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Possible habitat for life on Enceladus, a moon of Saturn

enceladus_layout_4_10

–  This graphic illustrates how Cassini scientists think water interacts with rock at the bottom of the ocean of Saturn’s icy moon Enceladus, producing hydrogen gas. Credit: NASA/JPL-Caltech

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Two veteran NASA missions are providing new details about icy, ocean-bearing moons of Jupiter and Saturn, further heightening the scientific interest of these and other “ocean worlds” in our solar system and beyond. The findings are presented in papers published Thursday by researchers with NASA’s Cassini mission to Saturn and Hubble Space Telescope.

In the papers, Cassini scientists announce that a form of chemical energy that life can feed on appears to exist on Saturn’s moon Enceladus, and Hubble researchers report additional evidence of plumes erupting from Jupiter’s moon Europa.

“This is the closest we’ve come, so far, to identifying a place with some of the ingredients needed for a habitable environment,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate at Headquarters in Washington. ”These results demonstrate the interconnected nature of NASA’s science missions that are getting us closer to answering whether we are indeed alone or not.”

The paper from researchers with the Cassini mission, published in the journal Science, indicates hydrogen gas, which could potentially provide a chemical energy source for life, is pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor.

The presence of ample hydrogen in the moon’s ocean means that microbes – if any exist there – could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water. This chemical reaction, known as “methanogenesis” because it produces methane as a byproduct, is at the root of the tree of life on Earth, and could even have been critical to the origin of life on our planet.

Life as we know it requires three primary ingredients: liquid water; a source of energy for metabolism; and the right chemical ingredients, primarily carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.

With this finding, Cassini has shown that Enceladus – a small, icy moon a billion miles farther from the sun than Earth – has nearly all of these ingredients for habitability. Cassini has not yet shown phosphorus and sulfur are present in the ocean, but scientists suspect them to be, since the rocky core of Enceladus is thought to be chemically similar to meteorites that contain the two elements.

“Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.

The Cassini spacecraft detected the hydrogen in the plume of gas and icy material spraying from Enceladus during its last, and deepest, dive through the plume on Oct. 28, 2015. Cassini also sampled the plume’s composition during flybys earlier in the mission. From these observations scientists have determined that nearly 98 percent of the gas in the plume is water, about 1 percent is hydrogen and the rest is a mixture of other molecules including carbon dioxide, methane and ammonia.

The measurement was made using Cassini’s Ion and Neutral Mass Spectrometer (INMS) instrument, which sniffs gases to determine their composition. INMS was designed to sample the upper atmosphere of Saturn’s moon Titan. After Cassini’s surprising discovery of a towering plume of icy spray in 2005, emanating from hot cracks near the south pole, scientists turned its detectors toward the small moon.

Cassini wasn’t designed to detect signs of life in the Enceladus plume – indeed, scientists didn’t know the plume existed until after the spacecraft arrived at Saturn.

“Although we can’t detect life, we’ve found that there’s a food source there for it. It would be like a candy store for microbes,” said Hunter Waite, lead author of the Cassini study.

The new findings are an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean. Previous results, published in March 2015, suggested hot water is interacting with rock beneath the sea; the new findings support that conclusion and add that the rock appears to be reacting chemically to produce the hydrogen.

https://saturn.jpl.nasa.gov/news/3024/nasa-missions-provide-new-insights-into-ocean-worlds-in-our-solar-system/

Animal Intelligence: Octopuses Do Something Really Strange to Their Genes

Octopuses Do Something Really Strange to Their Genes

Ed Yong, April 2017,  The Atlantic

https://www.theatlantic.com/science/archive/2017/04/octopuses-do-something-really-strange-to-their-genes/522024/

Octopuses have three hearts, parrot-like beaks, venomous bites, and eight semi-autonomous arms that can taste the world. They squirt ink, contort through the tiniest of spaces, and melt into the world by changing both color and texture. They are incredibly intelligent, capable of wielding tools, solving problems, and sabotaging equipment. As Sy Montgomery once wrote, “no sci-fi alien is so startlingly strange” as an octopus. But their disarming otherness doesn’t end with their bodies. Their genes are also really weird.

