# Double Rainbow

Can you see the double rainbow?

Ever noticed how the order of the colours changes for the second rainbow (red – yellow – green – blue – violet…)

Typically rainbows are formed when light is refracted (bent) through a raindrop.

A double rainbow forms when light is refracted twice in a raindrop, and occurs commonly when the sunlight is low in the sky. See here for more.

# Useful Resources

I will be adding resources here as I find them – mostly Maths and Physics themed.

## Online Mathematics Course

Loughborough’s Mathematics Education Centre runs a free, three-week MOOC – Getting a Grip on Mathematical Symbolism – designed for those students aspiring to become scientists or engineers but who lack mathematical confidence.

It will run again on the FutureLearn platform starting May 8th. Registration is open now:

https://www.futurelearn.com/courses/mathematical-symbolism

The course is designed for students who have some engineering or science knowledge gained through vocational qualifications or through workplace experience but who perhaps have not studied mathematics formally since leaving school. It will be appropriate for those who lack confidence but who need to establish a bedrock of knowledge in order to further their education.
This is a foundation, entry-level course and is not intended for those who already possess recent post-GCSE mathematics qualifications. It is highly recommended for those students going to university who have not studied maths beyond GCSE. Please share when appropriate.

Note that it is planned to run this course again shortly before the start of the new academic year in September.

Magnet Academy is an online resource provided by the National High Magnetic Field Laboratory — the largest, most high-powered magnet lab in the world. It has a wide selection of useful tutorials about electromagnetism for ages 5 upwards.

Interactive Magnetic Tutorials

# British Science Week

As part of British Science Week, Loughborough University hosts a ‘Community Day’ event where Loughborough locals are invited on campus to take part in various ‘science based’ activities.

This year it falls on 25th March I will be:

• Coordinating an Electrodough workshop – for which we’re looking for student ambassadors.
• Running a ‘Cold Science’ demonstration with liquid nitrogen.
• Working with the East Midlands Institute of Physics to deliver several ‘busking’ activities –  for which I’m looking for student ambassadors.

If you’re interested in getting involved please let me know.

# Liquid nitrogen

Roughly the same cost (weight for weight) as a pint of milk, it’s a common feature in science fiction films: the nitrogen dewar in the background that might at some point be used to freeze that alien chasing you down the corridor…

But how much liquid nitrogen would it actually take to do this?

Hint: Assume the creature weighs about 50kg and has a heat capacity of 2000 J/K/kg. Liquid nitrogen has a temperature of 77K and latent heat of 199 kJ/kg. For arguments sake, let’s say the creature becomes vulnerable at 250K…

Now let’s add another complication: the Leidenfrost effect. As a coolant, the low boiling point of liquid nitrogen (77K) typically means that it will boil off so fast on contact with another object much hotter than it, that a ‘protective’ layer of air is formed. This will insulate said object from the cooling effects of the liquid nitrogen, for example preventing cold burns for anyone crazy enough to stick their hand in a bucket of liquid nitrogen for a second or two. CAUTION: This effect will not stop you from getting burnt as more nitrogen is added.

For more see Wikipedia entry for liquid nitrogen

# The invisible rod

Challenge: How can you make a quartz rod invisible with some water, sugar and a beaker?

If we take something that is typically transparent (i.e. the quartz rod) you can normally see it quite clearly when placed in a liquid, by the way in which light is bent as it passes through.

This ‘bending’ of light – refraction – can be described by Snell’s law:

$n_{1}sin(\theta_{1})=n_{2}sin(\theta_{2})$

where ‘n’ is the refractive index of the material.

So you might imagine that if we can change the rod, or the liquid itself, so that light entering from behind the beaker does not refract further on entering the quartz rod, we can effectively make the quartz rod invisible. To do this we want to match up the refractive indices ($n_{1}, n_2$).

With water, as you add more and more sugar the refractive index increases, until finally it approaches that of quartz ~1.46.

# What is that goop? – Ferrofluid

If you’ve ever had a chance to play with ferrofluid then you’ll be familiar with the typical picture of a ‘flower-like’ pattern.

A ferrofluid is a suspension of tiny magnetic (iron) particles, covered in a substance called a surfactant (e.g. oleic or citric acid) that is mixed with oil.

The combination of these 3 substances leads to an animated response to magnetic fields:

1) The iron particles are attracted to the magnet.

2) The surfactant binds the iron particles and oil limiting how far the iron particles are pulled towards the magnet thus leaving it slightly mobile.

3) A combination of the magnetic field and surface tension of the ferrofluid mixture results in patterns that follow the direction of magnetic field lines.

Some mythbusters:

1) Yes, you can make a simple ferrofluid suspension with laserjet toner and oil, but it will never result in the patterns seen above unless a surfactant has been added. (This typically involves more detailed steps that I will not outline here but might be a great question/project for a chemistry teacher…)

There are several videos online (YouTube is a big culprit here) that suggest otherwise but I can only assume that they switch to storebought ferrofluid when they start the demonstrations. If anyone disagrees with this send me your recipe! I’d be happy to be proven wrong.

2) One of the YouTube videos above also suggests that you can mix the ferrofluid with water to create a more fluid mix. Again, this is pure myth. For a short term solution you could mix it with a water:propanol solution but this will slowly breakdown the ferrofluid itself and likely would not last more than a few weeks. If you want something more long term then you may need to buy it from a specialised company.

*Non-technical description of a surfactant: A chemical with ‘arms’ that can grab onto other substances.