Problem Solving Strategies

@Nobodyrecognizeable hi

Nobody recognizeable

@JohnRennie shouldn't c be correct?

I'm just answering a question in another room. Hopefully it won't be too long.

Nobody recognizeable

@JohnRennie OK.

@JohnRennie please ping me when you get back.

6:56 AM

@Nobodyrecognizeable constant temperature and volume suggests Helmholtz free energy since the Gibbs free energy is normally used at constant pressure not constant volume.

I think that if a system is held at constant temperature and volume then the Helmholtz free energy can only decrease not increase, so I guess the answer is (a).

7:30 AM

@JohnRennie: Hi sir :-)

@GuruVishnu hi :-)

@JohnRennie May I ask a small doubt?

@JohnRennie hi I am struggling with small optical instrument!

@GuruVishnu I'll be busy for a few minutes, but post the question now and I'll look at it as son as I'm free

@JohnRennie: Ok sir. Is there anything like "black" light? If yes, is it possible to specify its wavelength? Where is its position on the electromagnetic spectrum? For other colours, I'm able to find either that colour or consider it as a resultant of multiple colours in the spectrum.

7:33 AM

@YuvrajSingh... I'm working for a few minutes, but I won't be long.

OK sir.

@GuruVishnu no, black is the absence of light

@YuvrajSingh... I'm free now. What's the problem?

@JohnRennie my dad mom locked inside house.

I was studying. And when I reached the door they both are closed.

@JohnRennie Ok sir. Thank you. I made a person delete his answer by asking a question in the comment :-) Could you please answer that one sir?

@YuvrajSingh... You mean you are locked inside the house?

7:40 AM

@YuvrajSingh... Are you asking a doubt?

I have no phone with Sim, the interest I am using is through broad band.

@JohnRennie

@YuvrajSingh... climb out a window?

@Guru vishnu I am but right now I have bigger problem than that.

No window all are pack.

@JohnRennie :-)

@YuvrajSingh...: What's the problem in remaining inside a locked house? BTW in which floor are you in?

I don't think there's a lot you can do. Mail someone who knows your parent's phone number?

7:42 AM

@JohnRennie no I am at second floor.

@YuvrajSingh... Do you have parachutes?

:-)

@JohnRennie I mailed to three but no one reply.

@GuruVishnu now that isn't helpful

@Guru vishnu seriously?

@YuvrajSingh... No.

@JohnRennie Yes sir.

7:43 AM

I guess you just have to wait for your parents to return.

Should I scream for help!

Is there a problem? Can't you just stay in the room?

No water, no washroom, just a bed and study table.

How long are your parents going to be?

Do not know, father went to job and mom went to relatives.

7:45 AM

The trouble is that calling for help is going to cause embarrassment for your parents, and they won't be amused.

@YuvrajSingh...: As you said you don't have a sim in your phone. You may try some free online calling apps or messaging services to contact your parents.

@JohnRennie although I do not have social media account, but if I create them now it ask mobile otp.

That's a good idea. There are online services for you to send SMS messages. You could use them to message your father.

@YuvrajSingh... No need!

It's totally fine if you have a mic.

And internet of course.

@JohnRennie Better than parachutes right, sir?

@GuruVishnu I'm not sure the attempts at humour are appreciated.

7:49 AM

I wish I am the original owner of room! @GuruVishnu

Then I have right to kick you out.

@JohnRennie Understood sir. I thought it would help reduce stress.

@JohnRennie our maid coke around 2 o clock.

She has duplicate keys.

@YuvrajSingh... Ok. But my idea will work for sure.

That's only 40 minutes. Can you wait until then?

Let see she comes or not.

@JohnRennie can you ping yuvraj.

Through stack exchange

He has papa and mummy number.

Because I can, t invite my self.

7:51 AM

@YuvrajSingh... hi, Alesha needs to speak to you.

@JohnRennie is he is online? Or was?

Today!

