# Notes for Electromagnetic Spectrum

Here are the notes I had been preparing for you for the chapter on electromagnetic waves: Notes – Electromagnetic Spectrum

These notes are additional material for reading after you have gone through the chapter from the textbook. These notes should answer most of the questions some of you had raised after reading the textbook.

Advertisements

## 20 thoughts on “Notes for Electromagnetic Spectrum”

1. unnati says:

Sir could you also give us notes for electromagnetism please or atleast a good website because I am finding it difficult to understand that chapter.

• I will try preparing notes for that chapter. But it will be good to formulate your questions and ask.

2. Radhica says:

Few questions here- firstly I don’t yet understand how electric and magnetic field do not affect the EM waves?
Next light has a change magnetic flux so how come light is not electrical?
You said that the varying EM field is created by accelerating a charged particle so even though it will minuscule shouldn’t light have mass?
I have read only 1/4 part of it yet sir will other questions when I will finish the whole thing

• 1) I am giving you some analogies, see if they make some sense: Whether the ripples are produced on the surface of a swimming pool or on that of a large lake, the ripples do not get affected by the mass of water present, right?
Similarly, for em waves (where e- and m- fields are changing), a constant e- or m- field does not cause any influence. Remember it is the change in e-field which is producing the m-field and vice versa. A constant change in the fields. Change is the key word here. If you still don’t understand, then I’ll have to explain to you in person in school. I will be able to understand you more easily then.

2) Light is produced by a charged particle; it does not carry it. The same way as water ripples are produced by a falling stone which has some mass but the ripples do not carry that stone (or its mass). For example, in a candle, the electrons jump up and down several energy levels, and create the light waves while emitting energy. Electrons have mass. But light does not.

3. unnati says:

Sir but light has found to be affected by gravity right?

• that’s right. light was found to be affected by gravity in 1919, more than 90 years back.
however, the one has to understand theory of relativity (yes, from Mr. Einstein) to explain it.

4. Anshuman says:

Are you sure all the things in the notes are important?

5. unnati says:

That implies that it has some weight which means it has some mass.

6. Radhica says:

Unnati remember when we had gone to IUCAA they had explained vaguely why this happens. See if I remember correctly they said that it is light that bend due to gravity the gravitational field is so strong that space itself curve so therefore the light appears to be bending however in respect to itself it is not bending the space itself is bent. Also it is a mathematical impossibility for light to have weight as if u remember mass decreases with increase in speed and if it reaches speed of light then it won’t have any mass. Since light moves at speed of light (which I am assuming it does since it is light) it is impossible for it to have any matter.

7. Eniyan says:

sir, in our text book(pg. 135) It says that wavelength is the characteristic of color, so does it mean when light travels from one medium to another, the color changes? , since the wavelength is changing.

8. pareen says:

radhica correcting you, mass increases with increase in velocity, it doesnt decrease!!

9. unnati says:

Thank you pareen. Exactly why I am confused. You see if mass increases with velocity after it crosses speed of light then shouldn’t mass of electron till now, taken for granted , taken into consideration.Then since gravity affects mass (I believe), it should affect light.I may be completely wrong. This is what I thought of it.(I should also add out here I imagine electrons in light .I think this is where I am going wrong)

10. Radhica says:

Yup unnati that is where you went wrong. Even I asked the same question prabhat sir has answered it. He said that light is produced by a charged particle not carried by one so light doesn’t have mass and I have already explained it to you why it bends. Also correcting you gravity doesn’t affect mass gravity is a force which is produced by two bodies having mass. And pareen I do not think that you are correct I think mass decreases with increase in speed and prabhat sir can you please tell us which one is correct

• Pareen is correct. Let me explain this a bit:
In special theory of relativity, one comes across two types of mass – rest (or invariant) mass and relativistic mass. Rest mass is what is understood as ‘the mass’ in Newtonian mechanics or in our intuitive sense of the physical world. Relativistic mass depends on the velocity of the object and that increases with velocity according to the following formula (which is difficult to write in this replay-to-comments-window which is devoid-of-any-special-formatting-tools):

Relativistic mass = Rest mass / Square root of [1 – v^2/c^2]

So as ‘v’ increases, v^2/c^2 increases,
therefore 1- v^2/c^2 decreases,
therefore Square root of [1 – v^2/c^2] decreases,
therefore Rest mass / Square root of [1 – v^2/c^2] increases.

Some of you might also want to read this article titled ‘What is the mass of a photon?’: http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

11. unnati says:

I have very basic and dumb questions here.Where does the energy in the light come from?When speaking of light why do we speak of electrons too?

12. Radhica says:

Sir I still have doubts whether that is true or not but will ask when we come to school

13. satyajay says:

hello pk , sorry could not write report for sps i will get the notes i had made and will hand them to you if you want to read them.also i was not able to visit the blog as i was very busy studying from the textbook.anyway i will take them now and try to complete as much possible.