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Moza Bint Mohammed Preparatory Independent School for Girls

Science Department

GRADE 8: Physical processes 4

LightLESSON 2: REFLECTION

8.18.3 Describe how light is reflected at a surface and understand the difference between reflection by rough and smooth surfaces. Know the characteristics of an image formed in a plane mirror. Describe everyday applications of reflection.

STARTER ACTIVITYWork out possible ways for a dog to see if his bone is behind a tall wall without moving. Draw your ideas on picture.

Academic Year 2011-2012 1

Name :……………………….. Class: ………….. Date : …………………..


Science Department

THE LAW OF REFLECTION

Aim: To be able to correctly set up apparatus and accurately measure the angle of reflection.

Task: Reflect a light ray off a plane mirror and compare the angle of incidence withthe angle of reflection.

Keywords: Plane mirror : A flat mirror, drawn as a straight line with dashes underneath.Normal : An imaginary line that is draw straight up from where the light ray hits the mirror.Angle of incidence : The angle between the normal and the incident light ray.Angle of reflection - The angle between the normal and the reflected light ray.

Equipment:• Plane mirror• light ray box with single slit• pencil, ruler and protractor• Plain white paper

Method:1) Set up a ray box with single slit on a piece of paper.2) Reflect the ray of light from a mirror.3) Mark the position of the mirror by drawing a pencil line on its surface.4) Mark the position of the ray with dotted lines5) Remove the ray box and mirror6) Mark the non reflecting surface of the mirror with dotted lines7) Draw all the lines clearly using pencil and ruler8) Mark the direction of light with arrow.9) Use a protractor to measure 90 degree where the ray hits the mirror. Mark it and then measure the angle of incidence and angle of reflection.10)measure different angles.6E-WS7 THE LAW OF REFLECTION



Science DepartmentResults:

Angle of incidence Angle of reflection

Conclusion:What have you found out from this investigation?

___________________________________________________________________________

THE LAW OF REFLECTION!!

The angle of reflection is ............................. .................... to the angle of incidence.


HOMEWORK

Draw a perfect light ray diagram where angle of incidence = 45° and angle of reflection = 45°


Science DepartmentAim: To be able to use knowledge of reflection to answer a variety of questions

Questions on reflection

A1) Ambulances have the word ‘AMBULANCE backwards, why do you think this is?

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B2) White objects reflect every colour of light, yet we cannot see reflections in them.

Have a look at the following pictures.By referring to the picture explain why youthink that we can see reflections insmooth shiny surfaces but not on whiterough surfaces.

_______________________________

_______________________________

_______________________________

3) The beam of light from a torch is shone onto a mirror.The drawing below shows this.

surface ofm irror

torch

beam fromtorch

D C

B

A

(b) The light beam is reflected from the mirror.Which line, A, B, C or D, shows its direction? ................................................

4) Two mirrors at 90° to each other always reflect a ray of light back parallel to the



Science Departmentincident ray.

(a) (i) In the diagram below, a ray of light strikes mirror 1 at an angle of 45°.

Complete the diagram to show how the mirrors reflect the ray.Use a ruler and a protractor.

inc ident ray o f lightm irro r 1

m irro r 2

(ii) In the next diagram, a ray of light strikes mirror 1 at a different angle.

Complete the diagram to show how the mirrors reflect the ray.Use a ruler and a protractor.

in c ident ray o f lightm irro r 1

m irro r 2



Science DepartmentC) Aysha is using a periscope to watch the foxes in her garden. She ishiding behind a wall.

a) i) Draw a ray on the diagram to show the direction taken by the light when Aysha sees the fox.ii) Add an arrow to the ray to show the direction in which the light is travelling.iii) A fox cub has appeared at point X. What must Aysha do to see it?

LESSON 3: REFRACTION IN TRANSPARENT BLOCKS

8.18.5 Describe how light is refracted at a plane surface and describe everyday applications of refraction.

ACTIVITY

Predict: What happens when a pencil is put in a beaker of water?

Materials: Beaker, water, pencil

Procedure: Put the pencil in the beaker of water and observe.Observation:

Explain the inference:



Science DepartmentACTIVITY 5

Problem: How can we make the coin inside the beaker visible ?Predict :

.Materials: Beaker, coin, water

Procedure:Put coin in an empty beaker .Sit in such a way that you cannot see the coin. Pour water in the beaker till you can see the coin.

Observation:

Explain the inference:


The light ray at 50° to the normal - Complete the diagram, don't forget to draw in the normal when the light ray leaves the glass block.

The light ray at 0° to the normal (straight on) - Complete the diagram with what youobserve.


Science Department

Conclusion:Q1) When the light ray entered the transparent block did it move closer to the normal oraway from the normal?

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Q2) When the light ray left the transparent block did it move closer to the normal or awayfrom the normal?

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Q3) What happened to the light ray when it was shone straight on to the transparentblock?

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Q4) Did you notice anything about the angles made when the light ray entered and left theblock?

...................................................................................................................................................Q5) Did you notice anything about the angles made by the light ray and the normal when itwas inside the transparent block?...................................................................................................................................................

Facts about Refraction - Fill in the Blanks



Science Department

For refraction to happen a light ray must pass through a _ _ _ _ _ _ _ _ _ _ _ medium,

examples of transparent mediums are air, glass and _ _ _ _ _ . When the light ray passes from

a less dense medium (for example _ _ _ ) into a more dense medium (for example

_ _ _ _ _ ) then the light ray will slow down, which causes it to bend _ _ _ _ _ _ _ the normal.