A team of scientists led by Joshua Rosenthal at the Marine Biological Laboratory and Eli Eisenberg at Tel Aviv University have shown that octopuses and their relatives—the cephalopods—practice a type of genetic alteration called RNA editing that’s very rare in the rest of the animal kingdom. They use it to fine-tune the information encoded by their genes without altering the genes themselves. And they do so extensively, to a far greater degree than any other animal group.

“They presented this work at a recent conference, and it was a big surprise to everyone,” says Kazuko Nishikura from the Wistar Institute. “I study RNA editing in mice and humans, where it’s very restricted. The situation is very different here. I wonder if it has to do with their extremely developed brains.”

It certainly seems that way. Rosenthal and Eisenberg found that RNA editing is especially rife in the neurons of cephalopods. They use it to re-code genes that are important for their nervous systems—the genes that, as Rosenthal says, “make a nerve cell a nerve cell.” And only the intelligent coleoid cephalopods—octopuses, squid, and cuttlefish—do so. The relatively dumber nautiluses do not. “Humans don’t have this. Monkeys don’t. Nothing has this except the coleoids,” says Rosenthal.

It’s impossible to say if their prolific use of RNA editing is responsible for their alien intellect, but “that would definitely be my guess,” says Noa Liscovitch-Brauer, a member of Rosenthal’s team who spearheaded the new study. “It makes for a very compelling hypothesis in my eyes.”

Here’s how RNA editing works. Genes encode instructions in the form of DNA—in the sequence of four building blocks represented by the letters A, C, G, and T. For those instructions to be used, the DNA must first be transcribed into a similar molecule called RNA, which contains roughly the same building blocks. The RNA is then translated and used to build proteins—the molecular machines that carry out all the important jobs inside our cells. So DNA stores information, RNA carries it, and proteins are the result of it.

That’s the simple version. But the RNA often gets altered before it’s used to make proteins. Some of the changes are big—large sections are cut out, and the remaining pieces are glued back together. Other changes are small—sometimes, a single A gets converted into an I (which is functionally equivalent to a G). That’s RNA editing. It’s performed by a group of enzymes called ADARs, which recognize specific sequences of RNA and makes those A-to-I changes.

But to what end? RNA editing is still mysterious, and its purpose unclear. Technically, an animal could use it to change the nature of its proteins without altering the underlying DNA instructions. But in practice, this kind of recoding is extremely rare. Only about 3 percent of human genes are ever edited in this way, and the changes are usually restricted to the parts of RNA that are cut out and discarded. To the extent that it happens, it doesn’t seem to be adaptive.

In cephalopods, it’s a different story. Back in 2015, Rosenthal and Eisenberg discovered that RNA editing has gone wild in the longfin inshore squid—a foot-long animal that’s commonly used in neuroscience research. While a typical mammal edits its RNA at just a few hundred sites, the squid was making some 57,000 such edits. These changes weren’t happening in discarded sections of RNA, but in the ones that actually go towards building proteins—the so-called coding regions. They were ten times more common in the squid’s neurons than in its other tissues, and they disproportionately affected proteins involved in its nervous system.

Having been surprised by one cephalopod, the team decided to study others. Liscovitch-Brauer focused on the common cuttlefish, common octopus, and two-spot octopus. All of these showed signs of extensive RNA editing with between 80,000 to 130,000 editing sites each. By contrast, the nautilus—a ancient cephalopod known for its hard, spiral shell—only had 1,000 such sites.

This distinction is crucial. The nautiluses belong to the earliest lineage of cephalopods, which diverged from the others between 350 and 480 million years ago. They’ve stayed much the same ever since. They have simple brains and unremarkable behavior, and they leave their RNA largely unedited. Meanwhile, the other cephalopods—the coleoids—came to use RNA editing extensively, and while evolving complex  brains and extraordinary behavior. Coincidence?

Liscovitch-Brauer also found that around 1,000 of the edited locations were shared between the coleoid species—far more than the 25 or so sites that are shared between humans are other mammals. These sites have been preserved over hundreds of millions of years of evolution. “That’s pretty convincing evidence that these edits are integrated into the wiring of these genomes, and that disrupting this network of edits would be harmful in some way,” says Daniel Rokhsar from the University of California, Berkeley, who was not involved in the study.

