## optics

• ### Demo Highlight: Laser and Double Slit

One popular demonstration in our collection for introducing concepts of wave optics is M1-11: Laser Interference: Fixed Double Slits.

Collimated light waves come from the laser and pass through a pair of narrow slits in the slide; the light passes through and then projects on the distant screen. But light travels as an electromagnetic wave, so when the light comes out of the two slits, it forms two wavefronts, just like ripples from two stones dropped in a pond. These two wavefronts can interfere with each other, as we can model with this pair of overlapping concentric circles. Where two peaks or two valleys of the wave pattern line up, they add together, interfering constructively; when a peak and a valley overlap, they cancel out, interfering destructively. The same happens with light waves; the light from the two slits overlaps, and creates a pattern of bright spots (constructive interference) and dark spots (destructive interference).

When using this in class, we can adjust the slide to use different sets of slits, with different slit widths and different spacing between the slits. This is a good opportunity to challenge students to predict how changing these two variables will change the resulting interference pattern.

The spacing between the bright and dark fringes ultimately depends on three things: the distance between the slits and the screen, the wavelength of the light, and the spacing between the two slits. The first is probably obvious from your everyday experience – if you step farther away from the point that light spreads out from, it spreads out more!

Likewise, if you increase the wavelength, the space between the peaks of the waves gets larger, so it’s not a surprise that the spaces between their overlaps would get bigger, too. The effect of the slit spacing, though, takes a moment to think about. If the slits move closer together, the two wavefronts are more and more similar; so the differences between them, the points where they fully cancel out, are farther apart. So increasing the slit spacing decreases the spacing of the fringes, and decreasing the slit spacing increases the spacing of the fringes.

We can see this modeled in a ripple tank simulation here in the Physlet Physics collection at AAPT’s compadre.orghttps://www.compadre.org/Physlets/optics/prob37_7.cfm Use your mouse to measure the positions of the peaks relative to the double slit at the base of the image.

To experiment with this at home, check out this PhET Simulation at the University of Coloradohttps://phet.colorado.edu/sims/cheerpj/quantum-wave-interference/latest/quantum-wave-interference.html

Use the button on the right of the simulation screen to activate the double slit barrier. You will then be able to simulate precisely this experiment!

Light of a particular frequency is released from a source, passes through a double slit, and then is projected on a virtual “screen.” So you can see both the interference pattern as it is formed through space, and the final pattern that you can detect at a distance.

Carrying out the computations for this simulation is processor intensive, so it may run slowly; and as you can see in the image, the resolution is limited. There are limits to how well software can simulate reality!

But at the same time, we can use the simulator to try things we can’t easily do in the laboratory. The slider at the bottom will let you change the laser’s frequency, variable along the full visible spectrum and a bit beyond (fun challenge: is there anything remarkable about the spectrum selection here?).

The sliders at the right will let you change some of the physical parameters of the barrier. You can modify the width of the slits, or the separation distance between the two slits. The “Vertical position” slider lets you adjust the distance between the slits and the screen.

So here we can see the value of combining both real-world experiments and simulations – each alone is useful in learning, and each has its benefits and limitations, but the combination lets us see things we cannot with either one or the other on its own.

• ### Demo Highlight: Polarizers and Light Source

This week we’re exploring the physics of polarized light! We have several demonstrations of polarization in our collection; two of the most popular are perhaps the most straightforward: M7-03, which consists of two polarizing filters (or polarizers) and a light source; and M7-07, which adds a third polarizer.

Light is an electromagnetic wave, made up of oscillating electric and magnetic fields. We call a wave polarized when this oscillation has a particular orientation as the wave travels through space. The direction of the electric field defines the direction of polarization of the wave.

You can see two polarizers in action in this video starring Prof. Manuel Franco Sevilla.

A polarizer like this, also called a polarizing filter, passes only light of a given linear polarization. So it acts as a filter; if the first one is polarized vertically, it will block any horizontally polarized component of the light, and pass only the vertically polarized components. When the two polarizers are in line (which is to say that their axes of polarization are aligned), the second polarizer has very little effect on the light passing through. The first polarizer creates linearly polarized light; the second one, with the same polarization, passes nearly all the light that came through the first one. If we rotate the second polarizer, though, the axis of polarization, the direction in which it requires light to be polarized in order to pass through, rotates. So when the second polarizer is out of line with the first polarizer, it is only passing whatever component of the light from the first polarizer is also in line with the second one. As they rotate farther apart, that component is reduced. Once the two polarizers are fully 90 degrees apart, they no longer have any component in common, so together they pass no light at all! If the first one is polarized entirely vertically, and the second is polarized entirely horizontally, they are perpendicular.

