# Week 38 – Properties of Light

Some of our deepest scientific insights have come from the most basic of questions.  For our last lesson of the week, we will dig into the question:  What is light?

The short answer to the first question (what is light?) is that light is what we experience as a narrow band of waves of specific wavelengths within the visible part of the electromagnetic spectrum.  A particle of light is called a photon.  Visible photons (light) have properties of both a particle and a wave.  Photons travel in waves, and waves can be described mathematically by measuring wavelength, amplitude, period, frequency, and speed.

To visualize the parts of a wave, let’s bring in Bill Nye the Science Guy:

Thank’s Bill Nye!  Here’s what we learned:

In the vacuum of space, nothing moves faster than light.  In fact, we can say the speed of light is the cosmic speed limit.  In a vacuum (like space), light travels at 300 million meters per second (3.0 x 108 m/s).  Because this number does not change, it is a constant and is assigned the letter c (c = speed of light).  Side note: Thanks to Albert Einstein, you’ve probably heard of the equation E = mc2. In words, the equation says that energy (E) is equal to mass (m) times the speed of light (c) squared.  You already knew that c = speed of light!

Ever wondered how long it takes for light from the Sun to reach Earth?  Click here to work through the math and find out!

To complete our study of the properties of light, we need to introduce Planck’s constant (h):

Next, let’s revisit the parts of a wave and make some connections:

The notes above introduce Planck’s constant, h, which has units of Joule • seconds.  Planck’s constant (h) relates a photon’s energy (E) and frequency (f).  Frequency is defined as the number of complete waves that pass through a point in one second.  The faster a light wave is traveling (greatest speed, measured in meters per second, m/s), the higher the frequency (f, waves/second).  Therefore, the faster a light wave is traveling, the higher the energy (E) of the wave.  Energy has units of Joule • meters.  Waves with the shortest wavelengths (λ) have the highest frequency (f) and therefore have the greatest energy (E).

If you followed all that (and I have no doubt you did!) you are ready for an introduction to Quantum Mechanics.  Ars Technica has a fantastic 7-part series of articles focusing on Exploring the Quantum World.  The Crash Course videos below are well worth the watch as well.  Enjoy!

# Week 38 – Fetal Pig Dissection

While not nearly the same experience as dissecting in person, the videos below are the next best thing to learning about the anatomy of an organism with organ systems remarkably similar to humans: pigs!  Watch, learn, and consider following along with the Fetal Pig Dissection Lab guide.

The videos below are recommended but may not be accessible from school district computers due to age-restriction settings.

When finished, return to Week 38 – Dissection Lab and continue working.

# Week 38 – Star Spectra

We’ve reached the end our our learning this school year.  Appropriate, perhaps, that we end where we began: in the stars.  Way back in Unit 1, you learned that stars fuse lighter elements like hydrogen and helium to form heavier elements up through iron.  Elements with more protons than iron are created when stars go supernova. Plants and animals (yep – humans are animals) are made of star stuff – we are quite literally the product of exploding stars.

We also conducted flame tests, showing that metal cations are responsible for producing specific colors of flame when ionic compounds are burned.  Now we understand that our perception of color is a result of photoreceptors in our eyes being capable of detecting specific wavelengths of electromagnetic radiation.  When those receptors are activated, they send information to our brain which then decodes the signal into our perception of light and color.

When we look up at the stars, we are looking back in time, as it takes time for light to travel from its source to our eyes here on Earth.  The more distant the object, the further back in time we see.  It’s not too hard to imagine there might be organisms billions of light-years away that witnessed the supernova (singular) or supernovae (plural) that launched the atoms within you and me toward our remote location within the Milky Way galaxy.  The force of gravity eventually caused those atoms to coalesce to form our Sun and the planets that orbit it, including the Earth.  After 4.5 billion years, here we are, studying the stars:

Anyone who would like to invest further in their understanding of the stars should email me for a copy of the handout that goes along with the Star Spectra Gizmo.  This activity is purely optional and available for your own personal growth.  It will not be entered in the grade book.

# Week 38 – Light and Color

Welcome to Week 38!  For our final lesson of the 2019-20 school year, you will be exploring the connection between light and color.  Whether you are taking physics or any of our other science electives next year, this lesson will be a great preview.  Let’s get to it!

That’s it!  No new assignments this week (spoiler alert: no new assignments next week either).  Please make sure you have everything turned in by June 19.  It has been my absolute pleasure teaching you chemistry this year.  What a year to remember!

Remember, you can email me any time.  Office hours for Science are Tuesdays from 11am-12pm and Thursdays from 1pm-2pm.  Check your student Gmail for Zoom instructions.

# Week 38 – How long does light from the Sun take to reach Earth?

Our live are largely built around the rising and setting of the Sun in the sky each day.  Our biology is intimately connected to this via our circadian rhythms:

Have you ever wondered what would happen if the Sun just suddenly blinked out of existence?  When would you know?  Turns out, it takes time for light to travel from the Sun to the Earth.  We’ve just learned that light travels at a constant speed, c, in a vacuum like outer space.  We know that c = 3.0 x 108 m/s.  To calculate how long it takes light to travel from the Sun to Earth, we need to know the distance between the two.  While the orbit of Earth around the Sun is not a perfect circle, on average the Earth is about 93 million miles (mi) from the Sun.  Time for some math!

Have: c = 3.0 x 108 m/s and distance = 93 x 106 mi

Want: Time it takes light to travel from the Sun to Earth

Need: Connection between meters (m) and miles (mi)

Another quick Google search tells us there are 1609.34 meters in 1 mile.  We are in business!

