1 of 47© Boardworks Ltd 2008 Light. © Boardworks Ltd 20082 of 47 What is light?

1 of 47 © Boardworks Ltd 2008

Light

© Boardworks Ltd 20082 of 47

What is light?


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.


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.


Which materials let light through?


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.


How the eye works


Reflection


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:


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

Reading in mirrors

Time taken (s)

Natasha

Shani

Rajesh

Name1. 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.


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.


Reflection at a mirror


Reflection ray diagram


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]


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?


Reflection: summary


Reflection: true or false?


Refraction


Refraction at an air-glass boundary


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 [r2].

angle of incidence [i]

angle of refraction [r2]

angle of refraction [r1]

0 °

30 °

60 °


Refraction in a glass block


Explaining refraction


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


Refraction ray diagram


What happens during refraction?


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.


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


Refraction – true or false?


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.

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.


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.


The visible light spectrum


Recombining colours


Mixing coloured light


How do we see different colours?


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.


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.


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.


Seeing colours in coloured light


How do we see colours in coloured light?


The electromagnetic spectrum


Colour summary


Summary activities


Glossary


Anagrams


Multiple-choice quiz

1 of 47© Boardworks Ltd 2008 Light. © Boardworks Ltd 20082 of 47 What is light?

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Transcript of 1 of 47© Boardworks Ltd 2008 Light. © Boardworks Ltd 20082 of 47 What is light?