@YuvrajSingh... Try internet calling. It'll work for 1 min.

He was here earlier today.

But if it's only 40 minutes until the maid comes I would just wait.

@JohnRennie hi, I got late, what, s going on.

@YuvrajSingh... Alesha needs you to phone your mother and/or father. They have accidentally locked her in a room in the house.

7:56 AM

Let me call.

I don't think it was her fault

Yes sir.

20 mins ago, by Yuvraj Singh...

I was studying. And when I reached the door they both are closed.

@JohnRennie she is same, she always try to hide her mistakes.

She's your younger sister. Smile and pretend to be happy even if you're annoyed :-)

8:00 AM

@YuvrajSingh...: See. Try internet calling when ordinary call is not feasible. It'll work for 1 min. only. Try call2friends by searching on google. You could get some quick help. Be specific and quick.

Ok?

@GuruVishnu that's Yuvraj posting now, and he's going to phone to get someone to help.

@Guru vishnu sorry for her behavior,.

@JohnRennie I got the message aunt opens the door.

OK :-)

@JohnRennie Then who was it before? Are there two users with same username sir?

She owes you a favour now! :-)

@GuruVishnu Yuvraj's sister uses his account.

:53307725 I think she was a bit frightened by being locked in.

8:03 AM

@JohnRennie Ok sir.

From my age she's still a little girl. Take it easy.

Sorry sir I was rushed in anger.

When reading sincerely it'll hard to be concentrated on surroundings. So its natural.

It's OK :-)

@JohnRennie: Sir, may I ask a doubt sir?

8:05 AM

@GuruVishnu give me 10 minutes to do a bit of work an make a coffee.

@JohnRennie Ok. I'll ping you after that sir. I'm going to have my lunch now.

@GuruVishnu OK, speak to you after lunch.

@JohnRennie hi Alesha here, problem solved sir.

@YuvrajSingh... Yuvraj to the rescue then :-)

@JohnRennie he has done a favor on me I can see what he said in the room.

Anyway are you available to answer my question!

Actually there are ten question, and I could, n, t able to solve a single.

A far sighted person cannot focus distinctly object closer than 120cm (what it mean for me he can, t object closer than 120cm so his near point shift to 120cm am I right?) then lens that will permit Jim to read from a distance of 40cm will have a focal length equal to?

8:16 AM

Give me a moment to make a coffee ...

OK, coffee is made

A human's near point should be 25cm. If the near point is farther than 25cm it means the lens in the eye is too weak and the person needs glasses with a convex lens to increase the power.

If the near point is shorter than 25cm it means the lens is too strong and the person needs to wear glasses with a concave lens to reduce the power.

OK so far?

OK

In this case the person's near point is 120cm, i.e. > 25cm, so his lens is too weak and they need a converging lens in their glasses.

We call this being long sighted.

Yes

Lens create image at near pint 25cm right sir @JohnRennie?

Yes

So we get answer +30cm.

But answer is 60cm.

8:28 AM

Let me try the calculation.

@YuvrajSingh... 60cm is the lens needed to make Jim's eyesight normal i.e. to make his near point 25cm. Yes?

Oh yes, I have made sign can I post second question.

@JohnRennie

Cool :-) OK, what's the next question?

The lenses of focal length +10cm and - 15cm when put in contact behave like a convex lens, they will have zero longitudinal chromatic aberration if their disperse power are in the ratio

Ah, I don't remember how to calculate the total dispersion. I'll have to Google this.

@YuvrajSingh... Are you used to this schezophrenic feelings?

8:41 AM

@JohnRennie ok sir I will wait.

What's your real name @YuvrajSingh... ?

@JohnRennie I know little bit about chromatic aberration splitting of colors of light according to their wavelength so different wavelength colour form different images.

@YuvrajSingh... I can't find a simple equation, but I think dispersion is basically dn/dλ.

That, s for wave optics.

