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QUESTION: 1

Which one of the following statements is nottrue for cathode rays? [2002]

Solution:

Cathode rays are negatively charged rays.

Hence, when electric field is applied to them,

then they deflect in direction opposite to

electric field.

QUESTION: 2

When ultraviolet radiation is incident on asurface, no photoelectrons are emitted. If asecond beam causes photoelectrons to beejected, it may consists of [2002]

Solution:

Energy of photon of X-rays is more than

energy of photon of ultraviolet rays. Because

frequency of X rays is more than ultraviolet

rays.

QUESTION: 3

A photoelectric cell is illuminated by a pointsource of light 1m away. When the source isshifted to 2m then [2003]

Solution:

Power ∝ No. of electrons emitted (N)

QUESTION: 4

According to Einstein’s photoelectric equation, the graph between the kinetic energy of photoelectrons ejected and the frequency of

incident radiation is [2004]

Solution:

K.E. = hν – φ

∴ graph of K.E. versus ν is a straight line

with +ve slope h and K.E. > 0 for ν such that

hν > φ.

QUESTION: 5

A photosensitive metallic surface has workfunction, hv_{0}. If photons of energy 2 hv_{0} fallon this surface, the electrons come out with amaximum velocity of 4 ×10^{6} m/s. When thephoton energy is increased to 5 hv_{0}, thenmaximum velocity of photoelectrons will be

[2005]

Solution:

We know that

According to question

v_{2} = 2v_{1} = 2 × 4× 10^{6} = 8 × 10^{6} m/s.

QUESTION: 6

The work functions for metals A, B and C arerespectively 1.92 eV, 2.0 eV and 5 eV.According to Einstein s equation, the metalswhich will emit photoelectrons for a radiationof wavelength 4100 Å is/are [2005]

Solution:

So, only metals having work function less

than 3eV can emit photoelectrons for the

incident radiation of wavelength 4100Å.

QUESTION: 7

When photons of energy hv fall on an aluminiumplate (of work function E_{0}), photoelectrons ofmaximum kinetic energy K are ejected. If thefrequency of the radiation is doubled, themaximum kinetic energy of the ejectedphotoelectrons will be [2006]

Solution:

Applying Einstein's formula for photoelectricity

If we use 2v frequency then let the kinetic

energy becomes K'

So,

QUESTION: 8

In a discharge tube ionization of enclosed gas isproduced due to collisions between [2006]

Solution:

When electrons emitted from cathode collide

with gas molecules or atoms, they knock out

outer electrons and produce positively

charged ions. They become part of positive

rays.

QUESTION: 9

A photo-cell employs photoelectric effect toconvert [2006]

Solution:

A photo-cell employs photoelectric effect to

convert light energy into photoelectric

current.

QUESTION: 10

The momentum of a photon of energy 1 MeV inkg m/s, will be [2006]

Solution:

1 MeV = 10^{6} × 1.6 × 10^{–19} joule

Momentum of photon

QUESTION: 11

Monochromatic light of frequency 6.0 × 10^{14} Hzis produced by a laser. The power emitted is 2 × 10^{–3} ω. The number of photons emitted, onthe average, by the sources per second is [2007]

Solution:

Since p = nhv

QUESTION: 12

A 5 watt source emits monochromatic light ofwavelength 5000 Å. When placed 0.5 m away, itliberates photoelectrons from a photosensitivemetallic surface. When the source is moved to adistance of 1.0 m, the number of photoelectronsliberated will be reduced by a factor of [2007]

Solution:

Therefore, as distance is doubled, N_{E}

decreases by (1/4) times.

QUESTION: 13

The work function of a surface of aphotosensitive material is 6.2 eV. The wavelengthof incident radiation for which the stoppingpotential is 5 V lies in the: [2008]

Solution:

Work function, φ_{0} = 6.2eV.

Stopping potential V_{0} = 5V.

Thus the wavelength of the incident radiation

lies in the ultraviolet region.

QUESTION: 14

In the phenomenon of electric discharge throughgases at low pressure, the coloured glow in thetube appears as a result of: [2008]

Solution:

The coloured glow in the tube appears as a

result of excitations of electrons in the

atoms.

QUESTION: 15

A particle of mass 1 mg has the same wavelengthas an electron moving with a velocity of 3×10^{6 }ms^{–1}. The velocity of the particle is: [2008]

(mass of electron = 9.1×10^{–31} kg)

Solution:

Wavelength of

where v is the velocity of the particle.

Wave length of electron

QUESTION: 16

The number of photo electrons emitted for lightof a frequency v (higher than the threshold frequency v_{0}) is proportional to: [2009]

Solution:

The number of photoelectrons emitted is

proportional to the intensity of incident

light. Saturation current ∝ intensity.

QUESTION: 17

The figure shows a plot of photo current versus anode potential for a photo sensitive surface for three different radiations. Which one of the following is a correct statement? [2009]

Solution:

Retarding potential depends on the

frequency of incident radiation but is

independent of intensity.

