Physics: Waves

Definition of a wave

Waves carry energy or information from one place to another

Transverse waves

e.g. light waves

Travel perpendicular to the direction of energy travel

Up and down movements

Longitudinal waves

e.g sound waves

Travel parallel to the direction of energy travel

Across movements

What is the term for short and long wavelengths in a Longitudinal Wave

Short: compression

Long: Rarefaction

What frequency can humans hear?

20 - 20,000 Hz

Definition of Frequency

The number of waves passing a point in a second

Definition of ‘period’

P - time period

Time taken to pass a point (seconds)

Time taken for one complete oscillation

Variables for the speed of ripples on water practical

Control - Travel length, height dropped from

Dependant - Time to travel 2m

Independent- Water Depth

Equation for speed

S=D/T

Speed = Distance / Time

What is the ‘normal’?

A line drawn perpendicular to the mirror/surface to help measure the angle

When light reflects off a flat surface in a straight line, what is this called?

specular

When light reflects off a rough surface in different directions, what is this called?

Diffused

Info about sound waves

• Longitudinal (across)

• Vibrations travel through air

• Vibrations move to our ear drums which vibrate and cause the sensation of sound

Ultrasound

• Above 20,000Hz (above human hearing)

• Wave is partially reflected at the boundary between materials

What is ultrasound used for and how does it work?

Medical (foetus scan) - Sound waves bounce off skin, organs etc at different speeds. These echos are converted into images with a transducer.

Industrial (under oceans) - Time taken for reflections to reach a detector can be used to determine how far away a boundary is

Dog training (whistles)

What are seismic waves? Explain P waves.

P waves - longitudinal, pass through solids and liquids. Travel faster

Because they travel at different speeds in solids and liquids, they refract going in and out of the outer core (its liquid) leaving shadow zones.

Faint P waves can be detected in the P wave shadow zones (shows that inner core is solid)

What are S waves?

Secondary waves - transverse, Only travel through solids.

What is the Electromagnetic spectrum?

Moves energy from a source to an absorber

Lowest Freq:

Radio waves

Microwaves

Inferred Radiation

Visible light

Ultraviolet

X-ray

Gamma waves

Highest Freq

Wavelength

The distance from a point on a wave to the equivalent point on the next wave

(metres)

What Is the highest and lowest point on a wave called?

Peaks

Troughs

What can happen when a wave meets a boundary

Reflection

Transmission

Absorption

Refraction

What happens when waves are reflected off a surface

The angle of incidence is equal to the angle of reflection

What does Refracted mean?

Wave is reflected off a surface and changes direction

What happens when light travels from a material with a low to one with higher refractive index?

The light bends towards the normal

What happens when light travels from a material with a high to one with lower refractive index?

The light bends away from the normal

Why does Refraction happen?

• When a light wave enters a medium in which it travels slower at an angle,

• The first part of the wave to enter slows down

• The rest continues at a higher speed

• the waves changes direction towards the normal

Label this ripple tank

Wave speed equation

v=f x λ

wave speed = freq x wavelength

Label this diagram

How to determine wavelength with this practical

Total Length / number of half wavelengths x 2

How to draw the reflected ray?

What is a convex lens?

How do you show this?

Method for angle of refraction with ray box

1. place a glass block on a piece of paper

2. draw around the glass block

3. use the ray box to shine a ray of light through the glass block

4. mark the ray of light entering & exiting

5. join the points to show the path of the complete ray through the block

6. draw a normal line at 90° to the surface

7. use a protractor to measure the angle of incidence & refraction

8. use a ray box to shine a ray of light at a range of different angles (of incidence)

9. increase the angle of incidence in 10° intervals
   from 10° - 70°

What is the Leslie cube practical?

Discover how much infrared different surfaces absorb

1. Fill cube with hot water

2. Point infrared detector on each surface (same distances) can use thermometer but less accurate

3. Matt black → shiny black → white → shiny metallic (order of absorbsion)

Another method for the Leslie cube practical

• drawing pin held on with vaseline

• Falls off matt black side first because they absorb more infrared.

How are radio waves made?

When electrons oscillate in electrical circuits

Can be absorbed by electrical circuits in an aeriel, causing electrons in the circuit to oscillate

Creates an AC the same freq as the radio waves

Label a convex lens diagram

How to draw a ray diagram for a convex lens

• Real - because it is inverted & rays meet at a point

• Diminished - image is smaller than object

• Virtual - if the rays don’t meet and they are connected with dotted lines beghind the arrows

How to draw ray diagram of concave lens

How to draw concave lens diagram

What is black body radiation

A perfect black body absorbs all radiation - none transmitted or reflected

Best emmitter for radiation