Photoelectri e ec
1. What is the value of Planck‟s constant?
Planck's constant is a fundamental physical constant that gives the relationship
between the energy of a photon and its frequency. Its value in SI units is
defined as exactly **6.62607015 × 10 −34 joule second**¹².
2. What are the sources of error in this experiment?
Some possible sources of error in the photoelectric effect experiment are:
- Restraints on the voltage sensor, such as calibration errors, noise, or
resolution limits⁷.
- Restraints on the actual apparatus, such as alignment of the light source,
quality of the metal surface, or leakage currents⁷.
- Faulty averages of the stopping potential, such as outliers, rounding errors, or
human errors⁷.
3. What is the photoelectric effect?
The photoelectric effect is a phenomenon in which electrons are ejected from a
metal surface when light shines on it. The energy of the ejected electrons
depends on the frequency of the light, not on its intensity⁸.
4. Define “work-function”.
The work function is the minimum amount of energy required to induce
photoemission of electrons from a metal surface. The value of the work
function depends on the metal⁸.
5. What is the time lag between the arrival of light at a metal surface and the
emission of photoelectrons?
The time lag between the arrival of light at a metal surface and the emission of
photoelectron is very small, on the order of 10^-9 seconds or less⁸.
6. What do you mean by stopping potential/extinction voltage/cut off voltage?
The stopping potential, also known as extinction voltage or cut off voltage, is
the minimum negative potential applied to the metal surface that prevents any
photoelectrons from reaching the collector. It is proportional to the maximum
kinetic energy of the photoelectrons⁸.
7. What type of material should be chosen for photoelectron emission?
The type of material that should be chosen for photoelectron emission should
have a low work function, meaning that it requires less energy to eject electrons
from its surface. Some examples are alkali metals such as cesium or potassium⁸.
8. What is a photoelectric cell?
A photoelectric cell is a device that converts light energy into electrical energy
by using the photoelectric effect. It consists of a metal cathode that emits
electrons when exposed to light and an anode that collects them, creating a
current⁸.
9. What do you know about the structure of photovoltaic cells?
A photovoltaic cell is a device that converts light energy into electrical energy
by using the photovoltaic effect. It consists of a semiconductor material such
as silicon that forms a p-n junction, creating an electric field that separates
electron-hole pairs generated by light absorption⁸.
10. Can you name a recent method of a very accurate determination of Planck‟s
constant?
One recent method of a very accurate determination of Planck's constant is
based on measuring the frequency and wavelength of a laser beam using an
optical interferometer and an atomic clock. This method was used by NIST in
2017 to measure Planck's constant with an uncertainty of 9.1 parts per billion⁸.
11. Interpret thermionic emission in light of photoelectric effect.
Thermionic emission is a phenomenon in which electrons are emitted from a
heated metal surface due to thermal agitation overcoming the work function. It
can be interpreted as a special case of photoelectric effect where the incident
photons have very low frequencies and energies compared to visible light⁸.
12. In which phenomenon do you see the inverse photoelectric effect?
The inverse photoelectric effect is a phenomenon in which electrons are
absorbed by a metal surface when they have sufficient energy to overcome its
work function. It can be seen in phenomena such as electron capture by atoms
or molecules, secondary electron emission by metals, or Auger electron emission
by solids⁸.
Singl Sli Di ractio
1. Diffraction of light is defined as the bending of light around corners such
that it spreads out and illuminates areas where a shadow is expected¹. It
occurs when light passes through a small opening, comparable in size to the
wavelength of the light¹².
2. Diffraction differs from interference in that diffraction is the result of a
single wave interacting with an obstacle or an aperture, while interference is
the result of two or more waves overlapping and combining³.
3. Diffraction becomes appreciable when the size of the gap or slit is similar to
or smaller than the wavelength of the wave⁴. In this case, the wavefront on the
other side of the gap or slit resembles a semicircular wave⁴.
4. There are two classes of diffraction: Fresnel diffraction and Fraunhofer
diffraction². Fresnel diffraction occurs when the source of light and/or the
screen are at a finite distance from the diffracting object, while Fraunhofer
diffraction occurs when the source of light and/or the screen are at an infinite
distance from the diffracting object².
5. Fresnel and Fraunhofer diffraction differ in the geometry of the setup and
the complexity of the resulting pattern. Fresnel diffraction involves spherical
wavefronts and produces a complicated pattern that depends on the shape and
size of the obstacle or aperture². Fraunhofer diffraction involves plane
wavefronts and produces a simpler pattern that depends only on the size and
shape of the aperture².
6. In the present experiment what kind of diffraction occurs and how?
There are different ways to answer this question depending on what kind of
experiment is being performed. One possible answer is:
- In the present experiment, Fraunhofer diffraction occurs when light passes
through a single slit whose width is on the order of the wavelength of light¹.