Indeed, the edits seem to be so important that cephalopods have gone to unusual lengths to preserve them. To find and edit a particular RNA letter, ADAR enzymes rely on all the surrounding letters for clues. It’s like finding this single ‘A’ by looking for all the words in this paragraph. This means that editing sites effectively cordon off a large chunk of a cephalopod’s genome—between 23 and 41 percent by Rosenthal’s reckoning. Those sites must remain largely unchanged, or the editing enzymes won’t be able to find their targets. And this means that octopus and squid genomes evolve at a slower pace than those of other animals. They may be icons of flexibility and change, but their genomes are rigid and stagnant.

Rosenthal thinks that they pay for this sacrifice with a different kind of flexibility. By changing their RNA rather than their DNA, they might be more effective at adapting to challenges on the fly. From the same gene, they could produce proteins that, say, work better in hot temperatures or cold ones. And such changes would be temporary—the creatures could turn them on or off depending on the circumstance. Rosenthal wonders if they could learn or encode experiences in this way. “I’m working a lot on the squid ADAR enzymes and their distribution between cells,” he says. “It’s mind-blowing how variable they are. One neuron will have high levels but its neighbor will have nothing.”

“This study suggests that RNA editing and recoding is important in the function of the largest invertebrate brains,” says Carrie Albertin from the University of Chicago, who helped to sequence the first cephalopod genome. “By comparing vertebrate and cephalopod brains, we can understand how large nervous systems are put together.”

“It’s a really interesting phenomenon, but it’s unclear why they need so much RNA editing,” says Jianzhi Zhang from the University of Michigan. “It’s not absolutely clear if it has to do with behavior; humans have very complex brains and behaviors and in us, RNA editing is very rare.” The question isn’t just why coleoid cephalopods are unique in embracing RNA editing, but why nothing else has to the same extent.

So far, the team has a lot of correlations—compelling ones, but correlations nonetheless. Rosenthal’s next move is to develop ways of genetically manipulating cephalopods. If he succeeds, he could disable their ADAR enzymes, stop them from editing their RNA, and see what happens.

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Boolean logic

How to program in Scratch, using Boolean logic

  • Boolean operators include:
  • AND, OR, NOT,  < ,  = ,  >
  •    In other words, is one sprite touching some other thing? The answer by definition must be true or false.
  •    In other words, is one sprite touching something of a certain color? The answer by definition must be true or false.
  •  In other words, is a certain key being pressed?
  •  In other words, is the mouse being used?
  • Why use Boolean operators?
  •   To focus a search, particularly when your topic contains multiple search terms.
  •   To connect various pieces of information to find exactly what you’re looking for.
  • https://sites.google.com/a/onalaskaschools.com/tech/boolean-search-tools
  • .
  • https://ircutp.wordpress.com/utp-irc-faqs/boolean-operators/

 

A Boolean block is a hexagonal block (shaped after the Boolean elements in flowcharts)
The block contains a condition. The answer to the condition will be either true or false.
.
It’s important to determine if a statement (expression) is “true” or “false”.
Ways to determine TRUE and FALSE are prevalent in all kinds of decision making.
A mathematically precise way of asking if something is TRUE or FALSE is called a Boolean operation.
It is named after George Boole, who first defined an algebraic system of logic in the mid 19th century.
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Boolean data is associated with conditional statements. For example, the following statement is really
a set of questions that can be answered as TRUE or FALSE.
IF (I want to go to a movie) AND (I have more than $10) THEN (I can go to the movie)
.
We can combine several “boolean” statements that have true/false meaning into a single statement
using words like AND and OR, and NOT).
“If I want to go to the movie AND I have enough money, then I will go to the movie.”
BOTH conditions have to evaluate to true (have to be true) before the entire expression is true.
.
Some terms you already learned in math are really Boolean operators
Less than < [ ] < [ ] > Equal to < [ ] = [ ] > Greater than < [ ] < [ ] >
For example: (The height of a building) < 20 meters
For any building we look at, this statement will either be true or false.

Go through what each Boolean block does (page 68)

 

External links

Boolean: Lesson 13: GetCoding

Books

Book “Adventures in Coding”, Eva Holland and Chris Minnick, Wiley, 2016. Pages 50-59

Learning Standards

Massachusetts

Computational Thinking 6-8.CT.c.2 Describe how computers store, manipulate, and transfer data types and files (e.g., integers, real numbers, Boolean Operators) in a binary system.