The power of the combined lenses is D = D1 + D2, and for no chromatic aberration you want dD/dλ = 0, which means dD1/dλ = -dD2/dλ.

8:48 AM

My book says $f_r-f_v=wf_v$.

Where $w$ is angular dispersion.

That's the Abbe formula for the dispersion

@JohnRennie

@JohnRennie: Hi sir.

Yes.

The left hand side is basically df/dλ because it's the change in f when the wavelength changes from red to blue.

8:51 AM

OK!

I'm sure there's a simple equation for doing this. I can't remember it so I'm trying to derive it, so far with not much success.

Ah, see:

Achromatic lens

An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane. The most common type of achromat is the achromatic doublet, which is composed of two individual lenses made from glasses with different amounts of dispersion. Typically, one element is a negative (concave) element made out of flint glass such as F2, which has relatively high dispersion, and the other is a positive (convex) element made of crown glass such as BK7, which...

@YuvrajSingh... can I use MathJax or are you on the phone?

Yes sir I am on my new PC!

OK, the Wikipedia article gives the equation:

$$ \frac{D_1}{v_1} + \frac{D_2}{v_2} = 0 $$

@JohnRennie Are my messages visible?

as the condition for no chromatic aberration.

What it's basically saying is that the change in D1 with wavelength is cancelled out by an equal and opposite change in D2 with wavelength.

@YuvrajSingh... OK so far?

8:58 AM

Yes.

@JohnRennie

And a quick rearrangement gives:

$$ \frac{v_1}{v_2} = - \frac{D_1}{D_2} $$

Yes

So there's the ratio of the dispersions.

@AjayMishra hi, yes, I can see that post.

@YuvrajSingh... I guess that's something you just need to remember.

What, s D1 and D2 want to confirm!

@JohnRennie

D is the power of the lens i.e. D = 1/f

(including the sign)

9:01 AM

OK, then Thanks sir.

@JohnRennie I think @AjayMishra Mishra want to ask something your turn!

@JohnRennie though I have some questions but they are based on same. So I can go through them easily.

@YuvrajSingh... OK. I'm here for a couple of hours more if you want to ask anything.

@YuvrajSingh... What's your real name?

@JohnRennie Hi.

@GuruVishnu hi :-)

@AjayMishra if the person using Yuvraj's account didn't want to answer your first question I think you should let the matter drop.

@AjayMishra although I am not person, but calling my brother mental hurt me! @JohnRennie

9:08 AM

What's your brother real name?

@AjayMishra ask him! You will surely get a good reply!

@JohnRennie: Sir. Is it possible for you to read deleted answers. I posted a doubt in the comment and it made that user to delete the answer :-)

@JohnRennie Haha, I'm just curious, what can be a possible reason for not replying for the question of asking name?

@GuruVishnu what's the question?

Q: what wavelength is white light made of?

2

Some sources say that white light = R+O+Y+G+B+I+V while others = R + B + G my question is do they divide the wavelength range (400-700nm) when they talk about RBG eg 400-500 = B 500-600 = G 600- 700 = R and same for ROYGBIV but this time among 7 ...

9:11 AM

@AjayMishra sorry bahiya if you find it unrespectful, but it, s matter between you and right now he is not online, he is the one who can say his name! I do not know what kind of bonding you have.

@GuruVishnu yes I can see the deleted answer. What did you want to know about it?

The second answer (deleted one) had something like "superposition" of all light waves leads to white colour. I asked is it right to say "interference" of all waves in visible range leads to white colour.

@JohnRennie Did you see my comment, sir?

@YuvrajSingh... Wierd, you don't know his name. That's too much weird. Do you wanna play a game?

But the comment that user sent as a reply didn't answer my doubt sir. That's why I came here. Shortly before I was ready to send a reply, the answer got deleted.

@GuruVishnu yes, His answer looked fine to me so I don't know why he deleted it. White light is the combination of light with all the different wavelengths, but we don't get interference because light of different wavelengths can't be coherent.