The light from a coherent source (such as a laser) forms a distinctive pattern
on a screen that is far away from the slit. The pattern consists of a bright
central maximum and alternating dark and bright fringes on both sides. The
intensity of the fringes is a function of angle¹.
7. What will happen if the width of the slit is increased?
- If the width of the slit is increased, then the diffraction pattern will become
narrower and sharper, because less light will be diffracted at large angles¹. The
central maximum will also become brighter and wider, while the secondary
maxima will become dimmer and narrower¹.
8. What is the difference between a single slit and double slit fringe systems?
- The difference between a single slit and double slit fringe systems is that in a
single slit system, only diffraction occurs, while in a double slit system, both
diffraction and interference occur³. In a single slit system, there is only one
source of light that produces a broad pattern with a bright region at the centre
and alternating dark and bright regions on both sides³. In a double slit system,
there are two sources of light that produce an interference pattern with
equally spaced bright fringes separated by dark fringes³. The interference
pattern is superimposed on a diffraction envelope that modulates its intensity³.
9. What is the source you are using in your experiment? How does it work?
- There are different sources that can be used in diffraction experiments, but
one common source is a laser. A laser is a device that emits coherent light of a
single wavelength and colour by stimulating atoms or molecules to emit photons
in a process called stimulated emission². A laser consists of an active medium
(such as gas, liquid, or solid) that can be excited by an external energy source
(such as electricity or light), an optical cavity (such as mirrors or lenses) that
reflects and amplifies the emitted photons, and an output coupler (such as a
partially transparent mirror) that allows some photons to escape as a beam².
Newto ’ Rin
1. In the Newton's ring experiment, interference occurs when light from a
monochromatic source is reflected from the upper surface of a glass plate and
the lower surface of a plano-convex lens that is in contact with the glass plate¹.
The light rays reflected from the two surfaces have a path difference that
depends on the thickness of the air film between them¹. This path difference
causes constructive or destructive interference depending on whether it is an
odd or even multiple of half the wavelength of light¹.
2. The fringes are formed on the lower surface of the lens, where the
reflection takes place². They are observed by a travelling microscope that is
focused on that surface².
3. The fringes are circular because the thickness of the air film varies radially
from zero at the point of contact to a finite value at the edge of the lens¹. The
path difference between the reflected rays is proportional to the thickness of
the air film, which in turn is proportional to the square of the radius of the
circular ring¹. Therefore, the condition for interference depends only on the
radius of the ring and not on its angular position¹.
4. No, all rings are not equispaced. The rings are closer near the centre and
farther away from the center¹. This is because the thickness of the air film
increases more rapidly near the edge of the lens than near the center¹. The
spacing between two consecutive rings is given by
(Search )
5. An extended source is used in this experiment to ensure that there is enough
light intensity to observe the fringes clearly³. If a point source or a narrow slit
is used, then only a small portion of the lens will be illuminated and only a few
fringes will be visible³.
6. If a point source or an illuminated slit is used instead of an extended source,
then only a few fringes will be visible and they will be very faint³. This is
because a point source or a slit will produce parallel rays that will illuminate only
a small area of the lens and produce low intensity fringes³.
7. Yes, it is possible to observe Newton's rings if a wedge shaped film formed
by two glass plates is supplied instead of a lens⁴. In this case, the fringes will be
straight and parallel instead of circular⁴. This is because the thickness of the
air film between the glass plates varies linearly instead of quadratically with
distance from the point of contact⁴.
8. The central spot in this experiment is dark when viewed by reflection and
bright when viewed by transmission¹². This is because there is a phase change
of 180° when light reflects from an optically denser medium (glass) to an
optically rarer medium (air)¹². At the point of contact, where the thickness of
air film is zero, there is no path difference between
the reflected rays but there is a phase difference of 180° which leads to
destructive interference and hence a dark spot in reflection¹². In transmission,
there is no phase change and hence constructive interference and hence a
bright spot¹².
9. If you look at transmitted rays instead of reflected rays, you will see an
inverted pattern of fringes with respect to reflection¹². That is, you will see a
bright central spot and alternating dark and bright rings around it¹². This is
because in transmission, there is no phase change due to reflection and hence
constructive interference occurs where destructive interference occurs in
reflection and vice versa¹².
10. If a liquid of refractive index $$\mu$$ greater than that of
the lens and less than that of
the glass plate is introduced between
the lens and
the glass plate, then
the central spot will become bright when viewed by reflection and dark when
viewed by transmission⁵. This is because there will be no phase change when
light reflects from
the lens to
the liquid or from
the liquid to
the glass plate as they are both optically denser than air⁵. Therefore, at
the point of contact, where
the thickness of liquid film is zero, there will be no path difference or phase
difference between
the reflected rays and hence constructive interference and hence a bright spot
in reflection⁵. In transmission, there will be a phase difference of 180° between
the rays that pass through
the liquid film and those that do not as they encounter different refractive
indices along their paths⁵. Therefore, at
the point of contact, there will be destructive interference and hence a dark
spot in transmission⁵.
