logearav

Thanks for the replies. But i have reproduced the question asked in competitive exam held for recruiting teachers , verbatim. No additional details are given either. To ease out the confusion, i posted in this forum so that learned members can clarify

Two photons are travelling, one in the east and another in the west. Then the relative velocity between two is (a) 2c (b) c/2 © c^2 (d) c I know the velocity of light is constant, i presume the answer is option (d). But the term relative velocity confuses me. I look forward for revered members' help in this regard

What do you mean by Gain medium,sir?

Thanks again. Now the stimulating photon travel along the axis of mirror, it gets reflected from one side of the mirror and gets incident on other atoms and the lasing action starts. In what way, the reflection from the mirror helps in amplifying the laser signal?

Revered Members I have attached images of applications of Gauss' law namely 1) Electric field due to an infinitely long charged wire and 2) Electric field due to an infinite charged plane sheet. In both cases Gaussian surface is cylinder. In the first case, the electric flux is found for curved surface and it has been done for only side that is right side of cylinder in this case. In the second case, the electric flux is found for plane caps but it has been done for both the left plane cap and right plane cap. My doubt is , why we are doing for only one side for the curved surface and for both sides in the case of plane caps?

Thanks for the reply Swansont. Wikipedia says "At the nuclear level, binding energy is also equal to the energy liberated when a nucleus is created from other nucleons or nuclei. ". Why the energy released is more for binding nucleons for smaller nuclei when compared to heavier nuclei"

They are due to balancing of the Centripetal force and Electrostatic force of attraction

Revered Members, Binding energy for lighter nuclei is large and for heavier nuclei BE is less. Why? I know BE is the energy needed to bind protons and neutrons in a nucleus. Also it can be defined as the energy that must be supplied to disrupt the nucleus into constituent protons and neutrons. Take a lighter nuclei which has 3 protons and so ideally the energy needed to bind these 3 protons should be less when compared with the energy needed to bind 82 protons(say) in a heavier nuclei . So, why BE is less for heavier nuclei than lighter nuclei?

When we consider a beaker of water and two molecules, one completely inside the water and the other at the free surface, the water molecule at the free surface will experience net downward force of attraction and this causes the water molecule to behave like a stretched membrane, which is called as surface tension. Now to bring this molecule at the free surface to interior of beaker, one has to do work against the force of attraction and this is stored as potential energy. Water molecule acquires minimum surface area to have minimum potential energy. What is the relation between surface area and potential energy? More work done so more potential energy and more surface area. But how surface tension correspond to minimum potential energy. I can't understand.

Thanks a lot swansont. Now i understood by your analogy with water in a cup. Thanks a ton. Now i could grasp. People like you is an asset for this forum.

The Fermi Level is defined as the highest occupied molecular orbital in the valence band at 0 K, so that there are many states available to accept electrons, if the case were a metal. Thanks swansont. Fermi Level is the highest occupied molecular orbital. I can't understand " so that there are many states available to accept electrons". Let me have a try. If fermi level is occupied , then how it is possible to say many states are available to accept electrons, because fermi level is the highest level and also occupied so no state can exist above fermi level. Sorry for stretching you, i am a novice. Thats why i ask for explanation.

Sir, I can't understand. Could you elaborate, please?

Why none has answered my query?

Revered Members, Can i know what is meant by fermi level? I googled this, but i could not understand. One site says Fermi level as top of collection of electron energy levels at absolute zero. As far as my knowledge I know electron energy levels as K, L,M, N etc with energy values -13.6eV, -3.4eV etc. Top of collection means, is it O eV?

thanks swansont. My doubt is an electron which comes to metastable state has already has one photon absorbed in it which was supplied when the electron was in ground state. My question is, do we supply one more photon to this electron which has one absorbed photon already? I mean where the stimulating photon comes from? Do we supply externally?

