In order to describe the illumination of a scene produced by a source of light we need to use an appropriate unit (in the same way we measure distance in metres and time in seconds). Scientists use a unit called a LUX to measure and describe Illumination. On Earth when the Sun is overhead it can create an illumination of about 100,000 lux. On a cloudy day we might only get 35,000 lux.
However, an adequate illumination for normal working is reckoned to be only about 300 lux. It seems that even on a cloudy day the Sun produces over 100 times as much light as we actually “need”! The minimum illumination for viewing objects without too much difficulty is around 120 lux.
Most people are quite comfortable watching a floodlit sporting event at night. The lighting for such events is designed to give an illumination on the field of between 200 and 500 lux, depending on the sport. After the game if you were to take a romantic walk home by moonlight, you would be coping with an illumination of only 0.1 - 0.2 lux!
What these numbers show is the amazing ability of the human eye to adapt to an enormous range of light levels .
You will also notice that we have avoided using the word “brightness” even though it is normal and natural to ask “how bright is the Sun?” or “how bright is that lamp?” This is because scientifically we reserve the word “brightness” to describe the effect the brain perceives when light shines into our eyes – and that is a very complicated process. As we saw above, the eye and brain allow us to cope in our everyday lives with a huge range of illumination, but it is not a very good system for quantitatively measuring that illumination. We can accurately say that two lamps appear to produce the same illumination (they appear equally bright to us) but it is impossible for us to say if one illumination (produced by a lamp, say) is twice as strong as that produced by another “brighter” lamp. To answer that we need instruments that measure lux!
One of the things that can be really boring about science, especially at school, is that most times (well in exams especially) you are supposed to churn out the same old answers to the same old questions, just to get the marks. Think how much more fun it would be if you were allowed to think of imaginative answers for a change, and got credit for them! We don't mean just silly, non-sensical answers, although they can be useful in exploring a topic, but ones that show you know some science even though your ideas may not necessarily be all that practical.
Start a campaign against always having to give the standard 'gimme-a-mark' type answers!
An example we like and which we heard of many years ago was connected to a physics question about pressure in the Earth's atmosphere. Pupils had been learning about how atmospheric pressure decreases with height (see Pascal's factary entry) - the pressure at the top of a mountain is a lot less than at sea-level for instance. So in principle, if you have a barometer (an instrument for measuring pressure), then by measuring the pressure change from the ground floor to the top floor of a building, you could calculate the building's height. This was the answer examiners wanted to hear when they set the following question:
Q: Explain how, using a mercury barometer, you could determine the height of a multi-storey building.
One student is reported to have given the following answers:
Answer 1: I would get a long piece of string, as high as the building, and tie the barometer to one end. I would then go to the roof of the building and use the string and barometer as a large pendulum. By measuring the time the pendulum took to swing I would calculate the length of the piece of string.
Good physics! The period, T, ( the time in seconds for one complete swing) of a pendulum with length L metres is given by 2π√(L/g), where g is the acceleration due to gravity and is approximately 10 m/s². If the period of the pendulum was measured to be 10.0 seconds, the student would calculate the height of the building to be 25.3 metres. That would probably be a much more accurate answer than he would get by trying to measure the change in pressure from ground floor to top floor!
Answer 2: I would go to the top of the building and drop the barometer over the side. By timing how long it took to reach the ground I could calculate the height of the building.
Good physics again! Ignoring the effects of air resistance, if the barometer took T seconds to fall to the ground under the force of gravity, then the distance it fell is given by D = 5T2 metres. The question didn't say you had to return the barometer in good shape! If the building really was 25.3 metres high, the fall time would have been 2.25 seconds.
Answer 3: I would find the caretaker of the building and offer to give them a really nice barometer as a present if they could tell me how high the building was.
Doesn’t contain any good physics, but a great answer all the same. We hope the student got full marks even without giving the expected answer since thinking of new and original solutions to problems sometimes opens up whole new areas of investigation.