KS3 Physics. 8K Light. Contents. 8K Light. What is light?. Reflection. Refraction. Colour. Summary activities. What is light?. Light carries energy and travels as a wave.
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Light carries energy and travels as a wave.
Light travels at a speed of 300,000,000 metres per second, which is much faster than the speed of sound.
Light waves travel in straight lines.
1. Investigate how light travels using a glass trough filled
with smoke. Use a slit to
send rays of light into the tank and describe what you see.
2. Investigate which materials let light through by holding different materials between a lamp and a screen.
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?
Light travels in a straight line directly into the eye.
An object that does not give out light is non-luminous.
How does your eye see a non-luminous object such as a book?
Light from the light source hits the book and some of this light
is reflected into the eye.
Objects that reflect light well:
Objects that do not reflect light well:
do not give any images) because they reflect light irregularly.
Working in pairs, one person is the ‘timer’ and the other person is the ‘reader’.
The ‘reader’ turns their back and has to read a list of words using a mirror. They must read each word correctly before moving on to the next word. 
The ‘timer’ measures the time taken and the results for the whole class are recorded in a table like this:
1. Who read the words in the quickest time?
2. Plot a bar chart of the results.
3. What was the average time taken in the class?
A plane mirror reflects light regularly so it produces a clear image, which is the same size as the object.
The image appears the same distance behind the mirror as the object is in front of it.
What is different about the image compared to the object?
When an object is reflected in a plane mirror, left appears as right and right appears as left. This type of reversal is calledlateral inversion.
1. Fix a plane mirror to a piece of paper and draw around it.
2. Draw a normal (at 90°) through the centre of the mirror outline.
3. Use a ray box to shine an incident ray at the mirror – plot the incident and reflected rays.
4. Measure the angles of incidence [i] and reflection [r] and record the results.
5. Repeat for another four angles
6. What do the results show?
Reflection can be very useful.
High-visibility strips are very reflective and make sure that this cyclist gets noticed when there is little light.
How does a periscope use reflection?
The two plane mirrors must be positioned at 45° to each other.
Light is reflected at right angles from the top mirror onto the bottom mirror and into the eye of the viewer.
Where are periscopes used?
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?
The speed of light depends on the material through which the light is travelling.
When light enters a different material (e.g. from air into glass), the speed of light changes.
This causes the light to bend orrefract.
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.
Refraction causes visual effects such as this ruler appearing to be bent.
This happens because light from the part of the ruler in the water is refracted as it travels from the water into the air.
How does refraction make this stone look closer to the surface of the water than it really is?
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
light rays came from.
image of prey
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.
A prism splits a ray of white light into the colours of the rainbow.
This process is known asdispersion.
The colours that make up white light are called the spectrum. The order of the colours in the spectrum is always the same.
Use this phrase to remember the order of colours:
The colours made by mixing two primary colours are called
the secondary colours – magenta, yellow and cyan.
The three primary colours of light (red, green and blue) can be mixed in different amounts to make all other colours.
How do we see the different colours in this frog’s skin?
This skin absorbs all colours.
No colours are reflected and so it appears black.
This skin absorbs all colours except red and so is seen as red.
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.
A filter absorbs some colours of white light and lets other colours through to create coloured light.
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.
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.
e.g. when white light passes through a prism.