physics problems 2

1. This question has some ambiguity. The space between spectral lines cannot be called "dark fringes". A "dark fringe" implies a "black spectral line".

2. The angular position of a bright fringe is given by,
sin(a) = m入/d
where a is the angular position, 入 is the wavelength, d is the slit separation, m is the order of fringes .

Hence, angular separation between two bright fringes (a1 - a2) with orders m1 and m2 respectively is
(a1 - a2) = arc-sin(m1.入/d) - arc-sin(m2.入/d)
Because all parameters, 入, d are the same, the fringes separation will remains unchaged for two given m1 and m2.

3. It is in fact the force per unit length on the wire.

4. The answer is option A.
Relsolve the velocity v into x and y components. The x component is v.cos(theta), and y compount is v.sin(theta).
The y-component is perpendicular to magnetic field, hence by Fleming's Left Hand Rule, drives the charge to move in a counterclockwise direction (as seen at the direction of the observer).

Since the charge is in a circular motion, the centre of the circular motion does not drift in the y direction. It only drift along the x-direction.

5. The answer is option A.
The electron performs a circular motion. The electron changes its direction by 360 degrees.

6. It is simply because the translucent screen absorbs part of the light when light passing through it, thus lowering the intensity.


2014-09-24 16:28:06 補充：
Your further questions:
2. Your formula is only applicable for small angular position. Because we are considering larger angles, that formula doesn't apply. Instead, the basic equation sin(a) = m入/d should be used.

2014-09-24 16:38:54 補充：
Both A and D give the wrong physical phenomena. Hence, both are right answers.
(sorry, I made some misinterpretation in my original answer).

2014-09-24 16:39:41 補充：
The above statement refers to Q4.