Which is an important aspect of physics that this demo shows: that linearly polarized light can be treated as having separable components, just like we can separate the component vectors of linear motion of an object in space, and a polarizing filter passes only light components parallel to its polarization. So if we add a third polarizer, canted with respect to the other two, it can pass components parallel to its axis of polarization, however we choose to orient that. This can have some interesting results, as we see in the next video, starring Dan Horstman.

The passage of light depends on the orientation of the current wave’s polarization and the filter it encounters – so adding the third filter actually could allow more light to pass!

• ### Demonstration Highlight: Cosmic Ray Detector

Demonstration P4-04, the Cosmic Ray Detector, has recently been upgraded. PhD student Liz Friedman shows it in action in this video.

When energetic particles from space hit the upper atmosphere, they create a cascade of particles in the air around us. Even a single cosmic ray might create a whole shower of particles in our atmosphere. They’re invisible, but with the right apparatus we can detect them. Each of the two dark blocks in this device is a scintillator. When a particle passes through one scintillator, it makes a tiny spark of light, which is picked up by a sensitive photodetector. By checking for correlations between the two paddles, we can spot which sparks are being caused by particles passing straight through from space.

These particles stream around and through us all the time, harmlessly; but it’s pretty amazing that we can build a tabletop device that can measure them!

You can read more about Liz’s adventures in physics, hunting neutrinos in Antarctica, in last summer’s Odyssey magazine https://cmns.umd.edu/news-events/features/4642

• ### Demonstration Highlight: Focusing Heat

Welcome back! This week we’re experimenting with optics, as graduate student Naren Manjunath introduces us to demonstration L3-18, in which we focus light with a parabolic mirror. Check out his video below:

This demonstration uses one of the old overhead transparency projectors that focuses the light by a large parabolic mirror under the platform. These used to be standard issue in every classroom; but in the era of digital presentations, they only see use on special occasions.

The mirror focuses essentially all of the light striking it, both visible light and invisible (to our eyes) infrared light, to a single point. All of the energy carried by those light waves arrives at that point. When we place a piece of paper at that point, it absorbs that energy and heats up rapidly, bursting into flame.

Many old fashioned projectors like this would have a “heat filter” built in, a piece of glass treated to be transparent to visible light but reflect infrared light. This was to prevent exactly this from happening, since you don’t want your materials bursting into flame in the middle of class! Even some recent projectors have had similar filters, although modern light sources (such as LEDs) that produce light only in the visible wavelengths desired make them unnecessary.

• ### Demonstration Highlight: Gravitational Lensing Model

In astronomy, gravitational lensing is the phenomenon whereby gravitational forces around a mass bend light in a way similar to a conventional refracting lens does. When a large mass lies between an observer and the light source they're observing, sometimes that mass can bend the incoming light, causing the source to appear in a different location, or even in multiple locations at once. This can even allow an observer to see a light source that would otherwise be unobservable due to being directly behind another object.

We have a model of this in our collection, as demonstration E1-21, a glass lens that is specially shaped to produce a similar effect to gravitational lensing. Light is bent more the closer it is to the lens' center axis. As a light source moves behind the lens, you can see the source appear to be displaced, or even see one source appear to become several, or become a ring of light around the center of the lens. All of these phenomena can be seen from gravitational lensing in space as well.

In this drawing, you can see a cross section of part of the lens. The changing curvature produces the gravity-like effect of increasing refraction towards the center.

Try experimenting with this simulation https://slowe.github.io/LensToy/ to see it in action in a starfield!

• ### Demonstration Highlight: Laser Waterfall

This week’s Demonstration of the Week is L5-11: The Laser Waterfall. Check it out in this video below, presented by physics student Ela Rockafellow, a leader of our Society of Physics Students.

The laser here is illustrating internal reflection: depending on the index of refraction of the water and the angle the light hits it at, more light can be reflected back and forth within the stream of water than passes through it. At a certain point, it exhibits total internal reflection, where essentially all of the light is traveling along the stream rather than heading straight out the side.

