science world 2 – chapter 6

light & sound

ABOUT LIGHT & SOUND

Both light and sound are forms of energy. They can both be reflected (i.e. bounce off) from objects. With light, you see your reflection when you look in a mirror. With sound, you hear an echo when you shout in front of a tall building. Reflection is one of the common properties (i.e. characteristics) of both light and sound.

Another property of light is that it always travels in straight lines – it is not able to bend around curves.

REFLECTION

The reflection you see when you look in a mirror is called an image. The light rays bounce off the reflective surface at the back of the mirror and travel to your eyes. Images can be seen in other reflective surfaces (e.g. glass, water) as long as they are smooth and shiny.

When light strikes a reflective surface it obeys the law of reflection. This law states that “light striking a mirror at an angle reflects at the same angle”. It can be shown in the following diagram:

normal

angle of angle of

incidence reflection

incident rayreflected ray

i r mirror

Angle of incidence = Angle of reflection

REFRACTION OF LIGHT

Air and water are transparent substances. This means that light passes through them. Substances that don’t allow light to pass through them are called opaque (e.g. paper, wood).

When light passes from one transparent substance to another it changes direction. This bending of light is called refraction.

normal

angle of incidence

light ray

Lenses are pieces of glass or plastic curved on one or both surfaces. There are two main types of lenses: converging lenses refract light inwards, whereas diverging lenses refract light outwards. (Look at diagrams on p 121)

LIGHT & COLOUR

Although sunlight appears to be white, it is not actually a single, pure white colour. It is made up of seven different colours that you can see, called the visible spectrum – red, orange, yellow, green, blue, indigo and violet.

White light can split into a spectrum when refraction occurs. This process is known as dispersion. Dispersion can happen as sunlight passes through raindrops forming a rainbow or when light passes through a glass prism. It occurs because each colour is refracted slightly different amounts (i.e. violet light is refracted more than red light).

WHY ARE THINGS COLOURED?

White objects appear white because they reflect all of the colours of the sunlight. Black objects absorb all of the colours of sunlight, so no colour is reflected back into our eyes. The colour we see in an object depends on the colour (or combination of colours) that is reflected back to our eyes e.g. leaves reflect green light, which is why they appear green.

Coloured filters can also change the colour of light. When white light hits a red filter, it absorbs all the other colours and only allows the red light to pass through. The colour of the filter tells you what colour it transmits (i.e. allows to pass through).

MAKING COLOURS

There are two main ways of making different colours – either shine different colours together or mix different coloured paints together.

Making colours by adding different coloured lights together is called addition. If you add red, green and blue lights they will make white light. (Look at Figure 43 on p 128)

Making colours by mixing different coloured paints together is called subtraction because each paint colour subtracts or absorbs colours from white light. (Look at Figure 45 on p 129)

SOUND WAVES

Sounds are made by vibrating objects. The vibrations are then carried through the air as sound waves. These waves are made up of bands of high air pressure (called compressions) and lower air pressure (called rarefactions).

The energy from the vibrating source is transferred from particle to particle as the sound wave travels. The waves spread out in all directions and gradually become fainter as the energy of the waves decrease.

In air at 0C, sound travels at approximately 330 m/s. It travels even faster through liquids and solids as the particles are closer together and can transmit the sound waves even quicker. (Look at table showing different speeds of sound on p 133)

LIGHT WAVES

Light waves carry electric and magnetic energy through space and are called electromagnetic radiation. They do not need particles to transfer their energy – they are able to travel through the vacuum of space. The various types of electromagnetic radiation are different because they have different wavelengths. (Look at Figure 61 on p 134)

Light travels through a vacuum at a speed of about 300,000,000 m/s – this is almost a million times faster than the speed of sound!!! This is why you always see the lightning before you hear the thunder during a storm.

Light travels slower when it passes through substances that contain more particles (e.g. the speed of light in glass = 198,000,000 m/s). This is the reason why it refracts when it passes from one transparent substance to another – it is changing speed so it also changes direction.

SPELLING WORDS

No. / EASY / HARD
1 / light / transparent
2 / sound / translucent
3 / filter / opaque
4 / retina / reflective
5 / lens / refraction
6 / pupil / dispersion
7 / iris / parallel
8 / colour / absorption
9 / image / incidence
10 / focus / prism
11 / convex / spectrum
12 / concave / converging
13 / diverge / receptors
14 / inverted / electromagnetic
15 / normal / optical
16 / mirror / transmitted
17 / microwaves / addition
18 / absorb / vibration
19 / bright / amplitude
20 / dull / wavelength

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