Colour.
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Rod cells in our retinas
detect faint light but not
colours and cone cells detect
the primary colours of red,
blue and green. We see
combinations of primary
colours as secondary colours
(magenta, cyan and yellow).
White light is a mixture of
colours. White light can be
split up using a prism to give
a spectrum of seven colours
(red, orange, yellow, green,
blue, indigo, violet).
The splitting of colour into a
spectrum is called
dispersion.
We are able to see colours
because objects do not
reflect all the colours in
light. White objects reflect
all the colours, but a red
object only reflects red and
all other colours are
absorbed.
This idea applies to all
colours except black – black
objects absorb all colours.
Filters are used to make
coloured light. They
transmit one of the colours
in white light and let the
other colours through.
If you look at a coloured
object in coloured light, its
colour may appear to be
different.
How does light travel?
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What is transmission and
absorption?
Light travels in straight lines from a source.
Light travels as transverse waves.
It travels much faster than sound, and does not need a
substance to travel through.
Light travels through transparent objects but not through
opaque objects.
Shadows are made when light is blocked by an object.
Opaque objects block all light.
Translucent objects allow some light to pass through, but it
is scattered so you do not see a clear image.
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Key words:
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Absorb ‘To soak up’ or ‘to take in’.
Opaque: Material that does not let light through. It is not possible to see through an opaque
substance.
Ray: A narrow beam of light, or an arrow on a diagram representing the path
Reflect: To bounce off a surface instead of passing through it or being absorbed.
Shadow: A place where light cannot get to, because an opaque object is blocking the light.
Translucent: Material that lets light through but scatters it. You cannot see things clearly
through translucent materials.
Transmit: To pass through a substance.
Transparent: A material that light can travel through without scattering. (Note: transparent
substances may be coloured or colourless.)
Spectrum: The seven colours that make up white light.
Dispersion: The separating of the colours in light, for example when white light passes through a
prism.
Angle of Incidence: The angle between an incoming light ray and the normal
Angle of Reflection: The angle between the normal and the ray of light leaving a mirror.
Incident Ray: A ray of light going towards the mirror or other object.
Normal: An imaginary line at right angles to the surface of a mirror or other object where a ray
of light hits it.
What is refraction?
• When light hits something
transparent it changes speed
and direction. This is called
refraction.
• Refraction takes place at the
interface between two
substances.
• When light is transmitted
through glass it slows down and
changes direction towards the
normal. When it travels back out
it speeds up again and changes
direction away from the normal.
Transparent materials let light
pass straight through. We say they
transmit light.
Opaque surfaces can absorb or
reflect light.
White surfaces reflect most of
the light that hits them.
Black surfaces absorb light very
well and reflect very little. This is
why they look so dark.
What is reflection?
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Light rays are scattered by
rough surfaces (diffuse
reflection), which means that you
cannot see an image in an object
with a rough surface.
Mirrors and shiny materials such
as polished metals reflect light
evenly. This is called specular
reflection.
The angle of incidence (i) is equal
to the angle of reflection (r) –
this is known as the law of
reflection.
Angles are measured between
the light rays and the normal (a
line drawn at right angles to the
reflecting surface).
You can see an image in a mirror
because the reflected rays of
light appear to come from a point
behind the mirror.
The image in a plane mirror is
the same size as the object, and
the same distance away from the
mirror. In the image, left
becomes right and right becomes
left.
Revision Questions
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What are the 7 colours of the spectrum? Write them in the
correct order.
What does a blue filter do to white light? Include the word
absorb in your answer.
Why does a red book look red? Use the words reflect and
absorb in your answer. Also draw a diagram to show your
answer.
Draw a ray diagram to show how the light travels if a man is
looking at a ball in a field.
Why can you see your reflection in a mirror but not in a piece
of wood?
Name something which is opaque.
Name something which is translucent.
Name something which is transparent.
Draw a labelled ray diagram to show what happens when light
reflects off a mirror. You must include these words in your
labels: normal, angle, reflection, incidence, mirror.
Revision Questions
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What are the 7 colours of the spectrum? Write them in the correct order.
What rhyme do you use to remember them?
2. What does a blue filter do to white light? Include the word absorb in your
answer.
3. What happens if you shine green light on a red book? Explain your answer.
4. Why does a red book look red? Use the words reflect and absorb in your
answer. Also draw a diagram to show your answer.
5. Draw a ray diagram to show how the light travels if a man is looking at his
feet in a mirror.
6. Why can you see your reflection in a mirror but not in a piece of wood?
7. Name something which is opaque.
8. Name something which is translucent.
9. Name something which is transparent.
10. What does the word transmission mean? Name something which transmits
light.
11. What does the word emit mean? Name something which emits light.
12. Draw a labelled ray diagram to show what happens when light reflects off a
mirror. You must include these words in your labels: normal, angle,
reflection, incidence, mirror.
13. What is refraction? Where does it occur?