Photons: What is Light?

00:00:12.04 This time we're going to discuss about
00:00:14.08 what light is.
00:00:15.18 It's an interesting question
00:00:16.25 and it's an important question
00:00:18.06 because this is what
00:00:20.02 the entirety of light microscopy is based on.
00:00:22.12 But it also seems to be
00:00:24.09 straightforwardly answered
00:00:26.00 because since we were very young
00:00:28.00 we know that light comes in straight lines,
00:00:31.02 straight rays --
00:00:32.26 they don't make turns --
00:00:34.27 and the only debate
00:00:36.15 that ancient Greece had had
00:00:38.25 was whether the light rays originate
00:00:41.00 from the object coming to the eyes
00:00:42.26 or whether it originates from the eyes
00:00:46.07 and coming to the object.
00:00:47.14 Well, that debate has now been settled,
00:00:50.13 and in fact describing light as rays
00:00:53.07 has been a very useful
00:00:55.03 and simple tool
00:00:56.15 to understand optics.
00:00:58.07 For example, when we look at
00:01:00.18 a simple thin lens
00:01:01.29 and we know that
00:01:03.24 if we sent parallel light into the lens
00:01:06.13 we will focus the rays down
00:01:08.03 to a single point,
00:01:09.23 and the distance between the point and the lens
00:01:12.14 is called the focal length of the lens.
00:01:14.03 Well, if the light rays are not parallel,
00:01:17.12 instead diverging,
00:01:18.29 the lens can still focus the light rays down,
00:01:23.03 this time, further away.
00:01:25.05 And this is the case
00:01:26.28 when a lens is making an image of an object,
00:01:31.12 and the distance between the object
00:01:33.18 to the lens and the image to the lens
00:01:36.22 has this following relationship,
00:01:39.05 and that's called the lensmaker’s formula
00:01:40.26 -- we can use that to calculate
00:01:42.24 how images are formed
00:01:44.19 and what the magnification of the lens is.
00:01:47.09 We can have a more precise description
00:01:49.10 of individual rays, for example,
00:01:51.14 just looking at one of them,
00:01:53.07 when they come to cross the plane where the lens is,
00:01:55.23 then we can describe the position of the intersection
00:01:58.25 as well as the angle of the light ray to this plane.
00:02:02.02 Now, when we have this lens,
00:02:04.07 what it does is to change the angle,
00:02:06.12 and in fact this is a general way
00:02:09.12 of describing a lot of different optical elements,
00:02:13.19 which is a transformation from one set of interception
00:02:17.18 and the angle to a different set,
00:02:20.29 and once we can figure out how this transformation is done,
00:02:23.12 then we can easily describe
00:02:25.26 all the output elements in any of the microscopes, for example.
00:02:29.17 And that has been an extremely powerful way,
00:02:32.13 and we do ray tracing
00:02:34.24 either in designing optics,
00:02:38.27 for example, this is one of the ray tracing diagrams
00:02:41.12 taken from a ray tracing software website,
00:02:44.00 and we have ray tracing to do a lot more fun things,
00:02:47.04 for example, in animation
00:02:49.24 and figuring out how the lights reflect from the surface of a car
00:02:54.12 and all kinds of beautiful images
00:02:56.11 we can create out of our computer
00:02:58.04 by doing ray tracing.
00:03:00.01 Thinking of light as rays
00:03:02.00 has been the common concept of people
00:03:03.25 for many, many years, for example,
00:03:06.08 Newton has always been thinking of light
00:03:08.24 as a particle going from one position,
00:03:11.01 straight line to another position.
00:03:13.02 But later on,
00:03:15.07 people discovered that
00:03:17.13 not all the phenomena
00:03:19.18 can be explained by treating light as rays.
00:03:21.11 And that comes in another description of light
00:03:23.24 as waves.
00:03:25.07 We know that waves describe something
00:03:27.06 that's oscillating and propagating in space.
