Q1: What evidence is there that light travels like a wave?
A: Evidence for the wave nature of light includes:
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Interference Patterns: When light passes through a narrow slit or a double-slit, it produces an interference pattern, which is characteristic of waves. Bright and dark fringes appear on a screen due to constructive and destructive interference.
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Diffraction: Light bends around obstacles and spreads out when passing through narrow openings. This bending is a wave phenomenon.
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Polarization: Light can be polarized, meaning it can oscillate in specific directions. This behavior is similar to waves, which can also be polarized.
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Reflection and Refraction: The behaviors of light when it reflects off surfaces and bends as it passes through different media can be described by wave theories.
Q2: What evidence is there that light travels like a particle?
A: Evidence supporting the particle nature of light includes:
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Photoelectric Effect: Light can eject electrons from a metal surface when it possesses enough energy, demonstrating that light can behave like a stream of particles (photons). The energy of the ejected electrons is directly proportional to the frequency of the incoming light.
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Compton Scattering: When X-rays are scattered off electrons, the change in wavelength of the X-rays can be explained only by treating light as particles, showing that photons carry momentum.
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Blackbody Radiation: The quantization of energy levels led to the development of Planck's law, which treats light as a quantized phenomenon, emitting energy in discrete packets (photons).
Q3: Describe the double-slit experiment. Why is it important to our understanding of electromagnetic waves?
A: The double-slit experiment involves shining a coherent light source (like a laser) onto a barrier with two closely spaced slits. The light that passes through the slits is projected onto a screen, where an interference pattern of alternating bright and dark bands appears. This pattern indicates that light behaves like a wave, as the waves emanating from the two slits interfere with each other.
The experiment is crucial because:
- It shows how waves can interfere, elucidating the wave aspect of light.
- When conducted with individual photons, over time, a clear interference pattern still forms, suggesting that each photon interferes with itself, demonstrating wave-particle duality.
- This experiment reinforces the foundational principles of quantum mechanics, proving that light and, by extension, all quantum particles can exhibit both wave-like and particle-like properties.
Q4: How does the emission and absorption spectrum of an atom support the particle model of light?
A: The emission and absorption spectra of atoms support the particle model of light through the following points:
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Quantized Energy Levels: Atoms have specific energy levels. When an electron transitions between these levels, it either absorbs or emits a photon of a precise energy, corresponding to a specific frequency of light. This quantization aligns with the particle model, where light is seen as consisting of discrete packets (photons).
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Spectral Lines: The emission spectra show distinct lines rather than a continuous spectrum, indicating that only certain energies (or photons) are emitted or absorbed. This discrete nature can only be explained by treating light as composed of particles with specific energy levels.
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Photoelectric Effect Connection: The ability of light to eject electrons from a material only at specific frequencies (and not just at any intensity) underlines the particle nature of light, where each photon must have enough energy (related to its frequency) to affect the electrons.
In summary, the emission and absorption spectra definitively support the particle model by showing the quantized interactions between photons and atomic energy levels.