4 Marks Questions

Question 14 Marks

In $1909,$ Robert Millikan was the first to find the charge of an electron in his now $-$ famous oil-drop experiment. In that experiment, tiny oil drops were sprayed into a uniform electric field between a horizontal pair of oppositely charged plates. The drops were observed with a magnifying eyepiece, and the electric field was adjusted so that the upward force on some negatively charged oil drops was just sufficient to balance the downward force of gravity. That is, when suspended, upward force $qE$ just equaled $Mg$. Millikan accurately measured the charges on many oil drops and found the values to be whole number multiples of $1.6 \times 10^{-19} C$ the charge of the electron. For this, he won the Nobel prize.

$(i) $If a drop of mass $1.08 \times 10^{-14} kg$ remains stationary in an electric field of $1.68 \times 10^5 NC ^{-1}$, then the charge of this drop is

$(a)\ 6.40 \times 10^{-19} C$
$(b)\ 4.8 \times 10^{-19} C$
$(c)\ 3.2 \times 10^{-19} C$
$(d)\ 1.6 \times 10^{-19} C$

$(ii)$ Extra electrons on this particular oil drop $($given the presently known charge of the electron$)$ are

$(a)\ 4 $
$(b)\ 5$
$(c)\ 8$
$(d) \ 3$

$(iii)$ A negatively charged oil drop is prevented from falling under gravity by applying a vertical electric field $100 V m ^{-1}$. If the mass of the drop is $1.6 \times 10^{-3} g$, the number of electrons carried by the drop is $( g =10$ $\left.ms ^{-2}\right)$

$(a)\ 10^9$
$(b)\ 10^{18}$
$(c)\ 10^{12}$
$(d)\ 10^{15}$

$(iv)$ The important conclusion given by Millikan's experiment about the charge is

$(a)$ charge has no definite value
$(b)$ charge is quantized
$(c)$ charge is never quantized
$(d)$ charge on oil drop always increases

OR
If in Millikan's oil drop experiment, charges on drops are found to be $8 \mu C , 12 \mu C , 20 \mu C$, then quanta charge is
$(a)\ 20 \mu C$
$(b)\ 12 \mu C$
$(c)\ 8 \mu C$
$(d)\ 4 \mu C$

Answer

In $1909,$ Robert Millikan was the first to find the charge of an electron in his now $-$ famous oil-drop experiment. In that experiment, tiny oil drops were sprayed into a uniform electric field between a horizontal pair of oppositely charged plates.
The drops were observed with a magnifying eyepiece, and the electric field was adjusted so that the upward force on some negatively charged oil drops was just sufficient to balance the downward force of gravity.
That is, when suspended, upward force $qE$ just equaled $Mg$.
Millikan accurately measured the charges on many oil drops and found the values to be whole number multiples of $1.6 \times 10^{-19} C$ the charge of the electron.
For this, he won the Nobel prize.

$(i) (a) \ 6.40 \times 10^{-19} C$
Explanation: As, $qE = mg \Rightarrow q =\frac{1.08 \times 10^{-14} \times 2.8}{1.68 \times 10^5}=6.4 \times 10^{-19} C$
$(ii) (a) 4$
Explanation: $q = ne$ or $\Rightarrow n =\frac{6.4 \times 10^{-19}}{1.5 \times 10^{-19}}=4$
$(iii) (c) \ 10^{12}$
Explanation: For the drop to be stationary,
Force on the drop due to electric field $=$ Weight of the drop
$ qE = mg$
$q =\frac{ mg }{E}=\frac{1.5 \times 10^{-6} \times 10}{100}=1.6 \times 10^{-7} C $
Number of electrons carried by the drop is
$n =\frac{q}{e}=\frac{1.6 \times 10^{-7} C }{1.6 \times 10^{-19} C }=10^{12}$
$(iv) (b)$ charge is quantized
Explanation: charge is quantized
OR
$(d) \ 4 \mu C$
Explanation: Millikan's experiment confirmed that the charges are quantized,
i.e., charges are small integer multiples of the base value which is charge on electron.
The charges on the drops are found to be multiple of $4$.
Hence, the quanta of charge is $4 \mu C$.