CSTA K-12 Computer Science Standards

CT.L2-14 Examine connections between elements of mathematics and computer science
including binary numbers, logic, sets and functions.

CPP.L2-05 Implement problem solutions using a programming language, including: looping behavior, conditional statements, logic, expressions, variables, and functions.

 

Notebook checklist

You should have each handout, correctly filled out, in order, name on top of each page.

⃞  MCAS formula sheet
⃞  Metric conversions sheet
⃞  Table with Kinematic equations in 3 columns: Constant velocity, Changing Velocity, Circular motion
⃞  Concept Development Practice Page 13-3: Gravitational Interactions
⃞  Why study science historically
⃞  The Physics of Circular Motion and our place in the universe: How science has developed from ancient times to today
⃞  The Physics of Circular Motion and our place in the universe: Solar System: Medieval to modern times
⃞  Concept Development Practice Page 7-1 Momentum (1 sheet double sided)
⃞  Skill Sheet 3-C Momentum (1 sheet double sided)
⃞  Skill Sheet 5-A Work (2 sheets, 3 pages)
⃞  Work – review sheet
⃞  Skill Sheet 5-B Power (1 sheet double sided)
⃞  Cumulative Review worksheet: For each of the situations draw & label the Action and Reaction forces, Net force, and free-body diagrams, Circular motion, and friction
⃞  “The Nature of Science” – In which you had to choose 1 of 4 topics to write about.
⃞  Concept Development Practice Page 8-1; Work and Energy (1 sheet double sided)
⃞  Concept Development Practice Page 8-2 Conservation of Energy (1 sheet double sided)
⃞  10.2 Temperature scales (2 pages, double sided)
⃞  Specific Heat (1 page double sided)
⃞  Skill Sheet 13.1 Harmonic Motion (3 sheets, 5 pages)
⃞  Concept Development Practice Page 25-3: Wave Superposition. (1 page, double sided)
⃞  Concept Development Practice Page 28-1 Colored shadows (1 page double sided)
⃞  25.2 Color mixing with additive and subtractive color (1 page double sided)
⃞  Lab instructions on ray tracing lenses: trapezoid, biconvex, convex and biconcave.
⃞  How A Mirror Works (1 page single sided) (3/28/17)
⃞  Review sheet which begins “Graded notebook check during class”, and which has a table of equations at the bottom (3/28/17)
⃞  “How to do problem solving: Work in assigned groups” 6 steps: 8 review problems.
⃞  MCAS Doppler effect (1 page double sided) (3/30/17)

⃞  Cornell Notes, Chap 20, Sections 1 and 2. Cornell Notes style. (4/3/17)

⃞  Cornell Notes (double sided) Website -> Physics -> Electromagnetism -> Electric-fields-and-potential (4/5/17)

⃞  Cornell Notes Website -> Physics -> Electromagnetism -> Electric Current (4/7/17)

⃞  2016 MCAS Exam (taking on 4/17. It will be collected at the time, but then handed back after April break. We’ll go over the answers.)

⃞   Chap 20 study guide – 4 pages (4/11, 4/12)

⃞  Cornell Notes:   Chap 6B Static Electricity.  Chap 7A: Measuring Electricity

⃞  tba
⃞  tba
⃞  tba
⃞  tba
⃞  tba
⃞  tba
⃞  tba
⃞  tba
⃞  tba
⃞  tba

 

Bullet spin angular momentum

How can we see a bullet’s angular momentum?

Video of a bullet spinning on ice: angular momentum is conserved

Video of a bullet spinning on ice #2 , http://gif-finder.com/bullet-spinning-on-ice/

Spinning bullet on ice. Long video

Mythbusters tested this claim in episode 166 – “Spy Car 2”, May 18, 2011

Myth Statement: A bullet fired into the surface of a frozen lake can spin like a top on impact. Inspired by a viral video.

MythBusters: Give bullet room to bounce – clip

How does a bullet start spinning to begin with?