00:03:30.00 And the distance for the wave
00:03:33.09 to complete one oscillation
00:03:35.10 is called the wavelength,
00:03:37.06 and the number of oscillations in a unit time
00:03:39.04 is called the frequency.
00:03:40.19 When you multiply the two together,
00:03:42.07 that describes how much distance this wavelength...
00:03:45.15 this wave can travel in a unit time,
00:03:47.29 and that's the speed that it's traveling.
00:03:51.06 And so when you multiply the wavelength of light
00:03:53.27 and frequency of light together,
00:03:55.24 we get the speed of light.
00:03:57.13 And this is 299,792,458 meters per second,
00:04:04.07 in vacuum.
00:04:05.24 And this is an exact number.
00:04:07.22 In fact, in physics,
00:04:09.12 you don't get so many exact numbers,
00:04:11.02 but this is exact
00:04:13.16 because this is how a meter is defined.
00:04:15.11 So, for light waves in a visible range,
00:04:18.21 meaning the light that we can see by eye,
00:04:21.16 the wavelength is about somewhere
00:04:24.14 between 400 and 700 nanometers
00:04:26.03 -- this is a very, very small distance --
00:04:28.26 corresponding, in the frequency,
00:04:30.22 is in the 100s of terahertz range,
00:04:32.24 very fast oscillations.
00:04:34.25 So, we're talking about...
00:04:36.29 this is how light propagates in vacuum.
00:04:39.25 Now, we don't live in vacuum...
00:04:41.24 I mean, what if light is trying to go through some transparent media?
00:04:45.05 In this case, the light wave
00:04:48.06 can have some interaction with the medium,
00:04:49.22 and that can slow down propagation of the wave.
00:04:52.01 And it still has the same frequency
00:04:55.20 because at the interface
00:04:57.12 it goes with the same oscillation cycles
00:04:59.27 and inside the media it has to follow that,
00:05:02.08 so by slowing down the speed
00:05:05.25 what you get is shorter wavelength.
00:05:07.04 And to describe how this slowing down is happening,
00:05:11.21 we have our definition of refractive index.
00:05:14.16 That's defined as
00:05:17.08 the ratio between the speed of light in vacuum
00:05:19.26 and the speed of light in the medium
00:05:21.20 or, the same as
00:05:24.01 the wavelength of light in vacuum
00:05:25.25 over the wavelength of light in the medium.
00:05:27.27 We call this speed difference
00:05:30.01 as the refractive index
00:05:32.12 because that describes the phenomenon of refraction.
00:05:35.08 Basically, when you have a light ray
00:05:37.09 hitting the interface,
00:05:38.28 moving from one medium to a different one,
00:05:40.17 and that light ray can be bent
00:05:42.09 depending on the difference of the refractive index.
00:05:45.09 And here is a picture
00:05:47.05 showing the refraction of light
00:05:49.14 when going from air into glass.
00:05:53.09 And the refraction behavior
00:05:55.05 can be described in terms of Snell's law,
00:05:57.08 which says that the refractive index
00:05:59.15 multiplied by the sine of the incident angle is a constant.
00:06:03.10 But how is the speed of light associated
00:06:05.29 with the incident angle of light?
00:06:08.17 Well, we can explain this very simply.
00:06:10.00 We have a plane wave of light
00:06:12.04 propagating through the interface
00:06:13.23 and it has the two incident angles
00:06:15.21 θ1 and θ2,
00:06:17.20 and we know that at this time point
00:06:20.12 this light ray comes to the interface
00:06:22.18 and it's going to propagate
00:06:24.16 through the other medium,
00:06:27.00 and it's going to propagate
00:06:30.22 through this distance of l1.
00:06:32.26 And, in the same case,
00:06:36.02 light rays toward the other side is going to propagate
00:06:39.15 in a different medium
00:06:41.16 and reach this interface.
00:06:44.00 But it still maintains the plane wave
00:06:46.15 -- that means that the light wave
00:06:48.14 traveling from here all the way to here
00:06:50.22 is going to take the same time
00:06:53.27 traveling from here to this point.
