Photoelectric effect and photons

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Questions and Answers

In the photoelectric effect, if the intensity of incident light is increased while keeping the frequency constant, how would this affect the number of emitted photoelectrons and their maximum kinetic energy?

Increasing intensity increases the number of emitted photoelectrons, while the maximum kinetic energy remains unchanged.

Explain why different metals have different threshold frequencies in the photoelectric effect.

Different metals have different work functions, which represent the minimum energy required to remove an electron. This corresponds to different threshold frequencies.

Describe the relationship between the stopping voltage and the maximum kinetic energy of emitted photoelectrons in a photoelectric experiment.

The stopping voltage is the voltage required to stop the most energetic photoelectrons from reaching the collector. Therefore, $KE_{max} = eV_s$, where $V_s$ is the stopping voltage.

In an emission spectrum, what determines the specific wavelengths (or colors) of light that are emitted by a gas?

The specific wavelengths are determined by the energy differences between electron energy levels within the atoms of the gas. When electrons transition between these levels, photons with specific energies (and thus wavelengths) are emitted. Signup and view all the answers

Explain the difference between an emission spectrum and an absorption spectrum, and how they are produced.

An emission spectrum is produced when a gas emits light, showing bright lines at specific wavelengths. An absorption spectrum is produced when white light passes through a gas, showing dark lines at specific wavelengths where the gas absorbed the light. Signup and view all the answers

How can astronomers use emission spectra to determine the composition of stars?

By analyzing the wavelengths of light present in a star's emission spectrum, astronomers can identify the elements present in the star, as each element has a unique spectral fingerprint. Signup and view all the answers

Explain why the light emitted by neon signs or sodium vapor street lamps contains only a few particular colors (or frequencies).

When electricity is passed through these gases, electrons are excited to higher energy levels. When the electrons return to lower energy levels, they emit photons of specific energies, corresponding to specific colors/frequencies of light. Signup and view all the answers

If you shine white light through a gas and observe its absorption spectrum, what do the dark lines in the spectrum represent, and why are they dark?

The dark lines represent the specific wavelengths of light that were absorbed by the gas. They are dark because the photons of those specific wavelengths were absorbed by the gas atoms, exciting electrons to higher energy levels, and thus not transmitted. Signup and view all the answers

Describe how a diffraction grating is used to produce an emission spectrum from a gas.

A diffraction grating separates light into its constituent wavelengths by diffraction and interference. When light from a gas passes through the grating, each wavelength is diffracted at a different angle, creating a pattern of distinct lines corresponding to each wavelength present in the gas's emission spectrum. Signup and view all the answers

How does the work function of a metal relate to the energy of photons required to eject electrons from its surface?

The work function represents the minimum energy a photon must have to eject an electron. If a photon's energy is less than the work function, no electrons will be emitted, regardless of the light's intensity. Signup and view all the answers

Flashcards

Threshold Frequency

The minimum frequency of light required to eject electrons from a metal surface.

Emission Spectrum

The spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state.

Absorption Spectrum

A spectrum of electromagnetic radiation transmitted through a substance, showing dark lines or bands due to absorption of specific wavelengths.

Uses for emission spectra

Uses emission spectra to identify elements by flame tests and astronomers can identify elements in stars by analysing the frequencies present in the light given off.

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Work function

The energy needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface.

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Study Notes

Ultraviolet light with a frequency of 8.0 x 10^14 Hz shines on a metal surface that has a work function of 2.0 x 10^-19 J inside a photoelectric cell
It is possible to calculate the energy of the photons of incident light.
The threshold frequency of the metal can be found.
Solve for the maximum kinetic energy of the electrons emitted.
In an experiment, the cut-off voltage from a photoelectric cell was measured for several frequencies of incident light.
Voltage readings are used to calculate the maximum kinetic energy of emitted electrons at each frequency.
A graph of the maximum kinetic energy of the electrons against frequency can be plotted.
It is possible to calculate the gradient of the graph.
Use the graph to estimate the threshold frequency and the work function of the metal.
The experiment was repeated using another metal with a threshold frequency of 6.0 x 10^14 Hz.
The maximum kinetic energy of photoelectrons depends on several factors:
Work function
Emission spectra
Cut-off voltage

Atomic Line Spectra

Modern physics helped explain the atomic line spectra
Atomic line spectra - light given out by low-pressure gases when they are excited by heat or electrical discharge
Neon signs and yellow sodium vapour street lamps emit light produced this way.
The light from these sources contains only a few particular colours (or frequencies).

Fluorescent Lighting

High-voltage electrical discharge pass through hydrogen in a tube at low pressure results in a pale violet light
Passing the light through a diffraction grating spreads out the frequencies, producing an emission spectrum
The emission spectrum is similar to a prism spreading white light into a full spectrum of colours.
An emission spectrum for hydrogen: a series of narrow, coloured lines indicates only certain frequencies are emitted.
An absorption spectrum is created by shining white light through a gas, resulting in dark lines that correspond to the bright lines of the emission spectrum, showing the gas absorbs specific frequencies of light.
Each element has a unique pattern of lines in its spectrum
Emission spectra can identify elements in gas samples.
Chemists use them for flame tests and astronomers use them to analyze light from stars.
Flame colors:
Copper (Cu) is green
Potassium (K) is violet
Sodium (Na) is yellow/orange

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