Guns are rifled in order to impart spin to a bullet leaving the barrel.

bores rifle rifling

Rifling gives the bullet angular momentum, which stabilizes it
(keeps it from tumbling)

https://www.hunter-ed.com/alaska/studyGuide/Rifling-in-the-Rifle-or-Handgun-Bore/201001_700049202

 

 

New articles

A sudden decision to aim a handgun at some Houston County ice in 2003 yielded a video of spinning bullets that became an Internet sensation and led to a May 18 appearance on cable television’s “Mythbusters” show. The “myth” was born on a winter afternoon when Nate Smith of La Crescent, Minn., along with friends Nate and Andy Van Loon, were shooting bullets from a 9-mm Glock from a hillside toward a patch of ice on the Van Loons’ land near Houston.

“We just went out, not being very smart, and were shooting the ice and watching the pieces of ice fly, and we were recording ourselves,” Smith said. “I remember, after we shot a couple times, wondering what that crackling sound we were hearing was.”

After firing multiple shots, they found the bullets spinning on end on the ice surface, a few feet back from the point of impact. “You could see the trail from where it ricocheted and that’s how we found it,” Smith said. “And it went for about two minutes, so we just filmed that.”

The only explanation Smith could offer is rifling in the gun barrel caused the bullets to spin. Though Smith first put the video on the Internet in 2005, it took off in December 2009 when Smith’s friend Jason Nesbit asked if he could re-post it on YouTube. More than a half-million people have since viewed it.

 

When questions arose over the validity of the clip, Nesbit asked if he could re-create it on a local frozen lake. “We didn’t think anything would happen, so we said, ‘Sure, go ahead. Do whatever,'” Smith said. But Nesbit went further, sending the video to the “Mythbusters” website, where it caught the attention of the Discovery Channel show’s producers. Smith and Nesbit also were contacted by an Asian television show that paid them a small stipend to use the video. Smith watched May 18 as the “Mythbusters” hosts worked to prove or disprove what he had seen with his own eyes. “Somehow, they get their bullets to just spin like a top,” host Kari Byron said on the show. “Now that sounds like a myth,” co-host Tory Belleci added.

The key element was finding the correct angle to fire the gun into the ice, Belleci said. Too straight would deform the bullet too much to spin, too shallow would send the slug skipping off the ice.

The hosts set up nine 40-by-24-by-10-inch chunks of ice next to each other to replicate the frozen lake. The first shots became embedded in the ice and had no chance to spin. But a bullet was recovered that hadn’t deformed, a cause for cautious optimism. Yet the next several tests failed to yielded a spinning bullet, so the hosts tried firing at an angle to ricochet off a large piece of ice between 2 and 8 feet away from the initial contact point. The hit was clean, but the bullet was gone. Several more tries, several more lost bullets. The hosts decided to try again six months later on an actual frozen lake. But shot after shot and no luck. “This myth sucks,” Belleci said in the blizzard-like conditions.

For the final test, they gave the bullet room to ricochet backwards, as it had on Smith’s outing. After countless attempts, they found the spin they were looking for. It happened again and again, confirming the myth. Smith said the whole experience was somewhat surreal, since he once joked the video might someday make it to “Mythbusters.” “Eight years later, here we are and it was on TV,” he said. But he doesn’t recommend anyone try this at home. “Looking back at it,” he said, “it wasn’t very safe.”

  • article from Lacrosse tribune. ‘Mythbusters’ tackles Houston County video of spinning bullets. By Ryan Henry, Houston County News

 

Fair use

This website is educational. Materials within it are being used in accord with the Fair Use doctrine, as defined by United States law.

§107. Limitations on Exclusive Rights: Fair Use

Notwithstanding the provisions of section 106, the fair use of a copyrighted work, including such use by reproduction in copies or phone records or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use, the factors to be considered shall include:

the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
the nature of the copyrighted work;
the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
the effect of the use upon the potential market for or value of the copyrighted work. (added pub. l 94-553, Title I, 101, Oct 19, 1976, 90 Stat 2546)

 

Bullet spin kinematics

If a bullet is fired straight up in the air, will it return to ground level with the same velocity?

Some labs are not safe to perform in schools, but give valuable insight on physics: Thus we may watch a video of such an experiment.

http://www.discovery.com/tv-shows/mythbusters/?rfc=1

MythBusters Episode 50 – “Bullets Fired Up” – Busted, Plausible, and Confirmed

All guns are “rifled” in order to impart spin to a bullet leaving the barrel.

bores rifle rifling

This rifling gives the bullet angular momentum, which stabilizes it (keeps it from tumbling)

https://www.hunter-ed.com/alaska/studyGuide/Rifling-in-the-Rifle-or-Handgun-Bore/201001_700049202

 

How can we see a bullet’s angular momentum?

