Physics - EDEXCEL IGCSE - Waves

Waves:

Properties of waves:

Transverse waves - One which the vibrations are perpendicular to the direction of travel of the wave. e.g. a slinky jerked sideways, rope, water, the electromagnetic spectrum.

Longitudinal waves - One which the vibrations are parallel to the direction of the wave. e.g. sound waves, slinky jerked along its length. 

Time period = time / number of waves in that time

Frequency = 1 / time period

Wave velocity = frequency * wavelength

Waves transfer energy and information without movement of matter.

Wave behaviour:

Reflection - this is where a wave hits a surface and bounces back. The angle of incidence (angle at which the wave hits the surface) must be the same as the angle of reflection (the angle at which it  bounces back)

Refraction - this occurs as wavelength decreases when a wave goes from a less dense to a denser material. The light bends to the normal or away if from a dense to rare material. The way to calculate the angle of refraction is through the formula:

n₁ * sini = n₂ * sinr

where n = refractive index (speed of light / speed in material)

Diffraction - when waves move past an obstacle or through a gap it spreads out. This is called diffraction. Diffraction is largest when the gap is the same size as the wavelength.

The Electro-Magnetic Spectrum:

In order of increasing frequency and decreasing wavelength:

radio waves —> microwaves —> infrared —> light —> ultraviolet —> X-ray —> gamma rays

Uses:

• Radio waves - used in broadcasting and communications. Also used to transmit radio and TV.
• Microwaves - used in cooking and satellite transmissions.
• Infrared - used in heaters, optical fibre communication, remote controls and light vision.
• Light - used in optical fibres and photography.
• Ultraviolet - used in sunbeams, fluorescent lights, and security coding of bank notes.
• X-ray - used to produce internal structures of objects and materials. Used in medicine and security.
• Gamma rays - used for sterilisation and killing cancer cells.

Dangers:

• Microwaves - can cause internal heating of body tissue.
• Infrared - causes burning of skin if under the presence for too long.
• Ultraviolet - causes changes to surface cells, blindness and skin cancer.
• Gamma rays and X-ray - changes the DNA of cells and causes cancer and mutations.

Light:

Light waves are transverse waves that can be reflected, refracted and diffracted.

Examiners often like to ask for a part of the ray diagram to be drawn, drawing it similar to this will get you full marks on that question.

In an exam, the paper may ask to work out the real or apparent depth of an object in water, here is the formula to work it out:

real depth / apparent depth = n (the refractive index of the dense material)

Measuring the refractive index of glass:

1. Place a glass block on paper and draw around it.
2. Mark the normal and draw a few different angles of incidence.
3. Send the light rays along the angle and draw the emerging rays from the block.
4. Connect the ray and measure the angle of refraction.
5. Draw a graph of sini and sinr with sini on the y-axis and sing on the x-axis.
6. The gradient of the graph will be the refractive index.

Critical angle - the angle of incidence where the refractive angle is 90º. The equation to work it out is:

n₁ * sinc = n₂

Where n₁ is the refractive index of the denser material, c the critical angle, and n₂ is the refractive index of the rarer material.

Total Internal Reflection - this occurs when the angle of incidence is large than the critical angle when this happens the light reflects and does not refract. This principle is used in bike reflectors and fibre optic cables which are used in communication and endoscopy.

Sound:

Sound waves are longitudinal waves that can be reflected, refracted and diffracted.

Sound waves move through the vibrations of particles in the air. These vibrations are carried. However, although energy is transferred, matter is not.

The human range of hearing is 20Hz to 20,000Hz.

The Speed of sound is around 340m/s in air. This can be tested by performing the following experiment:

1. Measure the distance between 2 places.
2. Have a sound be made at one end.
3. As soon as the sound is made start a stopwatch.
4. When you hear it return stop the stopwatch.
5. The distance the sound travels divided by the time taken will give the speed.

Sound waves can be displayed by using a microphone to detect sounds and then feeding the information gained from it into an oscilloscope which displays the sound wave.

Pitch is controlled by the frequency of the sound wave.

Loudness is controlled by the amplitude of the sound wave.

Analogue and Digital:

Analogue - varies continuously in amplitude

Digital - only two different states exist - on or off

Advantages of digital:

• Less likely to be corrupted by noise as it is harder for the data to be changed by noise since it can only be either 1 or 0.
• Can be restored to its original form relatively simply.
• No loss of quality in amplification while in analogue signals it is very easy for noise to also be amplified therefore decreasing the quality.
• Can be stored in small memory and is processed very quickly by processors.
• More information can be carried by multiplexing.