Pho ’ Pendulu
1. What is eddy current?
Eddy current is a loop of electric current induced within a conductor by a
changing magnetic field or by relative motion between a conductor and a magnet.
Eddy current flows in closed loops within the conductor, in planes perpendicular
to the magnetic field¹².
2. Other than the eddy current damping force, what are the other forces that
are affecting your experiment?
Some possible other forces that are affecting your experiment are:
- The driving force of the external source that causes the forced oscillation of
the system³.
- The restoring force of the spring that tends to bring the system back to its
equilibrium position³.
- The frictional force of the air or other medium that opposes the motion of
the system³.
3. What do you expect if 0 = or 2 < 2?
If 0 = or 2 < 2, then the system is overdamped, meaning that it will return to
its equilibrium position without oscillating³.
4. What are your conclusions about the phase relationship between the driver
and the oscillator below and above resonance?
Below resonance, the phase difference between the driver and the oscillator is
less than 90 degrees, meaning that the oscillator lags behind the driver. At
resonance, the phase difference is exactly 90 degrees, meaning that the
oscillator is in quadrature with the driver. Above resonance, the phase
difference is greater than 90 degrees, meaning that the oscillator leads ahead
of the driver³.
5. Give a simple example of forced oscillation.
A simple example of forced oscillation is a pendulum that is pushed periodically
by an external agent, such as a hand or a motor³.
6. What is the physical reason for the large amplitude oscillation at the
resonance frequency?
The physical reason for the large amplitude oscillation at the resonance
frequency is that the driving force is in phase with the velocity of the
oscillator, meaning that it transfers maximum energy to the oscillator at each
cycle³.
7. Why does the resonance curve broaden for higher damping?
The resonance curve broadens for higher damping because the damping reduces
the sharpness of the peak and lowers the maximum amplitude at resonance³.
8. Check Equation 10 and 11.
Equation 10 and 11 are derived from applying Newton's second law to a forced
harmonic oscillator with damping and driving force. Equation 10 gives the
differential equation for displacement x as a function of time t:
m d 2x dt 2 + hddt + k x = F 0 cos ( ω t ) m\frac {d^ {2}x} {dt^ {2}}+b\frac {dx}
{dt}+kx=F_0\cos(\omega t) mdt2d2x+bdtdx+kx=F0cos(ωt)
where m is the mass of the oscillator, b is the damping coefficient, k is the
spring constant, F0 is the amplitude of the driving force, and ω is the angular
frequency of the driving force.
Equation 11 gives one possible solution for x as a function of time t:
x ( t ) = A cos ( ω t − δ ) x(t)=A\cos(\omega t-\delta) x(t)=Acos(ωt−δ) where A
is the amplitude of oscillation and δ is the phase difference between x and
F0cos(ωt). A and δ depend on m, b, k, F0 and ω.
Couple Pendul
1) The equations of motion (1) and (2) for the system shown in Fig. 1 are derived
from the balance of forces and torques acting on each pendulum. They assume
that the pendulums are identical, the oscillations are small, the spring is
weightless and linear, and there is no friction or energy loss. If these
assumptions are valid for the apparatus you are using, then you can agree with
the equations of motion. If not, then you need to give reasons why they are not
valid and how they affect the results.
2) Some possible sources of errors creeping in this experiment are:
- Measurement errors: These include errors in measuring the length of the
pendulums, the mass of the bobs, the spring constant, and the time periods of
oscillations. These errors can be reduced by using more accurate instruments,
repeating the measurements several times and taking the average, and using
proper units and significant figures.
- Nonlinear effects: These include effects that arise when the oscillations are
not small enough, such as nonlinear restoring forces from gravity and spring,
higher-order terms in the equations of motion, and chaotic behavior. These
effects can be reduced by keeping the initial displacements small enough (less
than 10 degrees), using a spring with a high spring constant, and avoiding
resonance conditions.
- Damping effects: These include effects that arise when there is friction or
energy loss in the system, such as air resistance, internal friction in the spring
and pendulum rods, and external disturbances. These effects can cause the
amplitudes of oscillations to decay over time and change the frequencies of
normal modes. These effects can be reduced by using a low-friction spring and
pendulum joints, minimising air currents and vibrations in the surroundings, and
applying a periodic driving force to compensate for energy loss.
Polarimetr
1. **Polarized light** is a type of transverse wave in which the electric field of
light is limited to a single plane along the direction of propagation¹. **Ordinary
light** is a type of transverse wave in which the electric field of light can
vibrate in multiple planes².
