66. The work functions for metals A, B and C are respectively 1.92 eV, 2.0 eV and 5 eV. According to Einstein's equation, the metals which will emit photo electrons for a radiation radiation of wavelength 4100 Å is/are

1. 1. none

   
   
   

   2. A only

   
   
   

   3. A and B only

   
   
   

   4. all three metals

   
   
   

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1. View Hint View Answer Discuss in Forum

   E =	hc
   	λ

   
   

   ⇒ E =	12375	eV
   	λ(in Å)

   
   

   ⇒ E =	12375	eV
   	4100

   
   ⇒ E ≈ 3 eV ​
   So, only metals having work function less than 3eV can emit photo electrons for the incident radiation of wavelength 4100 Å.

   Correct Option: C

   E =	hc
   	λ

   
   

   ⇒ E =	12375	eV
   	λ(in Å)

   
   

   ⇒ E =	12375	eV
   	4100

   
   ⇒ E ≈ 3 eV ​
   So, only metals having work function less than 3eV can emit photo electrons for the incident radiation of wavelength 4100 Å.

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67. The threshold frequency for photoelectric effect on sodium corresponds to a wavelength of 5000 Å. Its work function is

1. 1. 4 × 10^–19 J

   
   
   

   2. 1 J

   
   
   

   3. 2 × 10^–19 J

   
   
   

   4. 3 × 10^–19 J

   
   
   

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1. View Hint View Answer Discuss in Forum

   W0 =	hc
   	λ0

   
   

   =	6.63 × 10-34 × 3 × 108	= 4 × 10-19 J
   	5000 × 10-10

   Correct Option: A

   W0 =	hc
   	λ0

   
   

   =	6.63 × 10-34 × 3 × 108	= 4 × 10-19 J
   	5000 × 10-10

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68. A photoelectric cell is illuminated by a point source of light 1m away. When the source is shifted to 2m then

1. 1. number of electrons emitted is a quarter of the initial number

   
   
   

   2. each emitted electron carries one quarter of the initial energy

   
   
   

   3. number of electrons emitted is half the initial number

   
   
   

   4. each emitted electron carries half the initial energy

   
   
   

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1. View Hint View Answer Discuss in Forum

   Power ∝ No. of electrons emitted (N) ​
   

   P ∝	1	⇒ N ∝	1
   	r2		r2

   Correct Option: A

   Power ∝ No. of electrons emitted (N) ​
   

   P ∝	1	⇒ N ∝	1
   	r2		r2

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69. Einstein work on the photoelectric effect provided support for the equation

1. 1. E = hv

   
   
   

   2. E = mc²

   
   
   

   3. E = -	Rhc
      	n2

   
   
   

   4. K.E. =	1	mv2
      	2

   
   
   

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1. View Hint View Answer Discuss in Forum

   Einstein work on photoelectric effect supports the equation E = hν.It is based on quantum theory of light.

   Correct Option: A

   Einstein work on photoelectric effect supports the equation E = hν.It is based on quantum theory of light.

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70. When ultraviolet radiation is incident on a surface, no photo electrons are emitted. If a second beam causes photo electrons to be ejected, it may consists of

1. 1. infra-red waves

   
   
   

   2. X-rays

   
   
   

   3. visible light rays

   
   
   

   4. radio waves

   
   
   

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1. View Hint View Answer Discuss in Forum

   Energy of photon of X-rays is more than energy of photon of ultraviolet rays. Because frequency of X rays is more than ultraviolet ultraviolet rays.

   Correct Option: B

   Energy of photon of X-rays is more than energy of photon of ultraviolet rays. Because frequency of X rays is more than ultraviolet ultraviolet rays.

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