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## High school physics

### Course: High school physics>Unit 9

Lesson 2: Wave characteristics

# Properties of periodic waves

Learn about different properties of waves, including amplitude, period, frequency, and wavelength. See how these properties are related to one another, and how to use formulas to solve for one wave property when the other properties for that wave are already known,. Created by Sal Khan.

## Want to join the conversation?

• Is crest same thing as peak?
• Yup.
• Is the enrgy of a wave determined by amplitude or frequency? It seems that in physics, teachers say that amplitude represents the amount of energy wheras in chemistry (quantum theory), it always seems to do with frequency. Which is it? Is it different for a mechanical and electromagnetic wave respectively?
• The energy transported by a mechanical wave also depends on the frequency.
This can be shown for a rope of length L containing a stationary wave (wavelength = L).Knowing that a stationary wave can be seen as the sum of two non-stationary waves interfering, we can calculate the energy of one and double the result at the end (energy is a scalar quantity, so it can be simply added like this). For one wave, assume that the height of each point is given by:
y(x,t) = Acos(2pi*x/L + wt) , where A is the amplitude and w is the angular frequency.
The speed of a point x at a time t is
v(x,t) = -Aw.sin(2pi*x/L + wt)
Therefore the energy of a point (with mass dm) is given by
(dm)v²/2 = A²w².sin²(2pi*x/L + wt).(dm)/2 , and integrating for the whole mass of the rope (x = 0 until x = L) ,you have half of the energy of the oscillatory rope (the result stands for any time t)
E/2 = (1/4).A².w².m , where m is total mass of the rope.
This last result depends on both the amplitude and the angular frequency w, which is equal to 2pi.f, so E = 2A².f².m.pi²

For electromagnetic waves, the energy of each photon depends on the frequency, but the energy transported by a beam is a function of both the frequency (energy of each photon) as well as the amplitude (proportional to the number of photons being sent), so the total transported energy of an electromagnetic wave is also a function of both amplitude and frequency.
• What's the difference between a period and a wavelength? Both seem to be measured from (say) a trough to a trough. How can they ever be different? Are they measured in different units?
• You actually cannot find wavelength and period using the same graph. Wavelength is from the peak to peak of the graph of a wave when position is graphed against position (x-y graph). With period, it is the peak to peak of the graph of a wave when position is graphed against time (x-t graph).
• According to modern quantum theory of light emerged in which light is neither a wave nor a particle.So what is this?
• In quantum mechanics, sometimes light can act as both a particle and a wave, especially when it's being observed. Light has both wavelike properties-reflect, refract, and particle properties such as the photoelectric effect. Here's a video for the Double Slit Experiment done by Dr. Quantum...hopefully this helps!
• What type of waves is used in SONAR
• Ultrasonic (f > 20000Hz)sound waves are used in SONAR.
• just a question about wavelength......after getting refracted the wavelength of the light changes, since there is a change in velocity and frequency cannot change as the source is same, so the wavelength changes. so even the color of light should change? does that happen? for example- violet light is made incident on a glass slab. the wavelength changes sine the speed changes, does the color change??
• Color is our subjective perception of the frequency of light. You can't see the light inside the glass slab. You can only see light that exits the glass slab and reaches your eye. What we can say about the light inside the glass slab is that its velocity is reduced, its frequency is unchanged, and its wavelength is shorter.
• Why does the time period of oscillation of a spring not depend on gravity?
• nice question.

The strength of gravity only determines the SIZE of the force acting on the mass/spring. You could say that the spring opposes this force and, therefore it cancels out.

• Is periodic wave the same as transverse wave?