QUESTION: 18

Monochromatic light of wavelength 667 nm isproduced by a helium neon laser. The poweremitted is 9 mW. The number of photons arrivingper sec on the average at a target irradiated bythis beam is: [2009]

Solution:

λ = 667 × 10^{–9}m, P = 9 ×10^{–3}W

QUESTION: 19

A beam of cathode rays is subjected to crossed Electric (E) and Magnetic fields (B). The fields are adjusted such that the beam is not deflected. The specific charge of the cathode rays is given by: [2010]

Solution:

For no deflection of beam, eE = evB

⇒ v = E/B

QUESTION: 20

A source S1 is producing, 10^{15} photons persecond of wavelength 5000 Å. Another source S_{2} is producing 1.02×10^{15} photons per secondof wavelength 5100Å Then, (power of S_{2}) (power of S_{1}) is equal to : [2010]

Solution:

Energy emitted/sec by

Energy emitted/sec by

QUESTION: 21

The potential difference that must be applied tostop the fastest photoelectrons emitted by anickel surface, having work function 5.01 eV,when ultraviolet light of 200 nm falls on it, mustbe: [2010]

Solution:

QUESTION: 22

Photoelectric emmision occurs only whenthe incident light has more than a certainminimum [2011]

Solution:

For occurence of photoelectric effect, the

incident light should have frequency more

than a certain minimum which is called the

threshold frequency (v_{0}).

For photoelectric effect emission v > v_{0}

where v is the frequency of the incident

light.

QUESTION: 23

In the Davisson and Germer experiment, thevelocity of electrons emitted from the electrongun can be increased by [2011]

Solution:

In the Davisson and Germer experiment, the

velocity of electrons emitted from the

electron gun can be increased by

increasing the potential difference between

the anode and filament.

QUESTION: 24

Light of two different frequencies whosephotons have energies 1 eV and 2.5 eVrespectively illuminate a metallic surface whosework function is 0.5 eV successively. Ratioof maximum speeds of emitted electrons will be[2011]

Solution:

The maximum kinetic energy of emitted

electrons is given by

K.E = φ – φ_{0}

K.E1 = 1 eV – 0.5 eV = 0.5 eV

K.E2 = 2.5 eV – 0.5 eV = 2 eV

QUESTION: 25

Electrons used in an electron microscope areaccelerated by a voltage of 25 kV. If the voltageis increased to 100kV then the de–Brogliewavelength associated with the electrons would

Solution:

QUESTION: 26

In photoelectric emission process from a metalof work function 1.8 eV, the kinetic energy ofmost energetic electrons is 0.5 eV. Thecorresponding stopping potential is [2011]

Solution:

The stopping potential is equal to maximum

kinetic energy.

QUESTION: 27

The threshold frequency for a photosensitivemetal is 3.3 × 10^{14} Hz. If light of frequency 8.2 × 10^{14} Hz is incident on this metal, the cut-offvoltage for the photoelectric emission is nearly [2011M]

Solution:

K.E. = hv – hv_{th} = eV_{0} (V_{0} = cut off voltage)

QUESTION: 28

A 200 W sodium street lamp emits yellow lightof wavelength 0.6 μm. Assuming it to be 25% efficient in converting electrical energy to light,the number of photons of yellow light it emitsper second is [2012]

Solution:

Give that, only 25% of 200W converter

electrical energy into light of yellow colour

Where N is the No. of photons emitted per

second, h = plank’s constant, c, speed of

light.

QUESTION: 29

Monochromatic radiation emitted when electronon hydrogen atom jumps from first excited tothe ground state irradiates a photosensitivematerial. The stopping potential is measured tobe 3.57 V. The threshold frequency of the materialsis : [2012]

Solution:

QUESTION: 30

If the momentum of electron is changed by P,then the de Broglie wavelength associated withit changes by 0.5%. The initial momentum ofelectron will be : [2012M]

Solution:

The de-Broglie’s wavelength associated

with the moving electron

Now, according to problem

QUESTION: 31

Two radiations of photons energies 1 eV and 2.5 eV, successively illuminate a photosensitivemetallic surface of work function 0.5 eV. The ratioof the maximum speeds of the emitted electronsis : [2012M]

Solution:

According to Einsten’s photoelectric effect,

the K.E. of the radiated electrons

K.E_{max} = E-W

QUESTION: 32

For photoelectric emission from certain metal the cut-off frequency is v. If radiation of frequency 2v impinges on the metal plate, the maximum possible velocity of the emitted electron will be (m is the electron mass) [NEET 2013

Solution:

From photo-electric equation,

QUESTION: 33

The wavelength λ_{e} of an electron and λ_{p} of a photon are of same energy E are related by [NEET 2013]

Solution:

QUESTION: 34

A source of light is placed at a distance of 50 cm from a photocell and the stopping potential is found to be V_{0}. If the distance between the lightsource and photocell is made 25 cm, the newstopping potential will be [NEET Kar. 2013]

Solution:

Since, stopping potential is independent

of distance hence new stopping potential

will remain unchanged i.e., new stopping

potential = V_{0}.

QUESTION: 35

The de-Broglie wavelength of neutron in thermal equilibrium at temperature T is [NEET Kar. 2013]

Solution:

From formula

[By placing value of h, m and k)

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