# Electromagnetic waves and photons

1. Apr 3, 2012

### scientifico

Hello, why the electromagnetic waves move up and down instead of just travel straight?
When an electron go in a lower energy level it relases a photon, but where does this photon came from?

Thank you!

2. Apr 3, 2012

### sophiecentaur

Nothing goes "up and down" in an EM wave - that is to say there is no actual movement. There are varying electric and magnetic fields which are at right angles (i.e. transverse) to the direction of motion of the wave (the direction that the energy flows). There is 'NO WIGGLE'!!!

Where does a Photon come from? This is a Brand New Idea (well- only 100 year old haha) and is where Quantum Mechanics comes in. When light energy is emitted from an atom, it comes in specific bursts of energy, called quanta. The quanta of energy can be considered (very loosley and nothing at all like little bullets - never forget that) as being particles. These 'particles' are referred to as photons. When an electron in an atom changes its energy level, the amount of energy it loses is a specific value in the form of one of these particles. You ask where it comes from - well, it doesn't involve the atom actually losing a part of itself. As when you rub something to make it hot, it may not involve losing any actual material but heat still comes off it.

An EM wave consists of large numbers of these particles and, if it ever interacts with an object (another atom, a radio receiver, a rotating molecule etc.) it will always have the same amount of energy, which is directly related to the frequency of the em wave. All photons of a wave of a given frequency are identical and they all have the same energy, zero mass, they take up no room at all (no size) and travel at the speed of light. They will only interact with a system that can accept quanta with specific amount of energy.

3. Apr 3, 2012

### Drakkith

Staff Emeritus
The picture of squiggly waves you see on most pictures of light is a representation of the alternating electric and magnetic fields, not the actual motion of the light.

4. Apr 4, 2012

### scientifico

Yes I am referring to that rappresentation... so in the reality radio waves, for example, propagate themself in a straight line?

5. Apr 4, 2012

### Khashishi

No. In reality, the radio waves propagate as an expanding wave front. But, straight line is a good approximation when your obstructions are much larger than the wavelength of the waves.

6. Apr 4, 2012

### scientifico

What mean expanding wave front?
Do all electromagnetic waves propagate in the same way?

Thank you!

7. Apr 4, 2012

8. Apr 4, 2012

### sophiecentaur

Propagation is said to be in a straight line but this should not be confused with the wave spreading out. This happens, eventually, however you try to focus a beam aamof. A (surface) water wave spreads out in two dimensions and the power of the wave is spread over the circumference of a steadily increasing circle. A sound wave or EM wave will spread in three dimensions and the power is spread over the surface of a sphere - hence the 'inverse square law' will apply to the power falling on a given area as the distance increases.

It may be easier to justify the straight line propagation idea if you think in terms of shadows and obstructions. If you look at a star and put a line of small discs between the star and your eye, then if they all obstruct the star and the nearest disc obscures them all then they will all (not surprisingly) be in a straight line. This is despite the fact that the light waves are spreading out in all directions from the star and other observers can see the star even when you can't.

The representation doesn't show the path that the wave takes. It is a 'graph' of the strengths of the E and H fields, showing their magnitude and direction as they vary with the distance. It's important not to confuse the representation of a field with an actual displacement in space. If someone showed you a graph of temperatures as they varied over several days, you wouldn't confuse the wavy line with any actual movement, would you? It's the same with the way we 'draw' an EM wave.

9. Apr 4, 2012

### Drakkith

Staff Emeritus
Imagine an antenna that sends out radio waves. You can visualize this as dropping a rock into a pond. The rock will send ripples out in the water. Similarly the antenna will send out EM waves. This is what is known as the expanding wave front. The longer the wavelength the better the waves can go "around" objects.(Look up diffraction) This allows radio waves to be detected by your radio even though you don't have line of sight to the actual antenna. A smaller wavelength, such as visible light, cannot be detected around large objects, which is why you do have to be in line of sight with the light source. (Visible light can still diffract around tiny objects, but the size has to be approximately the same size as the wavelength or smaller, which is in the hundreds of nanometers for visible light. So very very tiny.)

10. Apr 10, 2012

### janu203

Is electromagnetic wave only varying magnetic field travelling through space or is it varying magnetic and electric fields both at right angles? because i have heard that varying electric field only happens inside of an antenna due to motion of electrons and after that changing magnetic field travel into space(no electric field). Help urgently needed.

11. Apr 10, 2012

### sophiecentaur

Forget 'electrons' when talking about EM fields and Electricity. They only cloud the issue. If you were talking about the vibrations on a spring, you wouldn't go 'molecular'. You'd be talking Youngs Modulus and density. Same with EM. Stick to macroscopic matters here.

How can you get a varying field 'inside' a piece of (ideal) wire? The fields must all be outside.

EM waves in space are just what the same suggests - electric and magnetic fields that vary in time and position. E and H vectors are at right angles to the direction of motion of the wave when it eventually gets far enough from the radiating structure. Their ratio is fixed in free space.
Inside a radiating structure the ratio can be anything (mostly electric or mostly magnetic, depending on the size and shape of the antenna.

12. Apr 10, 2012

### Bobbywhy

scientifico, the below Wiki page has two animated diagrams which show exactly how the EM wave looks as it propagates. It makes the process very clear, and I recommend you visit the page.

“Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D diagram shows a plane linearly polarized wave propagating from left to right”