P6: Waves Flashcards
What are waves?
Disturbances which travel through a medium, causing particles to oscillate and transfer energy to each other. Waves transfer energy but not matter.
What is the amplitude of a wave?
The maximum displacement of a point on a wave from its undisturbed position.
What is the wavelength of a wave?
The distance between the same point on two adjacent waves (e.g. trough -> trough).
What is the frequency of a wave?
The number of complete waves passing a point per second.
What is frequency measured in?
Hertz (Hz). 1 Hz = 1 wave/second.
What is wave speed?
The speed at which energy is transferred as a result of a wave (the same as the speed the wave is moving at).
What equation can be used to find wave speed?
What is the period of a wave?
The amount of time it takes for one full wave to pass a point.
The formula for period is given in the exam. What is the period of a wave with speed 8 m/s and wavelength 2m?
- Use v = fλ to work out the wave’s frequency: 8 = f x 2 so f = 4 Hz
- Period = 1/frequency = 1/4 = 0.25 s
What are transverse waves?
Waves whose oscillations are perpendicular to the direction of propogation of the wave (or the direction of energy transfer).
What are longitudinal waves?
Waves whose oscillations are parallel to the direction of propogation of the wave (or the direction of energy transfer).
Name 3 transverse waves.
EM waves, water ripples and S-waves.
Are electromagnetic waves transverse or longitudinal?
Transverse.
Name 2 longitudinal waves.
Sound waves and P-waves.
Are sound waves transverse or longitudinal?
Longitudinal.
What is the speed of sound in air?
≈330 m/s.
Describe how you could use an oscilloscope to measure the speed of sound in air.
- Attach a signal generator to a speaker.
- Connect 2 microphones to an oscilloscope.
- Place both microphones next to the speaker, slowly moving one away until the waves on the oscilloscope’s display are alligned, but have moved one wavelength apart.
- Measure the distance between the microphones to find one wavelength.
- Use v = fλ to find the wave speed - frequency is what the signal generator was set to.
- Check your result is about 330 m/s.
What three things can happen when a wave arrives at a boundary between two mediums?
- The wave is absorbed, transferring energy to the medium’s energy stores.
- The wave is transmitted (carries on travelling through the new medium. Most of the time, it is refracted.
- The wave is reflected.
What happens depends on the wavelength of the wave and the properties of the mediums.
What rule applies to all types of reflection?
Angle of incidence = angle of reflection.
θi = θr
When drawing reflection/refraction, what is the normal?
An imaginary line perpendicular to the surface at the point of incidence (the point where the wave hits the boundary).
What are the two types of reflection?
Specular and diffuse.
What is specular reflection?
Where a wave is reflected by a smooth surface and in a single direction.
What is diffuse reflection?
Where a wave is reflected by a rough surface, and the reflected rays are scattered in different directions.
This happens because the normal is different for each incident ray. But θi = θr still applies.
Why do some surfaces appear matte and some shiny?
- Shiny: taking a mirror for example, the surface is smooth, so specular reflection occurs, resulting in a clear reflection.
- Matte: this happens when the surface is rough, so diffuse reflection occurs, resulting in a reflection which is not clear.
Draw a diagram of reflection.
θi = θr
What is optical density?
A measure of how quickly light travels through a medium. The higher the optical density, the slower light travels.
What is refraction?
When light waves are bent when they enter a new medium (which is of a different optical density to the previous medium).
If a wave, when it is refracted, slows down, it bends __ the normal. This happens when the second medium is __ than the first.
- Towards.
- Optically denser.
If a wave, when it is refracted, speeds up, it bends __ the normal. This happens when the second medium is __ than the first.
Away from.
What happens to the wavelength and frequency of a wave when it is refracted?
The wavelength changes, but the frequency remains the same.
In what circumstances is a wave transmitted without also being refracted?
If a wave travels along the normal of a surface, it will hit the boundary of a new medium face on, so it carries on in the same direction (but at an altered speed, if the new medium is of a different density than the last).
Draw a ray diagram to show refraction where the second medium is optically denser than the first.
How would you measure the speed of waves in a ripple tank? Why would this setup be suitable for investigating waves?
- Attach a signal generator to a dipper rod on the tank.
- Dim the lights and turn on the lamp so that the wave crests create shadows on the screen below the tank.
- The distance between each shadow line is one wavelength, so measure the distance between 10 and divide by 10 to find the average wavelength.
- Use s = fλ to find the speed.
This is suitable for investigating waves because it allows you to measure the wavelength without disturbing the waves.
The following method can be used to create water waves of a specific frequency in a ripple tank:
Attach a signal generator to a dipper rod on a ripple tank. Turn on the lamp so that the wave crests create shadows on the screen below the tank. Measure the distance between the shadows of the wave crests to find the average wavelength. Use s = fλ to find the wave speed.
What would be the greatest source of error for this investigation? Suggest 2 methods to ensure the validity of the results.
- Random human error in measurement.
- You could take more measurements and find a mean, or you could dim the lights so that the shadows are easier to make out.
Practical:
Plan an investigation to find the speed of waves produced across a taut length of string.
- Create a “tightrope” by attaching a piece of string on one end to a vibration transducer, and on the other to a pulley and weight (see diagram).
- Attach a signal generator to the transducer, then turn on the generator. The string will start vibrating.
- Adjust the frequency of the signal generator until one whole wavelength fits exactly on the string.
- Measure the length of the string to find the wavelength.
- Find the speed of the wave using v = f λ. The frequency is whatever the signal generator is set to.
- Repeat this process, setting the generator so that different numbers of full wavelengths fit along the string. Calculate the speed - should be the same.
Practical:
Plan an experiment to investigate the refraction of light by different substances.
- Do the experiment in a dim room.
- Place a transparent block onto a piece of paper and trace around it.
- Use a ray box or laser to direct a ray of light at the middle of one of the block’s faces.
- Use a ruler to trace the incident and emerging rays.
- Join them up to show the path of the refracted ray through the block.
- Draw the normal to the point where the indicent ray entered the block.
- Use a protractor to measure the angle of incidence and angle of refraction.
- Repeat this process with blocks of different materials, keeping the angle of incidence constant throughout.
- You should find that the angle of refraction is smaller for more optically dense materials.