9:15 AM

@JohnRennie But coherence is not a mandatory condition for interference. I think it's needed only to observe a stable pattern like fringes.

Why is it wrong to consider white light as a result of interference of light waves in the entire spectrum?

Interference specifically means coherent light rays of equal wavelength interacting with each other.

@JohnRennie Ok sir. So superposition doesn't imply interference? I thought random addition of waves is also a type of interference.

In my book there was no clear demarcation of superposition and interference and I think that's the cause of my confusion.

Superposition just means adding the light. We tend to use the term interference when we get an interferenc pattern, and that requires coherent light.

@JohnRennie Sir hello

But I guess the distinction between the two terms is vague. You could argue that superposition of incoherent light rays does give an interference pattern, but that pattern is time dependent and changes too fast for us to see.

@Physicsfreak hi :-)

9:20 AM

@JohnRennie Ok sir. Then, I don't still understand how we (as well as cameras) perceive white light? Is there any physics based reason behind this or is it entirely a biological one. If it's the latter I'm not interested. If it's the former, kindly explain if possible sir.

@JohnRennie Today we had our physics practical. In India some unknown external examiner comes and asks us questions based on practicals

I have a doubt regarding that

Q: How does light combine to make new colours?

@GuruVishnu see:

60

In computer science, we reference colours using the RGB system and TVs have pixels which consist of groups of red, green and blue lines which turn on and off to create colours. But how does this work? Why would certain amounts of red, blue and green light make something seem yellow? Is this a bi...

optics visible-light vision biology perception

@Physicsfreak what's the question?

@JohnRennie Ok sir. Thank you.

@JohnRennie Quite simple. She asked me to define ohms law. My answer was that under constant conditions like temperature, units, and dimensions of conductor the current passing through it is directly proportional to potential difference across the conductor

She said that I was wrong

I should have said that the potential difference is directly proportional to current

Your description sounds a bit complicated but it seems fine to me. I would just have said V = IR

You said:

> the current passing through it is directly proportional to potential difference across the conductor

which is the same thing, but I would have answered as simply as I could.

I think the examiner was unfair not to accept your answer.

9:26 AM

@JohnRennie Yes sir

Current proportional to potential difference Or potential difference proportional to current is the same right?

Yes

But for example I don't remember Ohm's law saying R has to be a constant, so it can change with temperature. So your condition that constant temperature is required was wrong.

@Physicsfreak: I remember NCERT defining ohm's law stating the constant temperature condition.

If I were your externals then I'd have said your explanation was totally correct.

@JohnRennie But if resistance keeps changing then for different values of V diiferent current would be produced. How would we verify that the system follows ohms law

To me Ohm's law just means V = IR

But I agree with @GuruVishnu that the examiner should have accepted your answer.

@GuruVishnu Yes thats why I didnt want to take any chance

@JohnRennie Thanks for guidance. I could have argued with examiner but didnt want to annoy her

9:34 AM

@Physicsfreak Yes. That's a good move.

That was a good decision. It's rarely worth arguing with the examiner.

@GuruVishnu @GuruVishnu :)

@Physicsfreak :-)

Did you finish asking your doubts to sir?

@GuruVishnu do you want to ask another question? If so go ahead and post it.

@GuruVishnu Yes

@JohnRennie Thanks sir

9:40 AM

@JohnRennie Yes sir. Ok sir. Just a doubt from the linked answer.

@Physicsfreak Thanks.

@GuruVishnu yes?

Could you please decode this statement sir:
A reasonable model for colour is that the eye takes the overlap of the wavelength spectrum of the incoming light against the response function of the thee types of photoreceptors, which look basically like this:

That's an unnecessarily complicated way of answering. The eye has three types of colour receptors, and the sensitivity of the three types is shown in the diagram above.

i.e. one type responds mainly to blue light, one mainly to red light and one mainly to green light.