00:06:54.26 And we can calculate the time for the light wave to travel
00:06:57.15 as the distance over the speed of light,
00:06:59.28 and if the two points are separated by a distance of D
00:07:03.18 at the interface,
00:07:05.02 well then we can describe the two path lengths
00:07:08.15 as D times sine of the two angles.
00:07:12.09 And then we can put this
00:07:16.00 l1 and l2 into the time equation.
00:07:18.12 That's exactly what we get,
00:07:20.02 the Snell's law.
00:07:21.09 So this is a very simple explanation
00:07:23.10 of how the speed of light affects
00:07:26.07 the angles of refraction.
00:07:28.19 Because refractive index
00:07:30.28 tells how strongly the light interacts with the medium,
00:07:33.27 it is understandable that the refractive index
00:07:36.23 depends on the frequency of light,
00:07:39.09 and this is an effect called dispersion,
00:07:41.16 and this is classically seen as the
00:07:44.17 dispersion of a prism.
00:07:45.24 In this case, different colors of light
00:07:47.27 have different frequencies
00:07:51.16 and that has different refractive indexes,
00:07:52.28 and this will cause them to be
00:07:55.02 deflected by different angles,
00:07:56.10 and then we see this nicely colored rainbow,
00:07:58.07 or the same thing in nature.
00:08:02.00 This is exactly how rainbows are seen
00:08:04.19 after the storm.
00:08:06.28 These all look cool,
00:08:09.02 although these phenomena can still more or less
00:08:12.13 be explained by light as particles
00:08:14.09 traveling at different speeds in different media.
00:08:16.18 What light as a particle view
00:08:18.29 cannot really explain
00:08:20.13 is the phenomena of diffraction and interference.
00:08:23.04 This is unique to waves.
00:08:25.17 For diffraction,
00:08:27.11 we can see it as the propagation of a plane wave
00:08:30.11 hitting an obstacle with a small hole in there.
00:08:31.22 After the hole,
00:08:33.11 what will emerge is a spherical wave
00:08:35.26 -- light propagating in all kinds of directions.
00:08:39.01 And this can be very nicely illustrated
00:08:41.17 by a picture of a wave in the ocean
00:08:44.13 and passing through this gap,
00:08:47.02 and you can see the spherical wave front.
00:08:51.05 Interference is talking about
00:08:53.21 the effect of having multiple waves
00:08:56.24 coming together.
00:08:58.09 Now, here we have a propagating wave
00:09:00.20 going from the upper-left corner to the lower-right corner.
00:09:03.16 Now, if we have two waves
00:09:06.14 coming crossways to each other,
00:09:08.05 first, we will see that
00:09:11.14 they just go straight as if the other one is not around.
00:09:13.02 Now, you can see, after they met each other,
00:09:15.14 they just go straightforward.
00:09:17.14 The rule of interference should actually be called
00:09:19.25 the rule of no interference.
00:09:21.16 The really interesting thing happens, though,
00:09:24.02 at the space when the two waves
00:09:26.06 are overlapping with each other.
00:09:28.04 What you see is this pattern
00:09:30.03 caused by the superposition of the two waves.
00:09:34.15 And how do they add to each other?
00:09:36.13 Well, we have two sine waves
00:09:38.10 and for these two sine waves
00:09:40.07 they have fairly...
00:09:43.04 they have a fairly small difference
00:09:45.13 in the peak position.
00:09:47.15 In this case,
00:09:49.18 when you add the two together,
00:09:51.00 you get bigger amplitudes
00:09:52.22 -- this is called constructive interference.
00:09:54.25 If the two waves
00:09:57.15 have a fairly large difference
00:09:59.13 in terms of the peak position,
00:10:00.24 so that one peak overlaps
00:10:02.12 with the valley of the other one,
00:10:04.10 then the two will almost cancel out each other,
00:10:06.25 and this is called destructive interference.
