Transparent objects but not through opaque objects. Shadows

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Year 8 Light & Sound Topic 11 Summary Sheets
Objects which create light are luminous sources. Light travels in straight lines.
Light waves travel through transparent objects but not through opaque objects. Shadows are made because light cannot travel through opaque objects. Translucent objects show a glow of light through them.
Transmission and absorption

Transparent materials let light pass straight through. We say they transmit light. Opaque surfaces can absorb light. Black surfaces absorb light very well and reflect very little. This is why they look so dark.


Light rays are scattered by rough surfaces, and a reflection cannot be seen.
A plane mirror is a flat mirror. Light is reflected evenly by a plane mirror.

The angle of incidence is equal to the angle of reflection.
When light shines on to an object viewed in a mirror, the rays are reflected into the eye. They seem to come from a position behind the mirror. The image is the same size as the object and the same distance from the mirror. In the image left is right and right becomes left.

When light hits something transparent it changes direction. This is called refraction.
Refraction takes place at the interface between two substances. When light is transmitted through glass it slows down and bends towards the normal. When it travels back out it speeds up again and bends away from the normal.


White light is a mixture of colours. White light can be split up using a prism to give a spectrum of seven colours (red, orange, yellow, green, blue, indigo, violet).

The splitting of colour into a spectrum is called dispersion.

A rainbow is produced when water droplets in the air refract sunlight.
Different colours can be made by mixing light of the three primary colours (red, green and blue).
Coloured light can be made using a filter. A red filter lets red light through, but absorbs all the other colours.
We are able to see colours because objects do not reflect all the colours in light:
White objects reflect all the colours.

A red object only reflects red and all other colours are absorbed.

This idea applies to all colours except black.

Black objects absorb all colours.

Night and Day

We live on a planet called the Earth. The Earth gets heat and light from the Sun. The Earth spins on its axis once every 24 hours. The side of the Earth facing the Sun has daylight, and it is night on the side facing away from the Sun.

The Earth orbits around the Sun. It takes one year to go around once. A year is actually 365.25 days long, so every four years we have a leap year, when an extra day is added.

We can see the Moon because it reflects light from the Sun. The Moon seems to change shape during the month. The different shapes are called phases of the Moon. The phases happen because we cannot always see all of the part that is lit by the Sun.
Sometimes the Moon blocks the light from the Sun. When this happens we get a solar eclipse. If the Moon goes into the shadow of the Earth we get a lunar eclipse.

A solar eclipse.

A lunar eclipse.

Planets do not make their own light. We can sometimes see the planets because they reflect light from the Sun.
The Sun is a star. It is a ball of gas that gives out large amounts of heat and light energy. The Sun is like the stars you can see in the sky at night. The stars do not look very bright because they are a lot further away than the Sun. People often group stars into patterns called constellations.
The Sun is one of millions of stars in our galaxy, which is called the Milky Way. There are millions of galaxies in the Universe.
The stars are a very long way from Earth. Scientists measure distances to the stars using light years. A light year is the distance that light can travel in one year.

Sound and hearing
Sound vibrations and waves

Sound is a form of energy. Sounds are made when things vibrate. The vibrations are passed on by particles in solids, liquids or gases. Sound needs a substance to pass on the vibrations, so it can travel through solids, liquids and gases but not through a vacuum.

The speed of sound is faster through solids than liquids, and slowest through gases. This is because the particles are very close together in solids and so the energy is more likely to be passed from one particle to the next. The sound travels in all directions because the particles move in all directions unless something stops them.

Sound waves can be shown on an oscilloscope.

The frequency of a wave is the number of vibrations each second. The unit for frequency is hertz (Hz). If you listen to a sound with a frequency of 100 Hz, one hundred waves reach your ear every second. High pitched sounds have a high frequency, and low pitched sounds have a low frequency.
The distance between the waves is called the wavelength. It can be measured between any point on a wave and the same point of the next wave. It is often more convenient to measure it between the top of one wave and the next.

Half the height of the wave is called the amplitude. The loudness of a sound depends on the amplitude. Louder notes have more energy and the wave has a bigger amplitude.

Hearing and the ear

Sound waves travel through the air and into the ear. They cause the eardrum to vibrate. The vibrations are passed on to the cochlea in the inner ear, where they are changed to electrical signals called impulses. A nerve takes this message to the brain. When the message reaches the brain we hear the sound.

Sound can damage the ears if it is too loud or goes on for too long. Loud sounds can damage the eardrum or the cochlea. Unpleasant sound is often called noise.
We can measure how loud a sound is by using a sound intensity meter. This is an instrument which measures the loudness of a sound in decibels (dB). The threshold of hearing is the quietest sound we can hear and we say this is 0 dB.
Soft materials can absorb sound. Soft materials are used in soundproofing and for making ear protectors. Double glazed windows and soft materials like curtains help to reduce sound levels.
Sound and light

One major difference between light and sound energy is that light can travel through space (a vacuum) but sound cannot.

Light also travels much faster than sound. It is nearly a million times faster. Light travels at 300 million metres per second (or 300 000 km/s) and sound travels at about 330 metres per second.
Both light waves and sound waves can be reflected. We hear a reflected sound wave as an echo.

All waves transmit energy. Light waves raise the temperature of the object they fall on. Similarly sound waves also transmit energy.

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