Physics of light

A basic understanding of electromagnetism to allow for understanding of light and color. Links: units (radiant flux, spectral flux, radiant intensity, spectral intensity).

46 flashcards · Physics · by lakesare

1. What is dispersion?

2. Dispersion VS diffraction VS refraction?

3. What is refraction?

   The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell's Law. The bending of refraction can be visualized in terms of Huygens' principle. As the speed of light is reduced in the slower medium, the wavelength is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by medium changes.

4. What's the formula to find the index of refraction?

   The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium. Light that travels through transparent matter does so at a lower speed than c, the speed of light in a vacuum. The factor by which the speed is decreased is called the refractive index of the material.

5. State Snell's Law.

6. What is work W measured in?

   Joules Force * distance

7. What unit does energy E uses?

   Joules Work uses the same unit as energy, because work is just a change in energy.

8. What's black body?

   A black body is an idealized physical body that absorbs all incident electromagnetic radiation. We can model stars as black bodies. The thermal radiation spontaneously emitted by many ordinary objects can be approximated as black-body radiation. A perfectly insulated enclosure that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have negligible effect upon the equilibrium.A black body at ro…

9. What happens when black body is colder than its environment?

   Black body will continue to absorb all energy from the environment until it reaches the same temperature (then it's said to be in thermal equilibrium). If black body is hotter - it will emit the energy until it's in thermal equilibrium.

10. What is photoelectric effect?

    Emission of electrons when light (of certain frequency == energy) hits the material.

11. What's a photon?

    It's a quantum of the electromagnetic field. The invariant mass of the photon is zero; it always moves at the speed of light in a vacuum.

12. How does the wavelength of photon relate to its energy?

    The smaller the wavelength, - the bigger the energy. The energy contained within a photon is proportional to the frequency of its corresponding wave, with high energy photons corresponding with high frequency waves.

13. Explain what's shown on this graph.

    This is intensity of radiation of the blackbody at each wavelength FOR SOME TEMPERATURE of that blackbody. Radiation has a maximum intensity at a wavelength that depends on the temperature of the body.

14. What equation does this graph follow?

    Plank's equation, gives the intensity of radiation produced by black-body at a given temperature.

15. What unit is absolute temperature measured in?

    [K] Kelvin. The Kelvin temperature scale is an absolute temperature scale with zero at absolute zero. Because it is an absolute scale, measurements made using the Kelvin scale do not have degrees. The kelvin (note the lowercase letter) is the base unit of temperature in the International System of Units (SI). The Kelvin scale fulfills Thomson's requirements as an absolute thermodynamic temperature scale. It uses absolute zero as its null point. The kelvin is often used as a measure of the colou…

16. What is 1 Kelvin equal to?

    The size of the kelvin "degree" is the same as the size of the Celsius "degree.".

17. How many degrees Celsius is absolute zero?

    −273.15 °C

18. What is ultraviolet catastrophe?

    It's the prediction (made by deprecated Rayleigh–Jeans law) that the intensity of high-frequency light should eventually approach infinity:

19. What solved the ultraviolet catastrophe?

    It's the prediction (made by deprecated Rayleigh–Jeans law) that the intensity of high-frequency light should eventually approach infinity: Max Planck derived the correct form for the intensity spectral distribution function by making some strange (for the time) assumptions. In particular, Planck assumed that electromagnetic radiation can be emitted or absorbed only in discrete packets, called quanta, of energy: , where h is Planck's constant, and ν is the frequency of the radiation. Physicists…

20. What is: E - ? h - ? ν - ?

    E - energy h - Planck's constant (6.62... × 10-34 m2 kg / s) ν - is the frequency of the radiation Physicists now call these quanta photons, and a photon of frequency ν will have its own specific and unique energy.

21. Explain what's happening on this image:

    Sun can be approximated as the black body. Therefore, solar spectral irradiance is approximately equal to that of a blackbody radiation at 5578K!

22. How does Watt relate to Joules?

    W = J/s

23. What is radiant energy Qe?

    Energy of electromagnetic radiation [J]. The symbol Qe is often used throughout literature to denote radiant energy ("e" for "energetic", to avoid confusion with photometric quantities).

24. What is radiant flux of an object Φe?

    Radiant flux is the total energy emitted per second, and is measured [Watts or J/s]. The radiant flux of a 100W incandescent lightbulb is about 80W, with the remaining 20W being converted directly to non-radiated heat.

25. What is spectral flux of an object Φe,λ?

    Spectral flux of an object is radiant flux per unit wavelength [Watts/nanometer]. If we were to graph the spectral flux of our incandescent lightbulb as a function of wavelength, it might look something like this: The area under this curve will give the radiant flux. As an equation,

26. What is phase velocity?

    The red square moves with the phase velocity, and the green circles propagate with the group velocity.