00:10:09.01 In reality, we don't see light interference
00:10:12.02 that often,
00:10:13.17 because this delta-phi
00:10:15.16 has to comparable to the wavelength of light
00:10:18.03 and that's a very, very small distance.
00:10:21.18 And we do see that, for example,
00:10:23.22 when we look at the reflection from a film of bubbles...
00:10:27.08 a film of soapy water,
00:10:29.25 and very nice colorful horizontal stripes.
00:10:32.16 So, how is this happening?
00:10:35.04 Well, we have a very thin film of soap and water.
00:10:37.20 Light hitting this film...
00:10:40.07 some of it will be reflected from the front surface,
00:10:42.20 some will go through it,
00:10:44.16 and some will reflect from the back surface.
00:10:47.12 And light rays
00:10:50.27 that get reflected from the front and back surface
00:10:52.25 will have a phase difference
00:10:55.09 induced by this thickness of this film,
00:10:56.21 and that it how constructive or destructive interference can happen.
00:11:01.22 Now, we are holding this film,
00:11:04.16 like, upright,
00:11:06.08 so because of gravity
00:11:08.10 the soap film will be thicker near the bottom
00:11:10.06 and thinner at the top.
00:11:12.17 So, at different heights,
00:11:14.11 you do have different phase delay
00:11:16.23 of the front reflection and back reflection.
00:11:19.06 That's how you see these stripes.
00:11:22.09 And also because different colors of lights
00:11:24.10 have different wavelengths,
00:11:26.09 so the same physical distance
00:11:28.04 will mean different phase delay,
00:11:29.25 that's why the stripes are so colorful.
00:11:31.24 So, this is one of the not so common phenomena
00:11:35.08 of simple diffraction.
00:11:37.11 This is one of the very few cases
00:11:39.10 when you have pure interference.
00:11:41.07 In most cases, interference
00:11:44.03 always comes together with diffraction.
00:11:46.02 For example, even a simple propagation
00:11:48.25 of a plane wave...
00:11:51.01 at this wave front,
00:11:53.21 each point here is going to generate
00:11:56.28 small, tiny spherical waves,
00:11:58.24 so that's diffraction.
00:12:02.04 And all these spherical waves are going to propagate,
00:12:04.00 not interfere with each other,
00:12:05.21 and then superimpose on top of each other.
00:12:07.19 And here you can see these elements
00:12:10.08 from the new wave front,
00:12:12.02 that's the plane wave.
00:12:13.14 So this is interference.
00:12:15.00 And basically the propagation of light
00:12:16.23 is diffraction plus interference.
00:12:19.15 We can describe that more precisely
00:12:24.09 -- think about one point after this aperture
00:12:27.23 and you have all kinds of light waves
00:12:30.18 going from the aperture to this point,
00:12:33.13 and each one...
00:12:35.05 when coming to this point,
00:12:37.20 it's going to have an amplitude of A
00:12:39.13 and travel distance of l.
00:12:41.29 So, the total amplitudes that you get at this point, A,
00:12:48.06 is the sum of all these light rays
00:12:50.27 plus this phase difference.
00:12:53.00 And the intensity at this point
00:12:55.05 is basically amplitude squared.
00:12:57.29 And we can discuss one very simple,
00:13:02.04 very classical experiment,
00:13:03.18 the double-slit interference.
00:13:05.11 So, in this case,
00:13:07.07 we essentially cut the object
00:13:08.25 so that it will have one and two different points
00:13:11.03 that the light can go through,
00:13:12.29 and the separation is the distance of d.
00:13:16.14 Now, two spherical waves
00:13:18.27 are going to emerge from these points
00:13:21.26 and then they can come interfere with each other.
00:13:25.16 To simplify the math,
00:13:27.12 we can consider this extreme case.
00:13:29.25 In this case,
00:13:31.21 the two slits are very close to each other
00:13:34.18 with a separation of d
00:13:36.07 and the light rays can go some angle θ
00:13:38.18 off the optical axis,
00:13:41.14 and we have a screen
00:13:44.02 that's very far away, a distance of z,
00:13:47.20 from this slit.
