light, reflection, refraction, light dispersion

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• Why do we see lightning before we
hear thunder?

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Light and reflection
• Describe objects as being luminous, non
luminous, opaque, transparent, translucent
• Draw ray diagrams to show the
relationship between the angle of incident
and the reflected ray

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Light carries energy
and travels as a wave.
What is light?
Light waves travel in
straight lines.
Light travels at a speed of
300,000,000 metres per
second, which is much faster
than the speed of sound.
Light is produced by the Sun,
and by objects such as light
bulbs and matches.

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An object that gives out light is described as luminous.
How does light from a luminous object such as a light bulb
reach the eye?
How do we see things?
Light travels in a
straight line directly
into the eye.
LUMOS!
Make a list of 5
objects that are
luminous

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How do we see things?
An object that does not give out light is non-luminous.
How does your eye see a non-luminous object such as
a comb?
Light hits the comb
and some of it is
reflected into the eye.
Make a list of 5
objects that are
non- luminous

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Opaque: no light will
pass through
Transparent: all or
most of the light
passes through
Translucent: some
light passes through

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Light and Shadow
https://www.youtube.com/watch?
v=xgJdXpN9il4

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How can we change the size of
a shadow ? (TRY AT HOME)
Equipment :
1. ray box (to
produce a strong
ray of light).
2. card cut out
3. stand
4. screen at the
back (white
paper).

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How can we change the size of
a shadow ?
Method :
1.Use a ray box to produce a strong ray of light.
2.Make a card cut out and put it on a stand.
3.Put a screen at the back (white paper).
4.Measure the distance (from the light source to the
cut out) and the height (of the shadow)
5.Keeping the distance from the light to the screen the
same, move the cut out.
6. Repeat steps 4 and 5.
7.Record your measurements in a table.

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How can we change the size of
a shadow ?
Prediction :
When the object is ________ (near/ far) to
the light source the shadow is ___________
(taller/ shorter).
How are you making
your test fair ?

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Results table
Distance of the
object from the
light source (cm)
Height of shadow
(cm)
Is the shadow
sharp or blurred?

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Reflection

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Objects that reflect light well:
Reflective materials
 Have smooth, shiny surfaces
and are usually pale colours.
 Give clear images because
they reflect light regularly.
We see things when light bounces off
them and enters our eyes. Most things
reflect light in all directions. A mirror
reflects light evenly.
What is a plane mirror?
Flat, shiny mirror made of glass – with a thin layer of
aluminium or silver on the back

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Reflection at a mirror

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Reflection ray diagram

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Why is there a
reflection here?
but not
here?
Write your
thoughts on your
post it note

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Reflection
Lesson Outcomes
Say what causes a reflection in a plane (flat) mirror
Describe how an image is formed by a plane mirror.
Draw complex ray diagrams (level 7)
Why are you able
to see your
reflection in a
mirror?
Watch the clip, then
T.P.S.
Video clip
Key words:
Reflection
Angle of incidence
Angle of reflection
Plane mirror
Real image
Virtual image
Normal line
Wave front
Watch another clip!

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Complete the
ray diagrams
to show where
the image
would appear
in the plane
mirror

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How is reflection useful in every day life?
Try to come up with some examples by
discussing with your partner!

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Reflection can be very useful.
High-visibility strips are very
reflective and make sure that
this cyclist gets noticed when
there is little light.
Using reflection
The two plane mirrors
must be positioned at 45°
from the vertical. Light is
reflected at right angles
from the top mirror onto
the bottom mirror and into
the eye of the viewer.
How does a periscope use
reflection?

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Reflection: summary

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Reflection: true or false?

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• What is refraction?
• Why does refraction happen?
• What happens when light hits a glass
block at an angle?
• What can you say about the refracted ray
compared to the incident ray when light
travels through air in to a glass block at an
angle?

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glass
air
The speed of light depends on the material through
which the light is travelling.
Why is light refracted?
When light enters a different material (e.g. from air into glass), the
speed of light changes.
This causes the light to bend or refract.
The speed of light is affected by the density of the material
through which it is travelling.
When light enters a more dense medium (from air into glass),
its speed decreases and this is why refraction occurs.

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What would happen if the light ray hit the glass block
at an angle?

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Investigation in to what happens when light passes through air and then glass

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1. Place a rectangular glass
block on a sheet of paper and
draw around it.
2. Draw a normal at 90° to the top
surface of the block.
3. Shine light rays, with angles of
incidence [i] of 30°, 60° and 0°,
into the block at the point where
the normal meets the glass
surface. Record the angle of
refraction [r].
4. Repeat the investigation for rays
leaving the glass block.
5. What do the results show?
angle i
Refraction investigation
angle r
Angle of
incidence [i]
Angle of
refraction [r]
30°
60°
0°

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Complete sheet 1

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Refraction through a glass block:
Wave slows down and bends
towards the normal due to
entering a more dense medium
Wave speeds up and bends
away from the normal due to
entering a less dense medium
Wave slows down but is
not bent, due to entering
along the normal

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Refraction at the
air-glass boundary

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Refraction in a glass block

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What happens during refraction?

