© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2006 1 of 47 KS3 Physics 8K Light

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20061 of 47

KS3 Physics

8K Light


8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour


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.

What is light?


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.

Investigating light

Light waves travel in straight lines.

Transparent material lets almost all the light pass through. Translucent material only allows some of the light through. Opaque material does not allow light to pass through.


Which materials let light through?


An object that gives out light is described as luminous.

Light travels in a straight line directly

into the eye.

How do we see things?

How does light from a luminous object such as a light bulb reach the eye?

An object that does not give out light is non-luminous.

Light from the light source hits the book and some of this light

How does your eye see a non-luminous object such as a book?

is reflected into the eye.


8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour


Objects that do not reflect light well:

Reflective materials

Objects that reflect light well:

Have smooth, shiny surfaces and are usually pale colours.

Give clear images because they reflect light regularly.

Have rough, matt surfaces and are usually dark colours.

Give diffuse images (or 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:

Name Time taken (seconds) Natasha

Shani Rajesh

Reading in mirrors – instructions

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.

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 lateral inversion?

What is different about the image compared to the object?


1. Fix a plane mirror to a piece of paper and draw around it.

Angle of incidence [i]

Angle of reflection [r]

angle i

angle r

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 of incidence.

6. What do the results show?

Reflection investigation


Reflection ray diagram


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?

Using reflection

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?


Reflection summary


Reflection – true or false?


8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour


Refraction at the air-glass boundary


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°


Refraction in a glass block


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.


Refraction ray diagram


What happens during refraction?


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


The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf.

image of prey

prey location

The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

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.


Refraction summary


Refraction – true or false?


8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour


Passing white light through a prism


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 to remember the order of colours:


Colours of the spectrum


Recombining colours


Newton’s disc animation


Mixing coloured light


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 calledthe secondary colours – magenta, yellow and cyan.

red green

blue

magenta


Which primary colours?


How do we see different colours?


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.


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

A filter absorbs some colours of white light and lets other colours through to create coloured light.


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


Seeing colours in coloured light


How do we see colours in coloured light?


Coloured light activity


What colour does it appear?


8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour


Glossary

absorption – When light is taken in by a material.dispersion – The separating of the colours in light,

e.g. when white light passes through a prism.image – A copy of an object formed when light is reflected

from a mirror.light – A form of energy that is detected by the eyes. law of reflection – When light is reflected, the angle of

incidence equals the angle of reflection.prism – A block of glass, usually triangular, which separates

the colours in light.reflection – The bouncing back of light from a surface.refraction – The bending of light when it passes into a

different medium, e.g. from air into glass.spectrum – The range of colours that make up white light.


Anagrams


Multiple-choice quiz

Documents

© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2006 1 of 47 KS3 Physics 8K Light