When the light ray passes from a more dense medium to a less dense medium (for example

glass into air) then it will_ _ _ _ _ _ _ and bend away from the normal.

Aim: To be able to use knowledge of refraction to answer questions.

Hunting fish

Q1) Why do you think this is?

............................................................................................................

............................................................................................................

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Q2) Where do you think they will have to change their aim to? Above the fish, below the fish, to the left or to the right? Explain your answer using scientific terms.

.......................................................................................................................................................

.......................................................................................................................................................


Speed up Water Transparent Towards Air Glass

In some parts of the world people use spears in order to catch fish.Quite often when people try to do it for the first time they will miss,even if the fish is only a meter or so away. They quickly learn to notaim at the fish but just slight away from it.


Science DepartmentHOMEWORK

Look at these diagrams that show light rays hitting glass blocks

Which of these statements are correct? Mark the with a tick ( ) or a cross ( ).

a If a light ray strikes a glass block at 90º it will be refracted (bent).

b Light refracts (bends) outwards when it goes from air to glass.

c Light refracts (bends) outwards when it goes from glass to air.

d Light refracts (bends) because it moves at different speeds in the different materials.

LESSON 4: DISPERSION

8.18.6 Demonstrate how white light can be split into coloured light by refraction and explain examples of dispersion in everyday life (e.g. oil on water, rainbows).

Aim: To understand that white light is a combination of many colors and dispersion (“Splitting of white light into colors”)

Materials: Prism, Source of light, a white screen

Method: Pass the source of light through the prism in such a way that the ray coming out of the prism is focused on the screen.

Observation:

Inference :



Science DepartmentWhite light can be split up to form a spectrum by using a prism. A prism is a triangular block of transparent material like glass. Separating white light into colors is called dispersion.

The prisms were set up as shown in the diagram, with rays of white light shining through them. The prisms split the white light into colors as shown

w hite ray

w hite ray

position of screen

positionA

positionB

b lue

b lue

g reen

g reen

red

red

(i) A white screen was placed in position A as shown below.

w hite ray

w hite ray

screen

positionA

The rays of red, green and blue light are shown. Describe the appearance of the light on the screen.

..................................................................................................................

(ii) The white screen was moved away from the prisms to position B as shown below.



Science Department

LESSON 5: COLORS

18.7 Know that objects appear coloured when viewed in white light because some colours are reflected by the object but others are absorbed.18.9 Know the effect of superimposing red, green and blue colour filters18.10 Know that red, green and blue light, when superimposed, create white light and apply this knowledge to television screens and to colour vision.

ACTIVITY (18.9)

Color filters only let one color of light through it and stop the other colors.If a color is stopped by a filter we say that it is absorbed, and if a color passes through a filter we say that it is transmitted.

Aim: To understand color filters

Materials: Color filters, source of light

Procedure:Step 1.Take a color filter and note down its color.Place it in the path of light and observe. Note down what color of light is seen.Step 2.Take a second filter and note down its color.Place it in the path of first filter and observe. Note down what color of light is seen.

Color of filter 1 Color of light Color of filter 2 Color of light

Inference:



Science Department

Q1) The human eye detects red light, blue light and green light. A combination of red, green and blue light is seen as white. We ‘see’ other colours when different combinations of red, blue and green enter the eye. This is shown in the table.

light enteringthe eye

colour ‘seen’ by the eye

no light black

red red

blue blue

green green

red + blue magenta

blue + green cyan

red + green yellow

red + blue + green white

(a) Some magenta paint is illuminated by a combination of red, green and blue light. USE THE WORDS ABSORBED OR REFLECTED

Explain why the paint appears magenta.

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(b) A mixture of cyan paint and yellow paint appears green in a combination of red + blue + green light. Explain what happens to:

(i) the red light? .........................................................................................

(ii) the blue light? .......................................................................................

(iii) the green light? ....................................................................................



Science Department

(c) Most colours of paint can be obtained by mixing different combinations of cyan, magenta and yellow paints. What combination of these paints makes:

(i) blue paint?

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(ii) red paint?

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Q 2) Aysha uses different colours of light and different coloured balls.

Complete the table to show the colours that the balls appear to Naomi.

colour of ball colour of thelight

the colour theball appears to

Naomi

green

white red

white

(ii) Why does a black object appear black in any light?

.................................................................................................

Q 3) Ali puts on a pair of special glasses as shown below. The glasses have coloured filters in them.

(a) Ali looks at a lamp through the green filter.

The lamp gives out white light, but appears to be green.Explain how this is possible.

............................................................................................................

(b) Ali looks at a red lamp.

(i) What colour will the lamp appear to Ali, if he looks at it through the red filter?

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Explain your answer.

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Science Department

HOMEWORK (18.7)

Q 1)Two cyclists are riding along a dark road at night. One is wearing black clothes and the other is wearing light-colored clothes.

A car is driving behind the two cyclists. Light from the car headlamp shines on the cyclists.

(a) What happens to the light when it reaches the light-colored clothes?

……………………………………………………………………………………….

(b) On the drawing above, draw a ray of light to show how light from the headlamp reaches the driver so that he can see the cyclist in the light-colored clothes.Draw arrows to show the direction of the light.

(c) What happens to the light when it reaches the black clothes?

……………………………………………………………………………………….



Science DepartmentConcept map

Luminous objectEg:

Non luminous objectEg:

LIGHT

Reflection Refraction Dispersion

Academic Year 2011-2012

Colors

Primary colors Secondary colors

16

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