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Draw the diagram in your book adding angles for the incident
ray and the ray that comes out of the block!

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Refraction causes visual effects
such as this ruler appearing to be
bent.
Effects of refraction
This happens because light from the part of
the ruler in the water is refracted as it travels
from the water into the air.
Light rays from the stone are
refracted as they leave the water.
The brain assumes that these rays
have travelled in straight lines and is
fooled into forming an image where it
thinks the
image
How does refraction make this stone look closer to the surface of the
water than it really is?
light rays came from.
actual location

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The Archer fish is a
predator that shoots jets
of water at insects near
the surface of the water,
e.g. on a leaf. The Archer
fish allows for the
refraction of light at the
surface of the water when
aiming at its prey. The fish
does not aim at the
refracted image it sees
but at a location where it
knows the prey to be.
image of
prey
prey
location
watch: Effects of refraction –
the Archer fish
Light is refracted when it passes through air in to water (but not as much as
when it passes through air in to glass).

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Refraction summary

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Refraction – true or false?

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How the eye works

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Refraction in the eye

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Colours and filters
Learning Objectives:
Describe how white light becomes a spectrum of
colour (5)
Explain how a coloured filter works and why certain
objects look certain colours in different light (7)

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Passing white light through a prism

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A prism splits a ray of white
light into the colours of the
rainbow.
This process is known as
dispersion.
The colours that make up
white light are called the
spectrum. The order of the
colours in the spectrum is
always the same.
Splitting white light
Richard Of York Gave Battle In Vain
Use this phrase (mnemonic) to remember the order of colours:

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Colours of the spectrum

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Recombining colours

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Newton’s disc animation

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The three primary colours of light (red, green and blue)
can be mixed in different amounts to make all other colours.
Primary and secondary colours
The colours made by mixing two primary colours are called
the secondary colours – magenta, yellow and cyan.
red green
blue
magenta

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Mixing coloured light

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Which primary colours?

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How do we see different colours?

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How do we see the different colours in this frog’s skin?
Seeing different colours
This skin absorbs
all colours.
No colours are
reflected and so it
appears black.
How do we see the different colours in this flower?
This part absorbs
all colours except
red and green.
It reflects red and
green light, and so
appears yellow.
This part absorbs
no colours.
It reflects all the
colours and so
appears white.
This skin absorbs
all colours except
red and so is seen
as red.

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Colour you see Colour light that is reflected
white
Black
red
yellow
Green
Blue

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Recap on last lesson
1. What do we call the separation of white light
and how is it done? (2)
2. Name all 7 colours of the spectrum in order (1)
3. Which is refracted most, red or violet light? (1)
4. Give two ways of making white light again (2)
5. What are the three primary colours? (1)
6. Name the three secondary colours and which
primary colours make each one (3)

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A red filter absorbs
all colours…
A blue filter absorbs
all colours…
A green filter absorbs
all colours...
…apart from red light.
…apart from blue light.
…apart from green light.
Using filters of primary colours: watch
A filter absorbs some colours of white light and lets other
colours through to create coloured light.

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A magenta filter absorbs
all colours…
A cyan filter absorbs
all colours…
A yellow filter absorbs
all colours...
…apart from red and blue.
…apart from green and blue.
…apart from red and green.
Using filters of secondary colours

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Seeing colours in coloured light

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How do we see colours in coloured light?

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Coloured light activity

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What colour does it appear?

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• In different colours of light this kit would look different:
Red
light
Shirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue

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Some further examples:
Object Colour of light
Colour object
seems to be
Red socks
Red Red
Blue Black
Green Black
Blue teddy
Red Black
Blue
Green
Green camel
Red
Blue
Green
Magenta book
Red
Blue
Green

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Using filters
• Filters can be used to “block” out different colours of light:
Red
Filter
Magenta
Filter

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Investigating filters
Colour of filter Colours that could be “seen”
Red
Green
Blue
Cyan
Magenta
Yellow

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• This little light of
mine
• I’m going to let it
shine
• This little light of
mine
• Shines in straight
lines
• Let it shine
• Let it shine
• Let it shine
• Let it shine
This little light of mine
Aka the incident ray
This little light of mine
Reflected the other way
Let it shine
Let it shine
Let it shine
Let it shine

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Red
Magenta
White
Yellow
Blue Green
Cyan