Video of a bullet spinning on ice: angular momentum is conserved

Video of a bullet spinning on ice #2 , http://gif-finder.com/bullet-spinning-on-ice/

Rhett Allain offers a physics analysis of the “Bullets Fired Up” episode of MythBusters

http://kwc.org/mythbusters/2006/04/episode_50_bullets_fired_up_vo_1.html

Summary of results: Bullets Fired up

Annotated MythBusters: Bullets Fired Up analysis

Analysis: Bullets Fired Up – MythBusters

Results are complicated: In most cases the bullet lands with non-lethal velocities, despite confirmed cases to the contrary. How to solve this apparent paradox?

There are really two different cases:

Experiments show that bullets lose (*) spin (and angular momentum) via interactions with the air. Once they lose angular momentum they are no longer spin-stabilized. Without spin-stabilization they tumble, and thus they experience more drag (“air resistance”), slowing them down more.

Bullets fired at an angle can come back down fast enough to kill someone. Experiments show that this allows bullets to keep more of their spin/angular momentum. They stay spin-stabilized for more of the trajectory, and thus keep more of their energy upon landing.

(*) Both momentum and angular momentum are conserved quantities; when we say that an objects loses either, it is because the momentum is actually transferred at a microscopic level to the surrounding molecules of air. See our lesson on momentum.

Analysis: What if the bullet falls without tumbling? How fast would they go

PhysicsForum meber WhyIsItSo gives us the following:

In the original post, there was not sufficient information to say, for example, if the bullet still had kinetic energy from being fired, or if all the speed that was left was its terminal velocity. For the latter case – Assumptions:

1. Take the ideal case (unlikely) that the bullet is falling in a consistently “nose-down” attitude.

2. Air pressure is 1.29Kg/m^3

3. Use a .45 Cal bullet, mass 300 g, drag coefficient 0.228. The “nose down” attitude gives us a cross-sectional area of 0.0001026m2.

4. Still day [no winds].

5. Ignoring humidty.

Process: We can use the Quadratic drag formula.

http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/quadrag.html

m = mass of bullet

g = acceleration of gravity = 9.8 m/s2

C = drag coefficient

ρ = air pressure [This symbol is the Greek letter rho]

A = cross-sectional area of bullet

That comes out to about :

Velocity terminal = 442 m/s = 1,448 ft/s = 987 miles/hour.

Gun enthusiasts are probably thinking that number is higher than the muzzle velocity of most guns, which means that in this “ideal” scenario, a bullet fired straight up wouldn’t get high enough to attain terminal velocity on the way down.

Much more realistic would be to assume the bullet is more or less sideways on the way down. This will result in a drag coefficient more like 0.6 (a sphere is about 0.5), and a cross-sectional area – very roughly – twice the nose-on area, or say 0.0002052 m2

Plugging those numbers in gives:

Velocity terminal = 192 m/s = 630 ft/s = 430 miles/hour

Reference https://www.physicsforums.com/threads/how-fast-does-a-bullet-return-to-earth.14560/

 

How fast does a bullet return to Earth?

 

Ray Tracing

This lesson is from Rick Matthews, Professor of Physics, Wake Forest University.

Lesson 1, convex lens
The object is far from the lens.

Convex Lens ray tracing GIF

 

Lesson 2, convex lens
The object is near the lens.

Convex Lens ray tracing GIF Object near lens

The rules for concave lenses, are similar:

A horizontal ray is refracted outward, as if emanating from the near focal point.

A ray that strikes the middle of the lens continues in a straight line.

A ray coming from the object, far from the far focal point, will leave the lens horizontal.

Lesson 3, concave lens.
Note that object placement has little effect on the nature of the image.
The rays diverge.

Concave Lens ray tracing GIF

__________________________

In every case:

if the rays leaving the lens actually intersect then the image is real.

If the rays leaving the lens diverge then someone looking back through the lens
would see a virtual image:
Your mind would extrapolate where you think the image should be,
even though one isn’t really there, as shown below with the dotted lines.

concave-lens

image from Giancoli Physics, 6th edition

http://users.wfu.edu/matthews/courses/tutorials/RayTrace/RayTracing.html