Reflection and Making a Cellphone “Hologram”

 Reflection

The property of reflection may well be the property of light we are most familiar with and dependent upon. After all, without reflection we wouldn’t be able to see. Light, either from a natural or an artificial source, leaves the source and bounces off an object (reflects). If the light reaches our eyes, we can see the object.

Once we understand the various properties of light, such as reflection, we can develop technologies that use the properties. For example, we use reflection for mirrors, telescopes, and microscopes. Some makeup manufacturers even use an understanding of reflection to produce makeup that makes our skin look smoother and more perfect than it really is.

We can also have fun with understanding light—how about “holo” nail polish? There will have to be a separate posting on that, because the science is more involved than simple reflection. But the cellphone “hologram” does, in fact, use simple reflection.

Thanks to Randy Attwood for help making the video.

Making a Cellphone “Hologram”

There are many sites on the Internet that describe how to do this. Just search for “make a phone hologram.” Here’s what I did to make the cellphone “hologram” in the video.

Materials

You need a piece of clear plastic; it must be relatively rigid but doesn’t need to be 100% rigid. Some sites recommend a clear CD case, but that can be tough to cut using an Exacto knife, especially if you want to involve kids. I used the clear plastic covering, or lid, from a box of Christmas cards. You could also use a transparency, if you have one lying around. You’re going to cut out 4 pieces to make a trapezoid, which needs to be able to support itself when the pieces are taped together. So keep that in mind while choosing your clear plastic material. And it must be large enough that you can cut all 4 trapezoids from it. For an idea of size, see the image below; it has dimensions on it.

  • Pencil
  • Ruler
  • Graph paper is helpful but not necessary.
  • Scissors or Exacto knife (have kids use scissors; adults can use the Exacto knife)
  • Clear tape
  • Cellphone with Internet

Instructions

  • Draw four trapezoids on the clear plastic. See dimensions below. Don’t change the dimensions. We want to end up with a specific angle of reflection.

  • Cut them out.
  • Tape them together, with a slanted side attached to another slanted side. You want to end up with a free-standing object, so all sides are joined.

  • On the cellphone, search YouTube with the term “phone holograms”; for example, the video I used is from here: https://www.youtube.com/watch?v=BZ6fun_RKfk 
  • Make sure the brigthness setting on the cellphone is at its highest.
  • Have the cellphone on a table or desk such that people can crouch down and be eye level with the flat phone. Have the video on the cellphone ready to play. Place the structure, small opening on the bottom, on the phone over the video. Crouch down to be eye level with the phone. Start the video, and turn out the lights and watch the “holograms.” They are faint and best seen in a darkened room.

I keep using quotes when I refer to the cellphone “hologram” because these are not real holograms. They are light reflections. They just look like holograms, which makes them so cool. You need lasers, mirrors, and wave interference to make a real hologram.

The Science

The dimensions of the trapezoids are such that the sides are designed to be at a 45º angle to the surface of the phone. When you reflect an image in a transparent screen (the clear plastic) at that angle, you see a “virtual” image, which is just a reflection of what the cellphone screen is playing.

You can have your kids or students view the images from each side of the plastic trapezoid. They may be able to figure out that they see the image of the video on the side they are watching.

Fluorescence and Colour Models

Fluorescence

Some materials will glow when lit with an ultraviolet (UV) light. But when you take the light away, the glowing stops. A phosphorescent material will continue to glow after the light source is removed.

Let’s look at olive oil. The molecules in olive oil absorb the energetic UV light, from, say, a UV pen light. And the molecules get “excited.” When they are “excited,” they release energy often in the form of visible light (heat is released, too). In the case of olive oil, the light is red. As you can see in the video, the light emitted by the canola oil molecules is greenish, and the light emitted by the rock salt molecules is purplish.

Thanks to Randy Attwood for help making the videos.

Colour Models

In a novel I read recently (The Last Widow, by Karin Slaughter), one of the main characters described making a UV light from a cellphone. Will, the character in the book, coloured the light on his cellphone with a blue Sharpie, covered the blue with see-through tape, then coloured over the blue with a purple Sharpie, then covered that with tape. He then turned on the cellphone’s flashlight and used the “UV light” to read a message written with urine. (Urine is fluorescent under a UV light, but it would have been invisible to the bad guys in the story in white light.)

While an exciting application of science as well as a clever addition to the plot, unfortunately, the science does not pan out. It might have worked if Will used filters, such as filters used with a camera, because some light gets through the filter and the rest is absorbed. You can’t make UV light with a material that is coloured by pigment, such as paint, fabrics, and printer ink. The light just gets absorbed. I tried it anyways, by the way. It didn’t work. Some light got through the Sharpie markings but it certainly wasn’t UV. (But the blue colour from the Sharpie came right off the cellphone light; it may be permanent on some materials, but not the glass light.)

There are two models of colour that describe what I wrote above: the additive model of colour and the subtractive model of colour.

According to the additive model, we “add” the three primary colours of light to produce any colour of light, including white. Varying the amounts of each colour of light will give different hues and colours.

