# Can you measure the amplitude of a specific light wave?

1. Dec 11, 2015

### NoemMijJakob

Hello world,

I'm currently in my last year of high school and only just getting interested into quantum physics, still learning new things every day. I was wondering if theres a way to measure the specific amplitude of a light wave(Or electromagnetic wave) and what this says about the wave (does it maybe have something to do with its energy level/frequency??)

The 2nd thing I was wondering that if you can say certain electro magnetic waves have say an amplitude of 2 mm and they meet a hole of half that size, 1mm all around, do only some of the light waves pass through or none at all?

I'm sorry if this isn't the place to ask these questions, I'm kind of new on these forums,

I wish you all a great day,

Jakob

2. Dec 11, 2015

### PietKuip

The amplitude of an electromagnetic wave can be given as the amplitude of the electric field. It is expressed in units of volt per meter.

For frequencies that are not too high (radio waves), this can be measured with an antenna.

3. Dec 11, 2015

### NoemMijJakob

Thanks for the reply, can the amplitude of a single wave be measured however?

4. Dec 11, 2015

### .Scott

Welcome to this forum, Jakob

Light waves consist of photons. The energy of a photon is proportional to its frequency - roughly speaking its color. With blue more energetic that red.
If your wave does not have too many photons in it, it might be possible to could those photons as they strike a detector. In that case, the total energy of the wave would have been the energy per photon times the number of photons.

5. Dec 11, 2015

### PietKuip

The amplitude of radio waves can be measured. For broadcasting stations, those are typically millivolts per meter. https://en.wikipedia.org/wiki/Signal_strength

For light, one measures the intensity, in watts per square meter. It is proportional to the square of the amplitude.

6. Dec 11, 2015

### Staff: Mentor

Generally one should be very cautious about introducing photons into the discussion when the classical wave model of light is sufficient to answer the question. It is almost never correct to think of light as consisting of photons unless explicitly quantum mechanical effects are involved.

Here the question is about the amplitude of the wave, and it is measured using either of the two methods suggested by PietKuip. Photons don't come into the picture at all.

7. Dec 11, 2015

### .Scott

The first statement in the OP was an expressed an interest in "quantum physics". The title also asked about a "specific light wave" although the OP said he was also looking for the "specific amplitude". Perhaps I over-interpreted.

8. Dec 11, 2015

### Staff: Mentor

Just to make clear: the amplitude of an electromagnetic wave does not refer to a spatial size transverse to the beam direction. It refers to the strength of the field, measured in volts/meter for the electric field, or in tesla for the magnetic field. The electric field at a given fixed point in the wave starts at zero, increases to some maximum strength in one direction, decreases back to zero, increases to the maximum strength in the opposite direction, decreases back to zero, and repeats the cycle. Likewise for the magnetic field.

The sinusoidal graphs that you typically see for an electromagnetic wave are a graphical representation of how the strength of the field varies. Nothing literally moves in a sinusoidal fashion through space.

Last edited: Dec 21, 2015
9. Dec 12, 2015

### NoemMijJakob

Thanks! This really clarifies things for me.