines.

● Light can travel through a vacuum, like space.

● Light travels very fast – much faster than sound.

● Because light travels in straight lines, it can make an image on the

screen of a pinhole camera.

● Light enters the eye through the pupil, and makes an image on

the retina.

● Light is reflected in a mirror in a predictable way.

● Light is refracted when it moves from one transparent substance

to another.

● Refraction causes an object to appear to be in a slightly different

place than where it really is.

● Refraction can split white light into the spectrum of colors.

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SB U11.pdf