You can experiment with this phenomenon in this TIR simulation from Boston University. You can change the index of refraction of the outside medium (usually about 1 for air) and the interior (about 1.333 for water, around 1.5 for most glass, for example. Try different combinations and see whether the light stays in the stream or passes outside!

This is the same physics that lets fibreoptic cables carry signals over long distances – the light stays within the cable, traveling with very little loss, and allowing us to operate much of modern communications technology.

• ### Demonstration Highlight: More Fun with Polarization

Earlier this year, we took a look at new videos of our popular demonstrations of the polarization of light, demos M7-03 and M7-07. This week, we’re returning to the topic to check out some simulations that let you try this at home!

The first simulation, by Tom Walsh at the oPhysics site, lets you model a wave as it passes through a series of polarizing slits. You can independently adjust the angle of up to three such slit-filters, and see how the resulting wave responds. Experiment with it at https://www.ophysics.com/l3.html.

The second simulation, created by Andrew Duffy and hosted by Boston University, shows a graph of light intensity as it passes through a series of polarizing filters. Again, you can independently vary the angle of each of three filters, and now you can see how this changes the intensity of the light after each. Try it at http://physics.bu.edu/~duffy/HTML5/polarized_light.html.

Speaking of trying things at home, this isn’t a purely academic question – this is how polarized sunglasses cut the glare from sunlight reflecting off the road without preventing you from seeing where you’re going! Try rotating a pair of polarized sunglasses and see how their effect changes with angle. It may look something like the animation below. We do this in the classroom, too – check out demonstration M7-18

• ### Demonstration Highlight: Plane Mirror

Today we’re looking at a simple but powerful demonstration of optics, Demo L2-01: Optical Board and Plane Mirror.

A bright white light source is directed through a baffle with several slits, producing a set of rays. Lenses are used to collimate these rays, and they are then reflected off of a long plane mirror. If the lenses are adjusted such that the incoming rays are approximately parallel, the reflected rays will be as well.

Unlike light diffracted through a lens or prism, reflected light from a surface is unaffected by the frequency of the light. The reflection off of a flat mirror is dependent only on the angle the light strikes at. This demonstration shows that the angle of incidence is equal to the angle of reflection.

You can experiment with this at home – just find any flat mirror (there’s often one on the wall above the sink), shine a light on it with a flashlight or phone, and measure how the reflected light moves when you move the light.

To see this in mathematical detail, check out this simulation at oPhysics: https://www.ophysics.com/l9.html . You can see how light rays from the tips of an object will reflect off a surface. Try dragging the object (a big blue arrow) around to see how the reflection moves. The simulation can also trace out virtual rays – the light rays “behind” the mirror that aren’t really there, but are the geometric extensions of the reflected rays. Following the virtual rays lets you predict where you eye would “see” the reflected image of the object being.

• ### Demonstration Highlight: Reflecting Telescope Models

The most popular design of telescopes for astronomical research is the reflecting telescope. First developed in the 17th century, the typical reflecting telescope uses a curved primary mirror to focus incoming light, and a secondary mirror to direct that light to an eyepiece or sensor. There are many variations on the design, but the underlying principle is the same: light is focused largely by reflection, rather than refraction as in a lens-based Galilean refracting telescope, which allows them to avoid both the chromatic aberration common to lenses and the weight required to create very large ones. Reflecting telescopes have a long history in astronomy and astrophysics, from William and Caroline Herschel to the Hubble Space Telescope and beyond.

We have two models in our collection of how reflecting telescopes work. Demonstration L7-14 models the behaviour of light in a reflecting telescope on our optical board, which uses real optical elements to create a viewable two-dimensional ray diagram.

We also have a static model, demonstration E2-54, which shows the construction of a typical reflecting telescope with strings to represent the paths of light rays through the device. The two are best used in combination to show students how this system lets us observe distant objects.

You can experiment with this at home as well, with this simulation from JavaLab. You can adjust the angle of the incoming light and see how it reflects off the primary mirror and forms an image at the secondary mirror, and use an eyepiece lens to focus it on an observer. Try it out at https://javalab.org/en/newtonian_reflector_en/

• ### Demonstration Highlight: Refraction

Welcome back! This week, we’re taking a lot a three different demonstrations that are very valuable for introducing students to optical refraction. Refraction is the process by which light bends as it passes from one medium to another. Consider demonstration L4-03, with a metal rod sitting in a small tank of water; depending on the angle you view it from, the rod may seem to bend or break at the surface; there may even appear to be more than one of it, if you look in through the corner of the tank! And in demonstration L4-06, you can see a laser beam enter a similarly-sized tank of water, changing direction as it passes through the water’s surface.