2. **Optic axis** is a direction along which a ray of unpolarized light may pass
through a birefringent material without being split into two rays². **Uniaxial
crystals** are crystals that have one optic axis, such as calcite and quartz².
**Biaxial crystals** are crystals that have two optic axes, such as mica and
sapphire².
3. **Polarized light** is a type of transverse wave in which the electric field of
light is limited to a single plane along the direction of propagation¹. The plane
containing the electric field vector and the direction of propagation is called
the **plane of polarization**².
4. An **optically active substance** is a substance that can rotate the plane of
polarization of a beam of polarized light that passes through it². For example,
sugar solution and quartz are optically active substances².
5. The **optical activity** of a substance depends on its molecular structure,
concentration, path length, wavelength of light, and temperature².
6. The **optical activity** depends on (i) wavelength, because different
wavelengths of light experience different amounts of rotation by an optically
active substance². This phenomenon is called **rotatory dispersion**². (ii)
temperature, because temperature affects the molecular structure and
concentration of an optically active substance². This phenomenon is called
**temperature coefficient of rotation**².
7. The type of crystals that exhibit optical activity are **birefringent
crystals**, such as quartz and calcite². These crystals have different
refractive indices for different polarizations of light, and thus can split a beam
of unpolarized light into two beams with different planes of polarization².
8. The **optical activity** depends on the state of the substance, because
different states of matter have different molecular arrangements and
interactions that affect the rotation of the plane of polarization². For example,
liquid solutions are more optically active than solid crystals or gases².
9. **Specific rotation** is a measure of the optical activity of a substance,
defined as the angle of rotation per unit path length per unit concentration at a
given wavelength and temperature². It does depend on the wavelength of light
used and temperature of the solution, because these factors affect the optical
activity as explained above².
10. (i) **Molecular rotation** is the angle by which a molecule rotates the plane
of polarization of a beam of polarized light that passes through it². (ii)
**Rotatory dispersion** is the variation of optical activity with wavelength,
such that different wavelengths of light experience different amounts of
rotation by an optically active substance².
11. The specific rotations of pure liquids and pure solids are defined as the angle
of rotation per unit path length at a given wavelength and temperature, without
considering concentration².
12. **Natural rotation** is the rotation of the plane of polarization caused by
an optically active substance due to its molecular structure². **Magnetic
rotation** is the rotation of the plane of polarization caused by an external
magnetic field applied to an isotropic medium such as glass or water².
13. You cannot perform this experiment using white light instead of sodium
light, because white light contains multiple wavelengths that will experience
different amounts of rotation by an optically active substance due to rotatory
dispersion². This will result in a mixture of colors instead of a clear change in
brightness when viewed through an analyzer².
Transverse and Longitudinal wave
1. No, we do not get all the harmonics in all cases of stationary waves produced
on a string¹. We only get those harmonics that satisfy the condition that the
length of the string is a multiple of half the wavelength of the wave¹. These are
called resonant frequencies or natural frequencies of the string¹.
2. The phase velocity is the speed at which a point of constant phase (such as a
crest or a trough) travels along the wave². The group velocity is the speed at
which a group of waves (such as a pulse or a wave packet) travels along the
medium². The relation between the two velocities depends on the dispersion of
the medium, which is how the phase velocity varies with frequency². In a
non-dispersive medium, such as vacuum, air, or water, the phase velocity and the
group velocity are equal and independent of frequency². In a dispersive medium,
such as glass, metal, or plasma, the phase velocity and the group velocity are
different and depend on frequency². In general, either of them can be greater
than the other depending on the frequency range and the medium properties².
3. One possible way to improve upon the experimental setup to find the velocity
of rope wave without using a wave generator is to use a tuning fork to produce
vibrations at one end of the rope and measure the wavelength and frequency of
the resulting stationary wave³. The velocity can then be calculated by
multiplying the wavelength and frequency using
v=fλ
.
4. Some possible sources of error in the experiment are:
- The rope may not be perfectly uniform or elastic, which may affect its tension
and density³.
- The amplitude of vibration may not be small enough to neglect nonlinear
effects such as harmonic generation or wave breaking³.
- The measurement of wavelength and frequency may not be accurate enough
due to human error or instrument error³.
- The ambient temperature and humidity may affect the properties of the rope
and hence its velocity³.
5. If the string is plucked, it will vibrate with multiple frequencies that form a
harmonic series⁴. That is, it will vibrate with its fundamental frequency (the
lowest possible frequency) and its integer multiples (the higher harmonics)⁴.
The fundamental frequency depends on the length, tension, and mass per unit
length of the string⁴. The higher harmonics depend on how and where the string
is plucked⁴.