00:13:49.08 And then the light ray is going to hit a position
00:13:52.26 which is x away from the center.
00:13:54.12 So, what's the intensity there?
00:13:56.13 What's really important
00:13:58.25 is basically the path length difference
00:14:01.04 of these two light rays,
00:14:03.06 and this distance we can calculate
00:14:05.04 as d times sine of this angle θ,
00:14:10.05 and when θ is reasonably small,
00:14:13.15 we can approximate it as x/z.
00:14:17.15 For constructive interference,
00:14:19.26 we know that this distance difference
00:14:22.13 has to be an integer times the wavelength, λ,
00:14:26.14 and this will give us bright stripes
00:14:28.24 in the image plane.
00:14:30.15 And given that,
00:14:32.01 we can figure out the difference between...
00:14:34.05 the distance between these stripes
00:14:36.10 is exactly z/d x λ.
00:14:39.26 So, if z/d is a huge number,
00:14:43.02 we can actually magnify the wavelength λ,
00:14:46.14 which is less than 1 micrometer,
00:14:49.06 to some distance that we can observe,
00:14:52.02 like this image of light
00:14:55.28 going through two slits,
00:14:57.10 and we can clearly see
00:14:59.03 these bright and dark stripes,
00:15:00.16 and that's the classical double-slit interference.
00:15:04.21 So, here,
00:15:06.24 we know how we can describe light as waves.
00:15:09.10 More precisely, light is an electromagnetic wave
00:15:14.03 governed by the set of Maxwell's equations.
00:15:16.16 It looks complicated and scary,
00:15:18.13 but in fact it's quite elegant
00:15:20.10 and simple to understand.
00:15:22.06 The first equation is basically the Coulomb's law
00:15:24.25 and it says that charges
00:15:27.05 can generate static electric fields.
00:15:29.09 And the second equation says that
00:15:31.13 the magnetic force lines form closed circles,
00:15:34.18 or basically there are no equivalents of charge for magnetic fields.
00:15:37.00 The third equation says
00:15:40.00 that the changing magnetic fields
00:15:41.22 can generate an electric field,
00:15:43.03 and the fourth equation says that
00:15:45.13 electric current and changing electric fields
00:15:46.25 can generate magnetic fields.
00:15:48.08 Now, putting the third and fourth equations together,
00:15:50.21 we have a changing magnetic field
00:15:52.19 generating an electric field,
00:15:54.13 and that electric field can further generate magnetic fields...
00:15:57.02 so we end up getting
00:15:59.12 a wave propagating in space.
00:16:01.27 That's the nature of light
00:16:03.29 and all the other electromagnetic waves.
00:16:06.26 And in fact, the coefficient here,
00:16:10.01 that tells how strong the electric field
00:16:12.17 and the magnetic field are coupled together,
00:16:14.05 determines the speed of light.
00:16:16.09 So, now we know that light
00:16:18.19 is an electromagnetic wave,
00:16:20.07 and now we have one more property for light
00:16:23.05 -- polarization.
00:16:24.27 And this is basically in which plane
00:16:27.00 the electric field of light oscillates in.
00:16:30.25 And our eyes cannot see polarization
00:16:32.21 but we can have a polarizer,
00:16:35.10 which is an optical element
00:16:37.15 that allows only one oscillation direction to go through.
00:16:42.03 And natural light is more or less unpolarized
00:16:45.07 -- the oscillations go in any random direction.
00:16:47.23 If we have a polarizer,
00:16:49.25 we have polarized the light,
00:16:51.21 the oscillation is only in one direction.
00:16:53.12 If we have another polarizer in the perpendicular direction,
00:16:55.09 we get no light to go through,
00:16:57.07 and this effect has been utilized very frequently.