With the subtractive model, we are “subtracting” different colours through the processes of absorption and reflection: a red sweater is red because of the red pigment in the dye used to colour it. The sweater absorbs all the colours except red, and the red is reflected to our eyes.

Lunar Libration

The Moon only presents one half, or 50%, of its surface to us, what we call the near side of the Moon. (We never see any more than a small amount of the far side of the Moon from Earth because of the tidal lock between Earth and the Moon, but that’s for another blog post.)

However, we can actually see about 59% of the Moon’s surface over time, due to a phenomenon called libration. Libration is the real or apparent “wobbling” of an object, in the left-right direction and/or in the up-down direction. As a result of lunar libration, we get to see more of the Moon’s surface. Libration is noticeable to any observer, as shown in the two images.

The lunar libration predicted for January 2021 is based on data collected by NASA’s Lunar Reconnaissance Orbiter (LRO) satellite, which has been orbiting the Moon since 2009. We can see both the left-right libration and the up-down libration in these two images. On Jan. 1, 2021, Mare Crisium looks to be a bit higher and not fully visible as compared to the image for Jan. 25, 2021. Credit: NASA Scientific Visualization Studio and LRO

In the two images, notice how the size of the Moon seems slightly smaller on Jan. 25 than on Jan. 1. This isn’t a manipulation of the images. The difference is because the Moon’s path around Earth is not a perfect circle; it’s elliptical. Think of an oval, or a slightly flattened circle, with Earth positioned closer to one end than the other. So at one point the Moon is slightly closer to Earth, and at the opposite point it is slightly farther from Earth. The Moon’s elliptical path is important to the concept of left-right libration.

On Jan. 1, the Moon is 386,460 kilometres from Earth. On Jan. 25, the Moon is 397,371 kilometres from Earth. The difference is about 11,000 kilometres. So, because the Moon is farther away from us on Jan. 25 than on Jan. 1, it appears to be smaller in the sky to us, just as any object far away from us appears to be small. The size of the Moon itself does not change.

Back to the photos. So, what is happening here? Why can we see different positions of the Moon’s features? Why don’t we always just see the same features in the same spots?

Left-right Libration

The Moon rotates on its axis, just like Earth does. They both rotate west to east. So, if you could look down on Earth and the Moon from high above the north pole, you would see them rotating in a counterclockwise direction.

One Earth rotation takes 24 hours, our definition of a day. One Moon rotation takes about 27 days. It also takes the Moon just over 27 days to go around Earth once. As a result, we only see one side of the Moon. It seems like the Moon isn’t rotating at all, but it is.

Here is where the elliptical path comes in: in the late 1500s, a famous astronomer called Johannes Kepler figured out that objects such as Earth, the Moon, and the planets do not orbit in circles, but ellipses. Kepler then figured out that, because of the elliptical orbits, the objects move at different speeds, depending on where they are relative to the Sun, or Earth in this case. For example, when the Moon is at its farthest point from Earth, its speed is just under 1 kilometre per second. When it is at its closest point to Earth, its speed is just over 1 kilometre per second. (1 kilometre per second = 3,600 kilometres per hour.)

After the Moon’s closest approach to Earth, the Moon is still moving faster in its orbit. But the Moon’s rotation speed doesn’t change and it can’t keep up with the faster orbital speed. As a result, on the right side of the Moon we see a bit of the Moon’s far side before the Moon completes its full rotation.

After the Moon’s farthest distance from Earth, the Moon is still moving a bit slower in its orbit. So the Moon’s rotation is a little faster than its orbital speed. As a result, on the left side of the Moon, we see a bit of the Moon’s far side.

Up-down Libration

If you think of a line coming out of the centre of the Sun and going to the edges of the solar system, many planets would appear to sit on or near that line. The planets and other objects are said to be co-planar. Some don’t, though. Mercury, for instance. Mercury’s orbit crosses the imaginary line at an angle of about 7 degrees. And Pluto’s orbit is at an angle of about 17 degrees to the line. The Moon’s orbit is not co-planar with Earth’s orbit. The Moon’s orbit is inclined about 5 degrees to Earth’s orbit.

The concept of up-down libration is related to this 5 degree tilt. Also of note is the tilt of the Moon itself. Earth is tilted about 23.4 degrees, which causes our seasons. The Moon is tilted only about 1.5 degrees, so no seasons on the Moon! Anyways, back to libration. It’s so easy to move from topic to topic in astronomy because everything is so interesting!

As the Moon orbits Earth, sometimes it’s above Earth’s orbit, and sometimes it’s below Earth’s orbit. As a result, the Moon’s north pole is sometimes tipped toward Earth, and sometimes it’s tipped away from Earth. So at times we can see a little more of the south side as the Moon goes above our orbit, and a little more of the north side as the Moon goes below our orbit.

There are other factors that affect how much of the Moon’s surface we can see, such as our latitude on Earth and the Moon’s precession. But the main two factors are the left-right and up-down libration.

Watch the Moon with binoculars over a month, and try to see different positions of its features as shown in the NASA images.