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Chapter 12: Light
Travels in straight line
Consists of transverse waves in which the
electromagnetic vibrations are
perpendicular to the direction of travel.
Type of wave
Can they travel through
Can they travel through
a vacuum?
How are they detected
Can they be reflected
Can they be refracted
Speed of travel
The direction of vibration of the
waves is perpendicular to the
direction that the light travels
Light waves
Yes, (transparent/
Sound waves
Yes (solid, liquid, gas)
Eyes, cameras
3 x 108 m/s
Ears, microphones
300 m/s
Explain why you can see lighting before you can hear the thunder?
The light travels faster than sound. Therefore, the light from the lightning
travels to our eyes much quicker than the sound from the lighting.
We can see luminous objects because they are light sources, explain why we
can see non-luminous objects?
We can see objects because light travels from them into our eyes. Luminous
objects make their own light, e.g. the Sun, a light bulb and a candle. Most of the
objects do not make their own light. We can see them because they reflect
light to our eyes.
How does light form shadows?
Light ray travels in straight line, travelling from the light source to the object.
A shadow is made when an object blocks light, in which the object is opaque or
*Opaque objects make dark shadows;
translucent objects make faint shadows;
transparent objects make no shadow.
*If the object is moved closer to the light source, the shadow gets bigger.
If the objects is moved further away from the light source, the shadow
gets smaller.
*The Sun makes the longest shadows at the beginning and end of the day,
when the Sun is lowest in the sky.
The Sun makes the shortest shadows at midday, when the Sun is
highest in the sky.
Reflection of light
Light is reflected when it hit on the surface.
When the surface of an object is smooth and flat, e.g. mirror. All of the light
rays bounce off a mirror without being scattered. Thus, an image can be seen in
the mirror.
When the surface is rough, e.g. white paper, light are scattered in all directions.
Laws of Reflection
The angle of incidence is equal to angle of reflection.
The incident ray, the reflected ray and the normal at the point of
incidence lie in the same the plane.
Refraction of light
Any material that light can travel through is called a medium.
When light rays travel from one medium to another they bend.
The change of direction of light ray when it enters or leaves a transparent
How the refraction of light occurs?
The light bends because it travels more slowly in glass than it does in air.
A light ray bends towards the normal when it passes from air into a transparent
A light ray bends ways from the normal when it passes from a transparent
material into the air.
White light is a mixture and it can be is split up into a spectrum of colours,
which are the same colours as you see in a rainbow. Explain how the light is
split up into seven different colours when it passes through a prism?
A prism is a triangular glass block.
When the light enters the prism it bends/refracted.
The splitting up of white light into separate colours is called dispersion, which
occurs because of the light refraction.
It also bends as it leaves the prism and a spectrum appears.
The colours of the spectrum always appear in the same order:
Red, orange, yellow, green blue, indigo and violet
Explain why the spectrum always show the same order of colours?
When white light enter a block of glass, some colours bend more than others.
Violets bends the most; Red bends the least.
This means that the different colours travel off in different directions and so
we can see them separately.
Describe the effect of coloured filters on white light.
A filter is a piece of coloured plastic or glass.
It only allows one colour of light to pass through it; and absorbs the other
As we know, a prism divides white light into a spectrum. How to put white
light back together again?
Red, green and blue filters are the primary colours of light.
Adding the primary colour of lights together can make white light.
This is because each filter lets through one third of spectrum.
When you use them together, all colours of the spectrum are present and they
add up to give white.
How we see colour?
Coloured objects only reflect their own colour light.We see colours because
those are reflected off and object. For example, a red book looks red because
it only red light is reflected to our eyes but the other colours are absorbed so
we see red.
In red light, the red book still looks red because it reflects the red light. If the
book is placed in any other colour of light, it will absorb the light. Thus, no light
is reflected off the book into the eyes so we see black.
How do polarized sunglasses reduce glare?
Light usually scatters in all directions.
When the light is reflected from flat surfaces, it tends to become polarized
(travels in a more horizontal direction)
This creates an annoying and sometimes dangerous intensity of reflected light
that causes glare and reduces visibility.
Polarized lenses contain a special filter that blocks this type of intense
reflected light, reducing glare.
In these glasses only light perpendicular to the highway is allowed through. This
halves the amount of light reaching the eyes.
The pupil allows light to enter the eye.
The lens helps to focus the light
Light sensitive cells in the retina detect the light.
A nerve impulse travels from the retina to the brain along the optic nerve
where it is interpreted.
The iris helps to change the size of the pupil