Your brain processes the signals sent from the three receptor types to figure out what colour the light is.

Ok sir. Got it. Thank you :-)
I'm still reading that answer, and will ask if I've any doubts.

@YuvrajSingh... I wouldn't have done that. Ajay backed away when he was asked to.

9:48 AM

@JohnRennie hi.

@YuvrajSingh... hi

@JohnRennie no sir he was not, you can read the chat!

10:58 AM

deleted

@JohnRennie hello sir!

@JohanLiebert hi :-)

A: Overcoming the reaction force

@JohnRennie sir does this(👇) answer of mine look conceptually correct now.

3

Edit: I am modifying this post because I feel that I used words quite lousiy and so I would try to clarify that in this edit. Is the reaction force becoming overcome? No, not in the sense what we mean by reaction. Action reaction pairs are always equal and opposite (and act on different ...

@JohanLiebert yes that looks fine :-)

@JohnRennie actually Sir I previously put the words in wrong sense and I feel guilty that I built a wrong concept in the mind of visitors. What should I do to atone for what I did?

11:10 AM

I wouldn't worry about it.

@JohnRennie OK then thanks! :-)

If you realise you made a mistake then editing your answer is exactly the right thing to do. I've written wrong answers before.

(not deliberately :-)

3:33 PM

deleted

@JohnRennie: Hi sir :-)

Are you free now? I'm glad you're here now.

@GuruVishnu hi. I'll be eating lunch in ten minutes so ask quickly! :-)

Q: How does a pure and an impure spectrum look like in optics?

@JohnRennie Ok sir. I just now had my dinner. Sometimes I get amused when I realise different people have different times relative to the GMT.

1

The following text is from Concepts of Physics by Dr. H.C.Verma, from the chapter "Dispersion and Spectra", page 436, topic "Spectrum", sub topic "Pure and Impure Spectrum": [...] In an ideal situation, light of one wavelength should occupy one particular spatial position in the spectrum. In ...

optics spectroscopy dispersion

The above one sir. I wish to see a pure spectrum.

The comments I received were useful but I didn't get how it would look like.

So what are you asking? What the difference between a pure and impure spectrum is?

@JohnRennie Nope. How would a pure spectrum would look like.

3:45 PM

In most cases the pure and impure spectra will look so similar that you can't tell the difference between them.

@JohnRennie Fine sir. That's what I thought.

Any additional points to be remembered?

@GuruVishnu Suppose you have a single very narrow slit, then it produces the usual spectrum on the screen. Yes?

@JohnRennie Is the "usual" spectrum - impure or pure, sir?

I think it's impure.

@GuruVishnu a single very narrow slit produces an almost perfectly pure spectrum. What I'm working up to is trying to explain what is meant by an impure spectrum.

@JohnRennie Ok sir.

Is it time for you for lunch?

3:51 PM

Now suppose you have a second slit displaced from the first slit. It also produces a spectrum on the screen, and the two spectra overlap.

@JohnRennie Interfere?

No, just superimpose.

The two spectra add together, but because the slits are in slightly different positions the positions of the spectra on the screen are displaced relative to each other. Kind of like this:

Because slit 1 and slit 2 are in slightly different places their spectra appear in slightly different places on the screen. So when they add together they tend to blur the colours.

@JohnRennie So the impure spectrum we see is a result of superposition of multiple pure spectra, sir?

Yes, exactly.

@JohnRennie: Thank you very much sir :-)

3:58 PM

In this case two separate slits sum to give an impure spectrum.

@JohnRennie Understood sir. Thank you. I think it's time for your lunch. Sorry for disturbing in the last minute sir.

But if you have a single wide slit then the different parts within that slit behave as lots of narrow slits in different places, so a single wide slit gives an impure spectrum in the same way.

@JohnRennie Thank you sir. Now I got my doubt cleared :-)

I imagined this during your first explanation itself. It was interesting sir.

And now lunch! :-)