# P3 Revision Notes:Light and Sound

Just some notes on Light and Sound

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• Created on: 30-03-12 14:16

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REVISION NOTES: PHYSICS 3
LIGHT AND SOUND
Light Reflection
There are a number of different types of mirror. A plane mirror (or flat mirror) will show you an
exact mirror image of yourself if you look into it. Some mirrors bend outwards, these are called
convex mirrors if you look in one of these you see yourself being really tall and thin ­ like at a
funfair's Hall of Mirrors. Similarly, looking into a concave mirror which folds inwards will present
a shortened, fatter image than the true object.
Concave & Convex Mirrors
A concave mirror folds inward. For a distant object, the light rays are parallel when they reach the
mirror, as shown. They are then focused to a principal focus (or focal point), F, of the mirror. A
real image of the object is formed. The distance between the mirror and the principal focus is the
focal length, f, of the mirror.

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REVISION NOTES: PHYSICS 3
LIGHT AND SOUND
If an object is beyond the principal focus, an inverted image is formed (appears upsidedown) by
the reflected rays. The position and size of the image depends on the distance from the object to the
mirror. The diagram below shows how to use a ray diagram to locate the image:
The light rays from the tip of the object are used to locate the tip of the image.…read more

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REVISION NOTES: PHYSICS 3
LIGHT AND SOUND
Light Refraction
The change of direction in light rays is called light refraction.
Refraction applies to all types of wave, including light and sound. The diagram below demonstrates
refraction using waves of water in a ripple tank:
Light travels slower in glass than in air. When a light ray travels from air to glass, it refracts towards
the normal because it slows down upon entering the glass block.…read more

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REVISION NOTES: PHYSICS 3
LIGHT AND SOUND
Lenses
A lens works by changing the direction of light passing through it. The diagram below shows how a
lens refracts the rays so that they all meet as a point.…read more

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REVISION NOTES: PHYSICS 3
LIGHT AND SOUND
When the object is nearer to the lens than the principal focus, a magnified virtual image is formed,
but you can only see the image when you look into the lens from the other side to the object. The
lens acts as a magnifying glass in this situation.
Formation of a real image by a converging lens
The object must be beyond the principal focus, F, of the lens.…read more

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REVISION NOTES: PHYSICS 3
LIGHT AND SOUND
Sound
Any object which vibrates sends out sound waves
through the air, and vibrates your eardrums so you
hear sound. Sound waves cannot travel through a
vacuum. They are longitudinal waves, so vibrate in the direction they travel in, unlike
electromagnetic waves which are transverse (i.e. vibrate perpendicularly to the direction of travel).
Sound can be reflected in an echo. This can be heard in large rooms with bare, smooth walls.…read more

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REVISION NOTES: PHYSICS 3
LIGHT AND SOUND
Humans can hear frequencies between 20Hz and 20,000Hz ­ any sound waves with frequencies
above the human ear are said to be ultrasonic waves. We have many uses for ultrasound.
One example of its use is for ultrasound scanning, used for scanning body organs or babies in the
womb. A probe sends out ultrasonic pulses and detects pulses that are deflected back and when