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Question 24 Marks

LASER: Electromagnetic radiation is a natural phenomenon found in almost all areas of daily life, from radio waves to sunlight to x-rays. Laser radiation - like all light - is also a form of electromagnetic radiation. Electromagnetic radiation that has a wavelength between 380 nm and 780 nm is visible to the human eye and is commonly referred to as light. At wavelengths longer than 780 nm, optical radiation is termed infrared (IR) and is invisible to the eye. At wavelengths shorter than 380 nm, optical radiation is termed ultraviolet (UV) and is also invisible to the eye. The term laser light refers to a much broader range of the electromagnetic spectrum that just the visible spectrum, anything between 150 nm up to 11000 nm (i.e., from the UV up to the far IR). The term laser is an acronym which stands for light amplification by stimulated emission of radiation. Einstein explained the stimulated emission. In an atom, electron may move to higher energy level by absorbing a photon. When the electron comes back to the lower energy level it releases the same photon. This is called spontaneous emission. This may also so happen that the excited electron absorbs another photon, releases two photons and returns to the lower energy state. This is known as stimulated emission.
Laser emission is therefore a light emission whose energy is used, in lithotripsy, for targeting and ablating the tone inside human body organ.
Apart from medical usage, laser is used for optical disk drive, printer, barcode reader etc.
(i) What is the full form of LASER?
(a) light amplification by simultaneous emission of radiation
(b) light amplified by synchronous emission of radiation
(c) light amplified by stimulated emission of radiation
(d) light amplification by stimulated emission of radiation

(ii) The stimulated emission is the process of
(a) absorption of two photon when electron moves from lower to higher energy level
(b) release of two photons by absorbing one photon when electron comes back from higher to lower energy level
(c) release of a photon when electron comes back from higher to lower energy level
(d) absorption of a photon when electron moves from lower to higher energy level

(iii) What is the range of amplitude of LASER?
(a) 150 nm - 400 nm
(b) 700 nm - 11000 nm
(c) Both 150 nm - 400 nm and 700 nm 11000 nm
(d) 800 nm - 12000 nm
OR
LASER is used in
(a) Ionization
(b) Transmitting Satellite signal
(c) Optical disk drive
(d) Radio communication

(iv) Lithotripsy is
(a) Laboratory application
(b) An industrial application
(c) A medical application
(d) Process control application

Answer

LASER: Electromagnetic radiation is a natural phenomenon found in almost all areas of daily life, from radio waves to sunlight to x-rays. Laser radiation - like all light - is also a form of electromagnetic radiation. Electromagnetic radiation that has a wavelength between 380 nm and 780 nm is visible to the human eye and is commonly referred to as light. At wavelengths longer than 780 nm, optical radiation is termed infrared (IR) and is invisible to the eye. At wavelengths shorter than 380 nm, optical radiation is termed ultraviolet (UV) and is also invisible to the eye. The term laser light refers to a much broader range of the electromagnetic spectrum that just the visible spectrum, anything between 150 nm up to 11000 nm (i.e., from the UV up to the far IR). The term laser is an acronym which stands for light amplification by stimulated emission of radiation. Einstein explained the stimulated emission. In an atom, electron may move to higher energy level by absorbing a photon. When the electron comes back to the lower energy level it releases the same photon. This is called spontaneous emission. This may also so happen that the excited electron absorbs another photon, releases two photons and returns to the lower energy state. This is known as stimulated emission.
Laser emission is therefore a light emission whose energy is used, in lithotripsy, for targeting and ablating the stone inside human body organ.
Laser emission is therefore a light emission whose energy is used, in lithotripsy, for targeting and ablating the stone inside human body organ.
Apart from medical usage, laser is used for optical disk drive, printer, barcode reader etc.
(i) (d) light amplification by stimulated emission of radiation
Explanation: The term laser is an acronym which stands for "light amplification by stimulated emission of radiation"

(ii) (b) release of two photons by absorbing one photon when electron comes back from higher to lower energy level
Explanation: Einstein explained the stimulated emission. In an atom, electron may move to higher energy level by absorbing a photon. When the electron comes back to the lower energy level it releases the same photon. This is called spontaneous emission. This may also so happen that the excited electron absorbs another photon, releases two photons and returns to the lower energy state. This is known as stimulated emission.

(iii) (c) Both 150 nm - 400 nm and 700 nm - 11000 nm
Explanation: The term "laser light" refers to a much broader range of the electromagnetic spectrum that just the visible spectrum, anything between 150 nm up to 11000 nm (i.e., from the UV up to the far IR).
OR
(c) Optical disk drive
Explanation: An optical disc drive (ODD) is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or form optical discs.

(iv) (c) A medical application
Explanation: Laser emission is therefore a light emission whose energy is used, in lithotripsy, for targeting and ablating the stone inside human body organ.

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