467834040-light-1-ppt (1) light wave cambridge.ppt

light wave cambridge

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Light

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What is light?

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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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solar cell plants
Light energy
Light energy can be used to make other useful forms of energy.
It can be converted into electrical energy in a solar cell or
chemical energy in the leaves of plants.
Light is a form of energy and can be changed from one form
into another.

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Which materials let light through?

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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?
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.
Light travels in a
straight line directly
into the eye.

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How the eye works

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Reflection

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Objects that reflect light well:
Reflective materials
Have rough, matt surfaces
and are usually dark colours.
Give diffuse images (or do not
give any images) because
they reflect light irregularly.
This is called scattering.
Have smooth, shiny surfaces
and are usually pale colours.
Give clear images because
they reflect light regularly.
Objects that do not reflect light well:

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Working in pairs, decide who is the ‘timer’ and who is
the ‘reader’.
Reading in mirrors

Time taken (s)

Natasha
Shani
Rajesh
Name
1. Why are the words so
difficult to read in the mirror
– how do they appear?
2. What was the average time
taken in the class?
3. Plot a bar chart of results.
The ‘timer’ measures the time taken and the results for
the whole class are recorded in a table like this:
The ‘reader’ has to read a selection of words reflected in a
mirror. They must read each word correctly before moving
on to the next.

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A plane mirror reflects light
regularly so it produces a clear
image, which is the same size
as the object.
What is lateral inversion?
When an object is reflected in a plane mirror, left appears
as right and right appears as left. This type of reversal is
called lateral inversion.
What is different about the
image compared to the object?
The image appears the same
distance behind the mirror as
the object is in front of it.

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Reflection at a mirror

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Reflection ray diagram

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Fix a plane mirror to a piece
of paper and draw around it.
Reflection investigation
What do the results show?
Repeat for another four
angles of incidence.
Measure the angles of
incidence [i] and reflection [r].
Use a ray box to shine an
incident ray at the mirror – plot
the incident and reflected rays.
Draw a normal (a line at 90° to
the mirror) through the centre of
the mirror outline.
angle of
incidence [i]
angle of
reflection [r]

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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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Refraction

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Refraction at an air-glass boundary

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

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Refraction in a glass block

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Explaining refraction

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Why is light refracted?
The speed of light depends on the material through which the
light is travelling. When light enters a different material (e.g.
when moving 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, its speed
decreases and this is why refraction occurs.
air
glass

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Refraction ray diagram

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What happens during refraction?

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Effects of refraction
Light from the part of the pencil in the
water is refracted as it travels from the
water into the air, making it appear bent.
Light rays from the stone are
refracted as they leave the water.
How does refraction make this
stone look closer to the surface
of the water than it really is?
The brain assumes the rays
have travelled in straight lines,
and is fooled into forming an
image where it thinks the light
rays came from.

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The archer fish is a predator that shoots jets of water at
insects near the surface of the water.
Effects of refraction – the Archer fish
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

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Refraction – true or false?

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Colour

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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.
Splitting white light
Richard Of York Gave Battle In Vain
The order of the colours in the spectrum is always the
same. Use this phrase to remember the order of colours:
The colours that make
up white light are called
the spectrum.
Dispersion occurs because different colours of light refract
differently. Red light refracts the least; violet light the most.

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If there are water droplets in the air and the sun is illuminating
them from behind, then you may see a rainbow in the air.
Light enters the water
droplets and refracts. It
then reflects off the back
of the rain drop.
Natural dispersion
The red light refracts
the least and the
violet the most. This
causes dispersion of
the sunlight.

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The visible light spectrum

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Recombining colours

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Mixing coloured light

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How do we see different colours?

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How do we see the different colours in this frog and lily?
Seeing different colours
The frog’s red skin absorbs all of the
colours except red and so it appears red.
The black skin absorbs all colours. No
colours are reflected and so it appears
black.
The lily’s centre absorbs all colours
except red and green. It reflects red and
green light, and so appears yellow.
The leaves reflect all the colours and so
appear white.

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

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

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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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The electromagnetic spectrum

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Colour summary

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Summary activities

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Glossary

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Anagrams

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Multiple-choice quiz