Why does spinach go a muddy colour when cooked?

Why do some places have only one high tide a day and what makes cooked spinach lose its green colour. Plus the particular frustration of cycling into a headwind and the parsimonious nature of phosphorescence. Things that make you go “hmmm...”

Q Why do some places have only one tide a day?

A On the face of it this seems like quite a puzzle, but a little scientific sleuthing yields a simple answer.

“The main period of tidal oscillation is 12.42 hours [so] at most places you expect a high tide every 12 hours and 25 minutes,” Dr Thomas Adcock from the University of Oxford explains.

“The time of high tide will be slightly different everywhere as water moves around the coast. Thus, if high tide at Margate were at midday there would only appear to be one high tide in that calendar day.”

However, while some situations are a trick of the clock, other parts of the world do indeed have a diurnal tide (just one high tide a day).

“Examples of places with only one high tide are the Gulf of Mexico and Perth, Western Australia,” says Adcock. There are also locations that experience multiple high tides a day.

The reason for these surprising patterns comes down to the large lumps of land on the Earth’s surface. “[It] depends what particular [wave] frequencies the geometry of the coastline and sea-bed amplifies. If the earth were smooth and covered with water then everywhere would have two high tides.

“However, in some places the geometry damps these frequencies out and amplifies some of the other frequencies which are present,” Adcock says.

Q Why does spinach go a muddy colour when I cook it?

A The bright colour of spinach and other verdant veg is due to the presence of chlorophylls — green-coloured chemicals with a structure made up of a big flat ring and a long side chain. The ring looks a bit like that of the haem group in the haemoglobin of your red blood cells — but it has a slightly different structure and binds magnesium instead of iron.

When you cook up spinach the chlorophyll starts to break down. The side chains may fall off some of the chlorophyll molecules, making them water soluble, so a green colour leaches out into the liquid. Acids released as the plant cells break down also affect chlorophylls, kicking the magnesium out of the centre of the ring and replacing it with two hydrogens, thereby shifting the colour to more olive tones.

Q According to special relativity, from the photon’s perspective any journey is instant. So, can photons be said to exist if the length of their existence is zero? Isn’t what we call a photon just the interaction of two particles at an arbitrary distance?

A According to Dr Marika Taylor from the University of Southampton, photons need not trigger an existential crisis.

“Photons travel at the speed of light, which indeed means that the so-called “proper” length of any photon trajectory is zero,” she explains. “When we talk about the properties of particles or photons, however, we are usually referring to the properties measured by observers who aren’t moving at the speed of light themselves. Suppose a person is standing still and shines a laser beam at a reflecting wall. That person can measure the time according to their clock that it takes for the photons to travel to the wall and back again. This time is an appropriate measure of the time for which the laser beam exists.”

And seeing is believing. “Ultimately the existence of anything (particle or photon) can only be determined if one can observe it,” says Taylor. And photons are not simply the interaction to two arbitrarily spaced particles.

“Since the photon moves at the speed of light, a photon sent from one stationary particle to another will reach the second particle at a very specific time, which is the distance separating them divided by the speed of light,” says Taylor.

“Also the photon interacts with particles in very specific ways, described by the equations of quantum mechanics,” she says.

Q When riding a bicycle, does changing to a lower gear have the same effect when cycling into a strong headwind as cycling up a steep incline?

A Blustery days and imposing hills can make for a tough ride. But as Dr Alex Forrester from the University of Southampton explains, these challenges have a lot in common.

“When cycling, you are overcoming forces from aerodynamic drag, friction and weight (if you’re going uphill).

“The power required to overcome these at a certain speed should not depend on the relative amounts of these forces, eg 20 newtons from a headwind is the same as 20 newtons from an incline,” he says. Changing gear, however, can help. “Changing into a lower gear means that the same power can be produced at a lower torque, but pedalling faster, which might better fit a rider’s biomechanics and so feel ‘easier’,” he explains. “More likely is that the rider changes down, but retains a similar pedalling speed and so produces less power and slows down.”

This effect can also make the cycling seem “easier”. “Easier could mean less metabolic work, but perception is also important,” Forrester notes. “Cycling into a headwind is thoroughly demoralising, perhaps because it is an invisible force one is battling against, whereas hills are visibly conquered,” says Forrester.

“As such, changing gear perhaps just doesn’t seem to work as well into a headwind. Another problem with headwinds is that the rider’s position on the bike is very important,” he adds. “Standing up will vastly increase the aerodynamic drag and so make things harder. On a hill, at lower speeds and without a strong headwind, it is possible to change down a gear and stand up, which will certainly make things feel easier, albeit not necessarily faster.”

Q What makes stickers glow in the dark?

A These stickers and toys make use of a phenomenon known as phosphorescence. Unlike materials that absorb light and release the energy immediately as light (fluorescence) or as heat, phosphorescent materials “bank” some of the energy and release it very slowly over time, resulting in the familiar night-time glow.

This is a bit like getting your pay cheque and spending a small sum every day instead of instantly blowing the whole lot on a party. The “banking” effect arises because once a phosphorescent material has absorbed light it can re-jig its electrons to adopt a slightly more stable energised form that cannot easily lose energy. Typically, glow-in-the dark stickers contain the material strontium aluminate (with a little europium mixed in), which gives off an eerie green-blue glow after the lights are turned off.