## Sound Waves (Triple Physics Only)

Sound waves can travel through solids causing vibrations in the  solid.

Within the ear, sound waves cause the ear drum and other parts  to vibrate which causes the sensation of sound. The conversion of  sound waves to vibrations of solids works over a limited frequency  range. This restricts the limits of human hearing.

The range of normal human hearing is  from 20 Hz to 20 kHz

Question: There is no air in space.
Astronauts in space cannot hear sounds from outside their spacesuits.
Explain this. (2 marks)

Ultrasound waves have a frequency higher than the upper limit of  hearing for humans. Ultrasound waves are partially reflected when  they meet a boundary between two different media. The time taken  for the reflections to reach a detector can be used to determine how  far away such a boundary is. This allows ultrasound waves to be  used for both medical and industrial imaging.

The amplitude the reflected pulse is lower then the transmitted pulse because some of the energy is absorbed by the object as the sound wave travels through it.

The first pulse is transmitted into the object being investigated

The second plus is a reflection detected from a boundary between materials in the object

The time between the 2 pulses is worked out from the scale on the oscilloscope (0.1s per square) = 6 x 0.1 = 0.6s

The distance the wave travels in the time is 2 x the distance from the surface to the boundary that caused the reflection

If you know the speed you can use distance = speed x time to find the distance

Seismic waves are produced by earthquakes. P-waves are  longitudinal, seismic waves. P-waves travel at different speeds  through solids and liquids. S-waves are transverse, seismic waves.  S-waves cannot travel through a liquid. P-waves and S-waves  provide evidence for the structure and size of the Earth’s core.

Echo sounding, using high frequency sound waves is used to detect  objects in deep water and measure water depth.

The study of seismic waves provided new evidence that led to discoveries about parts of the Earth which  are not directly observable.

Question: What is ‘ultrasound’? (2 marks)

Question: Explain what scientists should do if they find evidence that ultrasound may be harmful to human health. (2 marks)

## Light (Triple Physics Only)

Waves can be reflected at the boundary between two different  materials.

Waves can be absorbed or transmitted at the boundary between two  different materials.

A lens forms an image by refracting light. In a convex lens, parallel  rays of light are brought to a focus at the principal focus. The  distance from the lens to the principal focus is called the focal length.

Rays that form images can be:

• Real: able to be projected onto a screen; always drawn with a straight, solid line with an arrow to show direction
• Virtual: rays which are used to construct ray diagrams to show image location that do not actually exist; always drawn with a straight, dotted line with an arrow to show direction

The image produced by a convex lens can be either:

• Real: formed where real rays intersect
• Virtual: formed where at least 1 virtual ray intersects

Ray diagrams show how light changes direction as it is refracted through lenses.

## Concave Lens

The image is always virtual, it can not be projected onto a screen.

## Convex Lens

The location of the image depends on the positions of the lens, object and focal point.

In this example the image is real, inverted (upside down) and magnified (a different size from the object)

The magnification produced by a lens can be calculated using the  equation:

magnification = image height/object height

Magnification is a ratio and so has no units.

Image height and object height should both be measured in either  mm or cm.

Question: A person uses a lens to read the letters on the back of a coin.
The image height of the letters on the coin is 9.0 mm
The magnification produced by the lens is 6.0
Calculate the height of the letters on the coin.

## Light & Colour (Triple Physics Only)

Each colour within the visible light spectrum has its own narrow band of wavelength and frequency.

## Specular Reflection

Reflection from a smooth surface in a single direction is called  specular reflection.

## Diffuse Reflection

Reflection from a rough surface causes  scattering: this is called diffuse reflection

The colour of an opaque object is determined by which wavelengths  of light are more strongly reflected. Wavelengths that are not  reflected are absorbed. If all wavelengths are reflected equally the  object appears white. If all wavelengths are absorbed the objects  appears black.

Objects that transmit light are either transparent (transmits all light) or translucent (transmits some light).

Colour filters work by absorbing certain wavelengths (and colour)  and transmitting other wavelengths (and colour). A blue filter transmits blue wavelengths and absorbs all others.

Question:A student wears a white T-shirt and a red baseball cap to a party.
Why does the T-shirt look white in white light?

Question:A white surface is viewed through a green filter.
What colour will the surface look?