The degree to which light bends depends on the difference in the index of refraction of each medium, which relates to the relative speeds of wave transmission in the medium. Prisms, lenses, and many other optical devices rely on carefully chosen angles and indices of refraction to create optical effects.

This public domain animation from Wikipedia does an excellent job of illustrating the effect. As the waves pass from one medium to another, the change in propagation speed induces a change in angle, as seen here.

You can see this happening on a larger scale in demonstration L4-01. Three rays of light are directed through a heavy slab of transparent plastic. The index of refraction of the plastic is much greater than that of air, so the light bends. In the photo below, you can see the beams change direction as they enter and exit the slab.

You can try it for yourself at home with this simulation in the PhET Interactive Simulations Collection: https://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html

• ### Demonstration Highlight: Tyndall's Experiment

Today we’re featuring an interesting experiment that explores the classic question of springtime: Why Is The Sky Blue?

Demonstration M7-31: Tyndall’s Experiment uses a chemical reaction to simulate light scattering through Earth’s atmosphere, but in a tiny container. Solutions of sodium thiosulfate and hydrogen chloride are dissolved in water, then mixed together in a glass tank in front of a light source. These chemicals react and form new particles. The resulting particles form a colloid, a liquid with particles suspended throughout, giving it optical properties that let it simulate our much thinner atmosphere on a smaller scale. Over the course of several minutes, as more and more particles form, we see more and more light scattered out the sides of the tank, while less and less passes straight through. As it goes through this transition, the light coming out the end changes from nearly white, to orange, to red, perhaps finally vanishing entirely.

In the atmosphere, light scatters off of molecules and other tiny particles in the air. The angle at which light scatters depends on the wavelength of the light; shorter wavelengths scatter farther than longer ones. Visible light consists of a spectrum of various wavelengths, with blue light having shorter wavelengths and red light having longer wavelengths. This means that blue light scatters more than red.

This property in the atmosphere was written about by physicists John William Strutt, Lord Rayleigh, and is commonly called Rayleigh Scattering as a result; but the experiment to observe similar effects in a liquid was developed by physicist John Tyndall, hence the name of the demonstration. John Tyndall was well known in his time for his interest in education, and gave public lectures with demonstrations, much like we do today! He also carried out important research on the effect of carbon dioxide in the atmosphere, what we now know as the greenhouse effect.

As the Earth turns, the angle we see the sun at changes. Early and late in the day, we’re seeing the sun through the atmosphere at a different angle, and looking through more atmosphere as a result. So at those times we mainly see red light; the shorter blue wavelengths have been scattered away. At midday, with the sun shining directly down on us, we see more blue!

A fascinating bit of physics, and also very beautiful.

• ### Demonstration Highlight: Visible and Invisible Spectra

A recurring favourite optics demonstration in many of our classes is N1-05 Spectra: Visible and Invisible. This seemingly simple setup can show us some important truths about electromagnetic radiation.

A carbon arc lamp is used to create a bright, broad-spectrum white light. This is an example of what is known (confusingly) as blackbody radiation, the light that an object emits due to its temperature. Technically, a hot object radiates light across many frequencies, but what we think of as its “color” is made up of the frequency ranges with the greatest intensity, which depends on the object’s temperature. This we see here a bright blue-white light, with high emission across all the visible frequencies.

Lenses right next to the source focus this light onto a narrow slit, which then passes a narrow beam of light, focused by an additional lens, to a prism. The prism refracts the light at different angles depending on its frequency. So projected onto the wall we will see, rather than a spot of bright white light, a spectrum of all the colors making up the light.

But here’s what’s interesting about this carbon arc lamp. Not all of the light is in that visible range! We have a fluorescent screen, which glows in the visible light range when it absorbs higher-frequency ultraviolet light; using this, we can see that there are bright bands of ultraviolet light off beyond the blue end of the visible spectrum on the wall.

