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Optics MCQ – image formation by concave mirror

John Abraham asks:

I am having trouble with a problem.

Q. In an experiment to determine the focal length (f) of a concave mirror by the u–v method, a student places the object pin A on the principal axis at a distance x from the pole P. The student looks at the pin and its inverted image from a
distance keeping his/her eye in line with PA. When the student shifts his/her eye towards left, the image appears to the right of the object pin. Then

(A) x < f

(B) f < x < 2f

(C) x = 2f

(D) x > 2f


The answer is B

(Try giving your explanation as comment. If not satisfied, ask again)


The image and the object will move together at 2f

The one closer to the eye will move faster

In the question, the situation occurs when the image is closer to the eye and this happens when the object is between f and 2f

Hence the answer is B

Formation of Beats

Abhay asks:

“What is meant by beats?discuss graphical method for formation of standing waves on streched strings?”


Beats are periodic variation in intensity of sound caused by the superposition of two waves slightly differing in their frequencies. Beats are heard when the difference in frequency is less than 10. (because human ear can distinguish two sounds distinctly only if they differ in time by 0.1 sec.)
Here are some links to understand beats better through interactive animations

Electric Field due to Polarisation

Sreeram asks:

“do charges produced due to polarization in a dielectric produce electric field outside?”


When a dielectric is kept in an electric field, it is polarized. The polarized ends are just like the two plates of a parallel plate capacitor. The electric field is confined between the two surfaces (except for some edge effect). So, in a macroscopic point of view, the answer is no.

(However, I agree that there is scope for disagreement and debate when we think and look at the microscopic level. Users of the site are requested to post comments)

A Question from Optics (awaiting answer)

Question: The difference between the speed of light compared to any other body on earth is negligible then if we take a car moving at 30m/s and i stand at a distance of 1m from it i am a stationary observer,if i assume the speed of light to be 3 x 10^8 then even i will observe the world at such a speed then when i look at the car say for one second then light has already reached my eye after bouncing off the car, by that time the car would have moved a very very small distance and this repeats until the car moves past me.what i cant understand is then why does the car whizz past me because at that rate it should take a lot of time to go past me and i should be able to see the cars every nanometer of motion just like in slow motion videos.i have read in a psychology book that it is because of the angle made by the retina of the eye and the objectively regardless of my human perception what could be the reason?

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