Revered Members Considering isolated Lithium atom which has 3 electrons, K shell will have 2 electrons with energy -13.6eV. L shell will have 1 electron with energy -3.4eV. Now , considering a small piece of Lithium metal containing 100 atoms, then there would be 100 K levels and 100 L levels differing in energy by extremely small amounts. We call it as energy band. Why there is small change in energy when there is energy band, while there is no energy change for isolated atoms?

Thanks swansont. But Mercury vapour which means mercury gas is also an example of many atoms system, isn't it?

Revered Members, Please see me my attachment. They are given in my textbook to depict paramagnetism and diamagnetism. When paramagnetic substances are placed in non uniform magnetic field, they have a tendency to move from weaker to stronger part Similarly, when diamagnetic substances are placed in non uniform magnetic field, they have a tendency to move from stronger to weaker part. Magnetic field is stronger at poles and become weaker when we move farther away from poles. But the diagram of diamagnetism shows that the liquid is bending towards the poles, that is stronger part of magnetic field which goes contrary against the property of diamagnetism. I feel the diagram is wrong. Am I correct?

Revered members, What is the difference between line spectrum and band spectrum? I have seen the line spectrum exhibited by Mercury vapour lamp and Hydrogen vapour lamp. My teacher says that band spectrum is similar to line spectrum and said that band spectrum is nothing but a line spectrum with large width. Is that true?

61H1 --> 2He4 + 21H1 41H1 + 21H1 --> 2He4 + 21H1 41H1 --> 2He4 Revered Members, First equation is given in my text book to explain fusion reaction. Now i attempted the second and third reaction. This is the fusion reaction happening in stellar energy. My doubt is , Can i cancel the 21H1 found in LHS and RHS of second equation to arrive at the third equation?

Revered Members, When light travels from rarer to denser medium and vice versa, velocity and wavelength changes but frequency remains constant. What is the reason? This question was asked in the physics quiz competition held in my school. I replied that since energy of the light remains the same and since energy is proportional to frequency, frequency remains constant. But the convener said the answer is wrong. Am i wrong?

Revered Members, An electron in ground state makes its way to an excited state upon absorption of photon of energy, equivalent to energy difference between ground state and excited state, and after some time, it decays by emitting the photon and returns to the ground state. This is called spontaneous decay. For Laser action, population inversion and stimulated emission should occur. Now let me explain the scenario 1) An atom(electron) in the energy level E1 absorbs a photon and goes to a state of higher energy say E3 2) While decaying from E3 to E1, it reaches a metastable state E2. Now, due to longevity of the stay in E2 than in E3, we can achieve population inversion in E2 My question is , 1) Do we supply a photon to the electron which stays in the meta stable state E2, so as to trigger stimulated emission? I mean, when the electron comes to E2, do we supply photon again externally or the photons come due to the spontaneous emission by other electrons which arrive to E2? 2) If yes, what is the energy of the supplied photon. Is it E3 - E1 or E2 - E1?

I dont understand mr.swansont

Thanks a lot swansont. Agreed, i am using cross product. My question is, why can't we measure the angle from Q to R in the arm QR and from S to P in the arm SP, instead of going the way as mentioned in my attachment?

Revered Members, In my attachment, while measuring the angle between current element I(QR) and magnetic field B, they are going in clock wise direction that is from R to Q(shown in Red line), so it is (90 - Θ ) and while measuring the angle between current element I(SP) and B, they are going in anti clockwise direction that is from P to S(shown in Red line), so it is (90 + Θ). May I get clarification from revered members as to why they have gone in different directions to measure the angle, instead of adopting the same direction, either clockwise or anti clockwise, for measuring the angle in the arms QR and SP? My friend told, the reason is,Because they measure the angle according to the direction of current. My query is The current starts from the point P and terminates at Q(for the arm PQ) and similarly it starts from point S and terminates at P(for the arm SP), so what harm in measuring the angle from the starting point rather the terminating point of current in both the arms(PQ and SP)?