4.6 - Standing waves - questions

1 - Describe what a standing wave look like !

2 - Describe how a standing wave can be formed !

3 - How is a standing wave formed in a stringed instrument ?

4 - Draw a picture of standing waves in a string and give formulas !

5 - How is a standing wave formed in an open pipe ?

6 - Draw a picture of standing waves in an open pipe and give formulas !

7 - How is a standing wave formed in a pipe closed in one end?

8 - Draw a picture of standing waves in a closed pipe and give formulas !

4.7 - Doppler effect - questions

9 - What is the Doppler effect ?

10 - What are the formulas for doppler effect and sound ?

11 - A police car is moving towards a wall that reflects its sound.
What frequency will the sound have that the police hear ?

12 - What is the formula for doppler effect with EM radiation such as light ?

13 - What is the formula for doppler effect if you work with wavelength ?

14 - What does Δf and Δλ in these formula tell you ?

4.8 - Diffraction at a single slit - questions

15 - Draw a picture of a diffraction measurement and the formulas.

4.9 - Resolution - questions

16 - What is the Rayleigh criteria ?

17 - Draw a picture of a resolution measurement and the formulas.

18 - Why can a DVD store more information than a CD ?

4.10 - Polarization - questions

19 - What is polarization ?

20 - What is a polarizor ?

21 - What is Malus's law ?

22 - What is Brewster's angle ? Draw a picture and give the formula.

4.11 - Uses of polarization - questions

23 - What is optically active materials ?

24 - Give an example of an optically active material.

25 - How does a Liquid crystal Display (LCD) work ?

4.6 Standing wave


A standing wave do not move sideways. Instead each point move up and down like an harmonic oscillator between a minimum and a maximum value. Some points do not move at all (the red points in the figure above). These points are called nodes.

A standing wave can be formed by two identical waves that moves with the same velocity in opposite directions.

When the string is pulled a wave travels along the string to one of the fixed ends. at the end it is reflected back. The reflected wave is superposed with the original wave to form a standing wave.

A soundwave in an open pipe is also reflected and superpositioned to form a standing wave but with antinodes (maximums) at each open end.

A soundwave in a pipe closed in one end is also reflected and superpositioned to form a standing wave but with a node (minimum) at the closed end and an antinode (maximum) at the open end. Remember that a soundwave is a longitudinal wave with the displacement along the pipe and not up and down as for a string.







4.7 Doppler effect

Doppler effect is when a light or sound source is moving with respect to an observer and the observed frequency is then changing. If the source and observer gets closer the frequency will increase. If they are moving apart it will decrease.

Look at the data booklet:  





Doppler effect for EM radiation

Look at the data booklet:  





Unlike Doppler effect for sound it does not matter if it is the observer or source that moves only if they get closer or further apart.

Distance increases:
The frequency gets smaller i.e. the observed frequency is:    fobserved = f0 - Δf
The wavelength gets longer i.e. the observed wavelength is:    λobserved = λ0 + Δλ
(This is called redshift because light from stars that moves away from us looks red as their wavelength gets longer)

Distance decreases:
The frequency gets larger i.e. the observed frequency is:    fobserved = f0 + Δf
The wavelength gets shorter i.e. the observed wavelength is:    λobserved = λ0 - Δλ

4.8 - Diffraction at a single slit



4.9 - Resolution





Both a CD and a DVD disc has small bumps that reflects laser light. The reflected laser light is measured by a detector. A DVD has many more bump but in order for the detector to be able to detect the light from the smaller bumps it must have a laser with a shorter wavelength.

4.10 - Polarization

An electromagnetic field such as light is said to be polarized if the electric field is in the same plane as the wave propagates. For unpolarized light the electric field vector can be in any direction.

A polarizer is a material that takes unpolarized light and only let through light that is polarized in one direction.

Look at the data booklet:  

If polarized light with intensity I0 hits a polarizor with an angle between the axis of the polarized light and the polarizor that is θ, then I is the intensity of the light that gets out from the polarizor.



4.11 - Uses of polarization

The rotation of the plane of polarization is called optical activity and materials showing this phenomena are said to be optically active.

A sugar solution is optically active. The higher the sugar concentration, the higher the rotation of the polarization angle (can be used for concentration analysis).

Some plastics become optically active when subjected to force. And the rotation of the polarization angle depends on the amount of force (stress analysis).

In a LCD, each dot (pixel) is a liquid crystal. These crystals rotate the polarization with 90 degrees. But when a battery is connected they don't. If one put a polaroid (analyser) over the crystal it looks like it goes black when the battery is applied.