00:16:59.17 For example, in photography,
00:17:02.07 the blue sky light is more or less polarized,
00:17:07.08 so we can use the polarizer to block some of this light
00:17:09.02 to get crispy images,
00:17:10.07 and that's also the same effect
00:17:11.24 that a polarized sunglass
00:17:13.09 can reduce the glare when we are driving.
00:17:16.17 So, beautiful equations...
00:17:18.21 problem solved.
00:17:20.11 In fact, that's how many people
00:17:23.00 in the late 19th century
00:17:25.05 thought for physics around that time.
00:17:28.02 Well, there are still two issues unresolved,
00:17:31.10 regarding light.
00:17:32.29 It's exactly these two issues
00:17:35.18 that have led to a fundamental change
00:17:37.10 of how we understood physics
00:17:39.05 in the early 20th century.
00:17:41.18 One of the issues is regarding the speed of light
00:17:44.22 and that has led to the birth of the theory of relativity.
00:17:46.29 The other issue is regarding
00:17:48.23 the energy of light
00:17:50.06 and that has led us
00:17:52.04 back to the age-old question
00:17:53.24 of whether light is a wave or a particle.
00:17:57.17 And then the birth of quantum mechanics.
00:18:01.10 A classic case for this issue is blackbody emission.
00:18:04.17 This is when you heat some object to a high temperature
00:18:07.29 and it starts to give out light,
00:18:09.14 just like filaments in lightbulbs.
00:18:12.03 With higher and higher temperature,
00:18:14.06 this object will first start to glow red,
00:18:16.13 then yellow,
00:18:18.08 then blue-white.
00:18:20.10 All the classic theories trying to explain this relationship
00:18:23.11 between color and temperature
00:18:25.18 are complete catastrophes.
00:18:28.05 Max Planck at that time
00:18:31.00 had a wild, wild, wild idea.
00:18:34.05 What if the energy of light is not continuous.
00:18:36.09 What if the energy of light comes in
00:18:38.20 discrete quanta with the energy of each quanta
00:18:41.02 proportional to the frequency of light.
00:18:44.13 Planck, at that time,
00:18:46.15 has absolutely no physical evidence
00:18:48.21 why the energy of light shouldn't be a continuous stream,
00:18:50.24 it's just a simple mathematic trick,
00:18:53.10 but this trick
00:18:55.20 solved the blackbody emission problem completely.
00:18:58.14 Albert Einstein had further extended this concept
00:19:01.08 to explain another experiment,
00:19:02.18 the photoelectric effect.
00:19:04.02 The photoelectric effect
00:19:06.00 is when you shine light on some metals.
00:19:08.01 The energy of light will kick out photoelectrons.
00:19:10.26 This is very much related
00:19:12.19 to how solar panels and digital cameras work.
00:19:15.27 Again, classic theory
00:19:18.11 failed to explain this effect,
00:19:20.22 because when you are trying to dump more energy
00:19:24.01 into the metal
00:19:25.25 by using a stronger or more intense light source,
00:19:28.28 you don't get electrons with higher energy
00:19:30.29 -- you simply get more electrons.
00:19:33.16 Einstein had found that,
00:19:36.05 if the light energy is discrete,
00:19:38.22 and if each quanta of light energy,
00:19:40.18 or each photon,
00:19:42.08 can kick out one photoelectron,
00:19:44.04 then the problem can be solved,
00:19:45.26 because the energy of the photoelectron
00:19:48.04 is more or less related to the energy of the photon,
00:19:51.15 which is related to the color of light.
00:19:54.03 And the intensity of light
00:19:56.08 is very much associated
00:19:58.13 to the number of photons,
00:20:00.04 and that's why it's associated
00:20:02.04 to the number of photoelectrons.
00:20:03.16 Although Einstein has been best known
00:20:06.29 for his contribution for the relativity theory,
00:20:09.06 the explanation of the photoelectric effect
00:20:11.02 is in fact what he got his Nobel Prize for,
00:20:13.15 and this is also how he got his name
00:20:16.12 into one of the units
00:20:17.28 -- 1 Einstein of light equals 1 mole of photons.
