Image copyright Reuters Image caption Thousands of members of the Yazidi sect, who have fled from jihadists, are trapped on the mountains.

RAF Hercules aircraft have dropped two rounds of "life-saving aid" to refugees trapped on a mountain in northern Iraq. But what exactly was dropped over Mount Sinjar?

"Airdrops are very challenging and it is the last resort really of humanitarian aid but these people are desperate - it's the only option at the moment," says Dylan Winder, head of humanitarian response at the Department for International Development (DfID).

Thousands of members of the Yazidi sect, who have fled from jihadists, are trapped on the mountains in northern Iraq.

So far, RAF planes have dropped 1,056 solar lanterns and 3,180 reusable water purification containers, each containing their full capacity of 5 litres of clean water.

Image copyright DfiD

The portable solar lanterns can also be used to charge mobile phones via a universal charging cable.

In a country in which more than 80% of people own a mobile phone, this is a crucial tool to help displaced Iraqis communicate.

And it will allow aid agencies to keep in touch with the refugees.

'Cheaper technology'

Users of the lanterns are instructed to "charge daily from dawn to dusk"

On a full charge, they can give out light for up to 11 hours or provide two hours talk time on a mobile phone, according to DfID.

Dropping the lanterns in Iraq is a brilliant idea. They've got a lot of sunshine there so it's going to work really well Solar technology author David Thorpe

The units are manufactured in India by Sunlite, which says that, in partnership with international aid agencies, its lanterns "have reached out to over a quarter of a million people across the globe catering to some of the world's largest humanitarian and disaster relief operations".

They include the 2011 tsunami in Japan, floods in Thailand of the same year and "the ongoing Syrian refugee crisis", it says.

Solar technology author David Thorpe says the technology behind the lanterns has been around for about a quarter of a century with solar modules "getting cheaper and cheaper in recent years".

The technology is the same as that found in solar-powered garden lights, he adds.

"Dropping the lanterns in Iraq is a brilliant idea," he says.

"They've got a lot of sunshine there so it's going to work really well.

"There's plenty of solar insolation and that's the key to these things working - it means they don't need to be supplied with kerosene or any fuel".

"It means they're self-sufficient in lighting."

He adds: "Nowadays, there are high-profile non-governmental organisations that specialise in getting these technologies out to places like Africa.

Media playback is unsupported on your device Media caption UK forces make Iraq air drop

"It's a fantastic tool because, in those kind of places, you get kids who can't study at night because there's no lighting - or they've got to use very dodgy kerosene lights, which are often homemade and dangerous and they're polluting for the environment.

"But with these lights it makes a fantastic difference to their education and outlooks."

The batteries in the units, which are charged by sunlight, could last between five and eight years before they would need replacing, while the solar panels themselves could last up to 30 years, he says.

Image copyright DFiD

DfID supplies

The UK has two hubs that hold stockpiles of kit - the biggest is in Dubai while another is in Kemble, Gloucestershire

The two hubs are vital to the UK's humanitarian effort and can be deployed at very short notice

The Dubai hub is ideally located for the same reasons the country is a global air and naval hub - it gives the best reach, allowing rapid despatch

DfID's Dylan Winder has said Dubai is "eight hours away from two-thirds of the world, which includes most countries at risk of natural disasters".

Goods permanently stored there include tents for about 17,500 people, plastic sheets for about 75,000 people and up to 36,000 blankets.

Source: DfID

'Disaster zones'

The sun's baking down on them, there's very limited food up there - we've had reports of them eating raw sheep Dylan Winder , DfID

The water purification "jerrycans" included in Saturday and Monday night's air drops are described by DfID as "a robust, point of use, fresh water nano-filtration device" that "requires no chemicals or power"

It says the Lifesaver cube, designed by manufacturer Lifesaver Solutions in conjunction with the charity Oxfam, "produces instant sterile drinking water, removing all viruses, bacteria, parasites and spores" and was made to be "easy to distribute, carry and use".

The cube, which was designed and manufactured in the UK, measures 20cm on each side and can be used to filter up to 5,000 litres of water before a new cartridge would be needed.

Its makers say this amount of water can be used "for drinking, washing, brushing your teeth and preparing food for a family of four, every day, for up to one year".

Instructions for use are simply "fill the cube, pump [with the unit's plastic pump mechanism], turn tap and drink".

These supplies will address the most lifesaving issues that they have at the moment Dylan Winder , DfID

Its maker says the cube has been used "in humanitarian disaster zones such as the Philippines and Mexico in 2013".

Members of the public can buy the cubes for £125, while the lanterns are not commercially available.

DfID said it could not reveal the prices it paid because they were commercially sensitive, adding that it negotiated a substantial reduction for humanitarian use.

Further RAF Tornado jets have now left the UK for Cyprus.

From there, they will help efforts to deliver more aid to refugees trapped on Mount Sinjar.

"The sun's baking down on them, there's very limited food up there - we've had reports of them eating raw sheep, for example," says DfID's Mr Winder.

"These supplies will address the most lifesaving issues that they have at the moment."