27. How does wavelength λ of a sinusoidal waveform depend on its phase velocity ν? The red square moves with the phase velocity (as opposed to green-circle group velocity).

    The wavelength λ of a sinusoidal waveform traveling at phase velocity ν is given by where ν is called the phase speed of the wave, and f is the wave's frequency. In the case of electromagnetic radiation—such as light—in free space, the phase speed is the speed of light, about 3×108 m/s. Thus the wavelength of a 100 MHz electromagnetic (radio) wave is about: 3×108 m/s divided by 108 Hz = 3 metres.

28. Which photon has more energy, - blue photon or red photon?

    Blue photon.

29. Give an equation for frequency ν and wavelength λ of a photon.

    c = ν * λ Where c is speed of light (3 * 108m/s)

30. You have a red photon. Find this photon's energy. Wavelength of this red is of 700nm.

    Used formulas: c = f * λ E = h * f Given: Phase speed of light c = 3 * 108m/s. Planck's constant h = 6.6 * 10-34 J*s Solution: f = c / λ = 3 * 108m/s / 700 * 10-9 = 0.4 * 1015Hz E = h * f = 2.64 * 10-19

31. What is the phase speed of light?

    The phase speed of light is about 3×108 m/s. The red square moves with the phase velocity.Phase speed is a magnitude of phase velocity.

32. What unit does Planck's constant have?

    J*s This unit can be derived from E = h * f formula.

33. What does this formula convey, physically?

    This is the size of packets of electromagnetic wave, - photons. Einstein's explanation was that light itself is quantized; that the energy of light is not transferred continuously as in a classical wave, but only in small "packets" or quanta. The size of these "packets" of energy, which would later be named photons, giving the modern version of the Planck–Einstein relation.

34. What happens when electron jumps from 3rd level to 2nd?

    Energy is emitted. A schematization of the Bohr model of the hydrogen atom. The transition shown from the n = 3 level to the n = 2 level gives rise to visible light of wavelength 656 nm (red), as the model predicts.

35. What does LED stand for?

    Light-Emitting Diode.

36. What's a diode?

    Diode is an electronic component that conducts current I primarily in one direction (asymmetric conductance). Diode has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. The triangle in the symbols points to the forward direction, i.e. in the direction of conventional current flow.Electron flow is the reverse of conventional current flow.

37. The angle of refraction of light depends on its wavelength, a phenomenon called ______.

    Dispersion

38. Humans at normal body temperature radiate chiefly at wavelengths around ______.

    10 μm (micrometers) Which is infrared light :-)

39. Why does the light dispersion happen?

    The index of refraction increases steadily with decreasing wavelength. Any type of wave can exhibit dispersion. Sound waves, all types of electromagnetic waves, and water waves can be dispersed according to wavelength. Dispersion occurs whenever the speed of propagation depends on wavelength, thus separating and spreading out various wavelengths.

40. Why wave speed = frequency × wavelength?

    Because frequency is the number of vibrations (wavelengths!) a wave makes in a second.

41. How does this graph explain that metal is: red when warm yellow → bluish when very hot?

    Links: https://www.youtube.com/watch?v=gcDC9Gxg0HA.

42. What is luminescence?

    Spontaneous emission of light by a substance not resulting from heat. Fluorescent minerals emit visible light when exposed to ultraviolet light. The following are types of luminescence: Chemiluminescence, the emission of light as a result of a chemical reaction Crystalloluminescence, produced during crystallization Electroluminescence, a result of an electric current passed through a substance Mechanoluminescence, a result of a mechanical action on a solid Photoluminescence, a result of absorpt…

43. Incandescence VS luminescence?

    Incandescence - is the emission of light from a hot body as a result of its temperature. Luminescence - Luminescence is emission of light by a substance not resulting from heat.

44. What is fluorescence?

    Fluorescence happens when invisible light shines through certain materials, and they give off visible light. Ultraviolet light shines one these liquids, and they give off a visible light. The most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can be seen only when exposed to UV light…

45. Draw lateral inhibition.

    Calculating second-level neuron output: 10 * 1 + 10 * (-0.2) + 10 * (-0.2) = 10 - 2 - 2 = 6. Lateral inhibition refers to an architecture for neural networks in which neurons inhibit spatially neighboring neurons. Lateral inhibition occurs in a two layer network when each neuron in the first layer excites a corresponding neuron (or set of neurons) in the second layer but inhibits the second-layer neurons lateral to the excitatory connection. In this diagram, excitatory connections (arrowheads)…

46. How is it determined whether an infant can discern colors?

    By the viewing preference. The basic question is whether an infant can tell the difference between two colors. To find out, using the preferential looking paradigm, the infant is shown two displays, with one color on the left and the other on the right. The predominant direction of the baby's gaze during the fixed time period of a trial is judged by a hidden observer who does not know which stimulus is on which side. On half the trials, the left/right positions of the stimuli are reversed to be…