So does this mean there’s something off beyond the red end as well? To check this, we have a thermopile, a horn containing a series of sensors that sense when they get warm. Using this, connected to an audio oscillator that changes pitch when the thermopile senses heat, we can scan across the wall… and indeed, we can hear the pitch change when the horn is in the dark area past the red end of the spectrum. There is infrared light hiding here, frequencies too low for us to see!

You can vary the temperature of your source and see how that changes not only the intensity of light, but its color – or, more accurately, its distribution of color. A light source can radiate light across a broad range of frequencies, which may be centered within, above, or below the range we can see. Try it out for yourself, compare the spectra of a household lightbulb to the Sun or another star, and see if you can guess the temperature of the arc lamp we use here!

• ### Quantum Demonstration and Simulation: The Hydrogen Atom

We love our demonstrations, but there are some things you can’t easily demonstrate in the classroom, either because the physics isn’t compatible with that environment, or because the scale is beyond what we can practically see. This is where simulations can be valuable, in letting us go beyond what we can do on the tabletop and look inside the black boxes.

The quantum nature of the hydrogen atom is a good example. We can demonstrate the emission spectrum of hydrogen with the Balmer Series demonstration P3-51, and we have simple models of electron orbitals for more complex atoms, but how can we look at the structure of the hydrogen atom itself?

Here are some simulations available for looking inside our smallest atom.

• ### Space News: The Nancy Grace Roman Space Telescope

NASA announced last month that their upcoming infrared observatory project had been named the Nancy Grace Roman Space Telescope. Today, we’re going to take a brief look at this project, Dr. Roman, and the role of physics in astronomy.

Nancy Grace Roman (1925-2018) was the first Chief of Astronomy for NASA, and the first woman ever to hold an executive-level position there. A graduate of Western High School in Baltimore, and later of Swarthmore College and the University of Chicago, she began her career in astronomy research specializing in the emission spectra of stars.

Spectroscopy is the study of the spectrum of light, the individual frequencies and colors that make up the light we see. Every element and compound emits its own distinctive pattern of frequencies of light, based on the structure and energy of the electrons within the atom. By analyzing the light from stars, we can use these distinctive patterns as a kind of fingerprint to identify the chemical makeup of distant stars and planets. Our demonstration collection at UMD Physics has many demonstrations about spectra and spectroscopy; be sure to click here and check them out!

Not all light is visible to our eyes; lower frequency light is below the range that we can see, in infrared wavelengths. Space telescopes are often valuable for observing this invisible light without interference from our atmosphere, just as they are in the visible spectrum.

Despite her skill as a researcher, the widespread discrimination against women in the sciences made it difficult for Dr. Roman to advance her career in academia. Eventually, she moved to working in government instead, first joining the Naval Research Laboatory and then NASA. She essentially created the astronomical science program at NASA, plotting its course for decades to come. She was a key player in the development of many of NASA’s research satellites, including Uhuru, the first X-ray astronomy satellite; and she was a leader in the creation of the Hubble Space Telescope. She advocated that NASA science should be for everyone, and ensured that their research and data were publicly available.

Dr. Roman retired from NASA, and began a second career in scientific computing. She learned programming at Montgomery College and went on to work as a contractor specializing in scientific data management, eventually returning to NASA as a contractor to manage the Astronomical Data Center at Goddard Space Flight Center.

Dr. Roman worked hard to inspire more women to become scientists and leaders in science, and we can hope to follow in her footsteps.

• ### STEM News Tip: Raman Spectroscopy for COVID-19 Testing

At the APS March Meeting, Prof Miguel José Yacamán of Northern Arizona University reported on their NSF-funded research to develop a new method of rapid testing for COVID-19 and other coronaviruses by using surface-enhanced Raman spectroscopy.

• ### STEM News Tip: Titan and its Atmosphere on APOD

Today's Astronomy Picture of the Day shows seven different views of Saturn's moon Titan. The surface of Titan is ahrd to see at most visible wavelengths, due to the extreme scattering of light due to particles in the atmosphere. But scattering is wavelength-dependent; anyone who has seen our demonstration M7-31: Tyndall's Experiment can report that you can see very different scattering of light at different frequencies - this is what gives us our rich red sunsets and the brilliant blue sky overhead at noon. To see the surface of Titan, astronomers can use even longer infrared wavelengths, to penetrate the clouds and see what's happening down there.