Calculation: (93 x 106 mi) x (1609.34 m / 1 mi) x (1 s / 3.0 x 108 m) = 499 s

Analysis: It takes about 499 seconds for light to travel from the Sun to the Earth!  Divide 499 by 60 and that gives us about 8.3 minutes.  So if the Sun blinked out right now, at this very instant, we wouldn’t know until 8.3 minutes from now.  The Sun is our nearest star, and it still takes 8.3 light-minutes for its light to reach us.

The second closest star to Earth is called Alpha Centauri.  Alpha Centauri is actually a triple star system (three stars in orbit around each other) located approximately 4.37 light-years from Earth.  Traveling at the speed of light, c, it would take 4.37 years to reach Alpha Centauri.  How many miles away is that?

4.37 light-years x (365 days / 1 year) x (24 hours / 1 day) x (60 minutes / 1 hour) x (60 seconds / 1 minute) x (3.0 x 108 meters / second) x (1 mile / 1609.34 meters) = 2.57 x 1013 miles, or 25.7 trillion miles away!

That’s a long way!  It also means that if you look at Alpha Centauri in a telescope (or just look in the right part of the night sky), you are actually seeing light that left the star system 4.37 years ago.  You are literally looking back in time!  In fact, every time you look up in the night sky, you are looking back in time.

If traveling 4.37 years at the speed of light seems like a long time, don’t despair.  The distance from Earth to the nearest planet outside our solar system is a bit less.  Discovered in 2016, the planet Proxima Centauri b orbits Alpha Centauri and is “only” about 4.2 light-years from Earth.

Last week, MIT Technology Review announced the likely discovery of an Earth-like planet around a Sun-like star. (To be more accurate, and to give you a sense of how the scientific process works, the exoplanet was observed, the findings were written up into a scientific article and submitted to the scientific journal Astronomy & Astrophysics on October 16, 2019, and after successfully completing the peer-review process, the article was accepted for publication on May 3, 2020 and then published by the journal on June 4, 2020.  Here is the link to the published article.)  The exoplanet is named KOI-456.04 and is 3,140 light-years from Earth.

While humanity hasn’t yet engineered a solution for how to accelerate a large spacecraft to near the speed of light, the video below introduces some important concepts regarding near light-speed travel.

Finally, if you think a bit more about the idea that looking at the stars is like looking back in time, the same hold true for Earth.  An alien pointing a telescope at Earth would be looking back in time at Earth as it was when the light left Earth.  If the alien is currently 65 million light-years from Earth, then the light they are observing today through their telescope left Earth 65 million years ago.  Is there a sufficiently powerful telescope that would allow the alien to actually see dinosaurs on Earth?  So glad you asked!

# Week 38 – Career Exploration: Butcher

Every year when we do dissections, there are always students who are shocked to find out they are absolutely fascinated by anatomy.  In fact, the students who are often the most reluctant before the dissection lab tend to be the most excited about it by the end.  Understanding the anatomy of different animals not only helps us better understand the world around us, it helps us better understand ourselves.  An appreciation of animal anatomy can open our minds to considering a variety of career paths.  Science and Medicine have vast numbers of careers that require varying levels of understanding anatomy: doctors, nurses, veterinarians, dentists, scientists, and all of the technical staff that support them.  But there are so many other careers out there.  Chefs and Head Cooks have to understand how to prepare different cuts of meat.  Butchers are even closer to the source: they are experts in the field of removing meat from a source animal and providing those cuts for us to eat.

It’s important to understand where our food comes from, and to appreciate the people who make it possible for us to eat without having to go out and catch our own food.  In the future, the career of butcher may transform into a career of food scientist specializing in the production of lab-grown meat.  Check out that future career below:

When finished, return to Week 38 – Dissection Lab and continue working.

# Week 38 – Electromagnetic Spectrum

When we think about light, we think about what can be seen.  If you’ve ever looked through a prism, you understand that white light is actually a collection of all the colors of the rainbow.  The visible spectrum consists of all of the light that we can see with our eyes.  Let’s go back to the rainbow.  The acronym ROYGBIV is a helpful way of remember the colors of the rainbow in “order” where R=red, followed by Orange, Yellow, Green, Blue, Indigo, and Violet.  It turns out that red light has a wavelength range of 620-750 nanometers (nm), while violet light has a wavelength range of 380-450 nm.  Remember, the shorter the wavelength, the greater the energy.  Therefore, because violet light has a shorter wavelength than red light, violet light is higher energy than red light.  We have specialized photoreceptor cells in our eyes that are excited by specific wavelengths of light.  When white light strikes an object, some wavelengths of light are absorbed by the object while other wavelengths are reflected. The color of an object is actually the wavelength of light that object does not absorb!  When reflected light is detected by our eyes, we see color.

Now for the really interesting part: visible light only comprises a small part of the larger electromagnetic spectrum.  The shortest wavelength of electromagnetic radiation is on the scale of 10-12 cm (smaller than the diameter of an atom).  Remember, the shorter the wavelength, the greater the energy.  Photons with the shortest wavelength are called gamma rays and they are powerful enough to shred DNA.  We learned about gamma (γ) rays earlier in the school year during our study of nuclear decay (Lesson 15).  Viewing the night sky with gamma ray detectors gives us a very different perspective about the structure of space compared to looking with our eyes.

At the other end of the electromagnetic spectrum are the radio waves, with wavelengths on the scale of 104 cm (the height of the Statue of Liberty).  Radio waves are emitted by stars and planets and can be detected with radio telescopes.  When the night sky is scanned using a radio telescope, we once again see structures in space that are invisible to our eyes.

To learn more about the visible spectrum, gamma rays, radio waves, and all the rest of the electromagnetic spectrum, visit NASA’s Tour of the Electromagnetic Spectrum and prepare to be amazed with the richness of the Universe!