# Physics A- Long answer questions

• Created by: Lucy
• Created on: 05-01-13 16:38
How does the existence in the spectrum of light of lines of a definite wavelength support the view that atoms have discrete energy levels?(2)
Photon energy is related to wavelength. Discrete wavelengths suggest fixed energy states of atoms.
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Explain in terms of energy why the threshold frequency exists. (2)
The energy of each photon increases with frequency. Electrons need a minimum amount of energy to leave the metal.
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Why does a photon theory of light provide a better explanation of the photoelectric effect than a wave theory of light?(5)
The amount of energy required is the work function. This suggests that the electrons are given energy in 1 discrete event (1electron interacts with 1 photon). So the light is concentrated. P.E occurs immediately after light falls on the metal.
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Explain how energetic electrons cause the atoms of the vapour of an element to emit light(3)
Inelastic collisions between electrons and atoms take place. Energy of the atoms increases. As the atom de-excites, the atom emits photons and returns to its original state.
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Explain why the kinetic energy of the emitted electrons has a maximum value (3)
hf is the energy available/received. Energy required to remove electrons varies. Kinetic energy will never be more than hf minus the work function.
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Explain why there is a minimum energy of the photon for pair production to take place and what happens when a photon has more energy than this value.
The ray must provide enough energy for the rest mass. The rest of the energy will be used as kinetic energy.
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Why does the existence of a threshold frequency support the particle nature of EM waves. (part 1)
If f is below the threshold f, there is no emission even if the intensity is increased. This cannot be explained by wave theory as wave energy increases with intensity. Light travels as photons. Photons have energy that depend on f.
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Why does the existence of a threshold frequency support the particle nature of EM waves. (part 2)
If frequency is above the threshold frequency, photons have enough energy. Also,the lack of time delay cannot be explained by wave theory.
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Explain how the existence of a spectrum consisting of lines of a definite frequency supports the view that atoms have discrete energy levels. (4)
Photons have energy hf. Photons of characteristic frequencies are emitted from atoms of a particular element. This is because atoms have discrete energy levels, which are associated with particular energy values.
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## Other cards in this set

### Card 2

#### Front

Explain in terms of energy why the threshold frequency exists. (2)

#### Back

The energy of each photon increases with frequency. Electrons need a minimum amount of energy to leave the metal.

### Card 3

#### Front

Why does a photon theory of light provide a better explanation of the photoelectric effect than a wave theory of light?(5)

#### Back ### Card 4

#### Front

Explain how energetic electrons cause the atoms of the vapour of an element to emit light(3)

#### Back ### Card 5

#### Front

Explain why the kinetic energy of the emitted electrons has a maximum value (3)

#### Back ## Comments

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