00:20:21.12 A classic example is if we have some system,
00:20:24.20 like a molecule,
00:20:26.11 that can have some resting, low energy state,
00:20:30.11 that we call the ground state,
00:20:32.24 and... for quantum...
00:20:34.16 quantum physics we know that
00:20:36.21 you can have some excited states
00:20:38.08 and the energy spectrum is not going to be continuous.
00:20:40.20 And so when you have a photon coming in,
00:20:43.29 only when the energy of this photon
00:20:47.00 -- Planck's constant times the frequency --
00:20:50.15 matches the energy difference,
00:20:52.05 the system will absorb this photon
00:20:53.23 and it goes from the ground state to the excited state.
00:20:56.15 The same thing happens for emission,
00:20:59.00 although for emission it's much more complicated,
00:21:01.01 so I'm not going to go through the details in that,
00:21:04.02 but this tells us that
00:21:07.20 the photon is the energy unit
00:21:11.08 for light to interact with matter.
00:21:13.12 And, well...
00:21:16.01 we have been describing light as waves,
00:21:19.22 now we're coming back to the view of light as particles,
00:21:22.28 and this has been a philosophical debate
00:21:27.11 for physics in the early 19th century...
00:21:29.16 for physicists in the early 19th century
00:21:31.11 for quite, quite, quite a long time.
00:21:34.16 And eventually people have been settling down with...
00:21:37.23 okay, light is both a wave and a particle
00:21:41.26 at the same time...
00:21:43.18 it's very, very hard to imagine,
00:21:47.01 but we have one experiment
00:21:48.24 to show how this can come in.
00:21:51.17 Well, it's still coming back to
00:21:54.03 the double-slit diffraction experiment...
00:21:55.28 double-slit interference experiment.
00:21:59.03 For better detection,
00:22:00.25 we are not using photons,
00:22:02.15 we are using electrons.
00:22:04.07 It's another thing that has
00:22:06.00 wave and particle features at the same time.
00:22:08.13 So, we have a screen
00:22:11.15 behind the two slits
00:22:13.26 and we have just a few electrons
00:22:15.27 shooting through a slit,
00:22:17.24 and here with 11 electrons
00:22:20.03 we have sprayed particles -- electrons --
00:22:22.27 everywhere, randomly scattered.
00:22:24.22 We don't see much of detail.
00:22:27.05 But then we have more and more and more electrons
00:22:32.12 when we collect...
00:22:34.05 and in this you can clearly see
00:22:36.14 the interference pattern becomes more and more clear.
00:22:39.08 So, in this case,
00:22:41.28 the electrons still are particles,
00:22:44.08 but the wave comes in
00:22:46.16 in describing the probability
00:22:49.15 of spatial distribution of electrons,
00:22:52.13 and this is kind of
00:22:54.29 how you can simply understand
00:22:57.07 why something can have both a wave
00:22:58.28 and a particle nature at the same time.
00:23:01.26 So, that's the end of the talk
00:23:04.02 and I hope, with this,
00:23:05.28 you can understand a little bit more
00:23:08.19 of what light really is
00:23:10.06 and, as well,
00:23:12.07 how we can use light for all our
00:23:15.04 microscopy experiments.

This Talk

Speaker: Bo Huang

Audience:

Researcher

Recorded: April 2012

Talk Overview

This lecture discusses the many aspects of light. Light can be described as rays, waves, or particles. Various aspects of all three of these descriptions are presented, such as ray tracing, wavelength, frequency, refraction, dispersion, diffraction, interference, the photoelectric effect and photons as the quantized energy of light.

Speaker Bio

Bo Huang

Dr. Huang’s research focuses on using super-resolution microscopy and single-molecule imaging to understand how proteins form large complexes and how proteins interact to regulate signaling. Huang is an Assistant Professor in Pharmaceutical Chemistry and in Biochemistry and Biophysics at UC San Francisco. Continue Reading