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Why is the speed of light constant?

Asked by: Alan Edgington, Ramsgate

It isn't. When it passes through some mediums, such as water, it slows down considerably. In the case of diamond, its speed is cut by over 50 per cent. But according to Einstein's Special Theory of Relativity, the speed of light in the vaccum of empty space is said to be the same for all observers, at just short of 300,000km/s.

This is undoubtedly weird, as every other speed is measured relative to something else. For example, a train can move at 150km/h relative to someone on a platform, but to the train's passengers its speed is pretty much zero. The speed of light is no ordinary speed, however: it's a universal constant that emerges from the laws of physics.

Specifically, it's the speed at which electromagnetic waves travel through the vacuum of space - and its value can be predicted by equations unifying our understanding of electricity and magnetism, as discovered over 150 years ago by the Scottish physicist and mathematician James Clerk Maxwell.

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What is the intensity of light? & how to best measure it

One rainy day you go to your home office to get some work done. Since the sun was hiding behind the gray clouds, you had no choice but to switch on the lights. As you did, nothing happened. You tried again and again, but the room stayed dim and gloomy. It wasn’t long before your mood began to match the overcast day.

If this happened to you, lighten up – you don’t need to revert to using candles and oil lamps. Chances are, your bulbs aren’t bright enough. If you’ve been wondering about the intensity of light and how to measure it, continue reading this article as we explain it in detail.What is light?Light is just a small fraction of electromagnetic radiation. It’s also the only part of the spectrum humans can see, thanks to the photoreceptors in our eyes. Plants contain a substance called chlorophyll which can ‘harvest’ visible light and use its energy to convert inorganic matter into biomass.Each wavelength of visible light has its specific color. When they combine, it appears white. Objects absorb certain wavelengths and reflect the rest. We perceive these reflected wavelengths as colors.What is the intensity of

Required practical - effect of light intensity on photosynthesis

can be investigated in water plants. Use Cabomba or Elodea, which are sold in aquarium shops.

The plants will release bubbles of oxygen - a product of photosynthesis - which can be counted.

A lamp with an LED bulb is set up at different distances from the plant in a beaker of water:

• an LED bulb is best as this will not raise the temperature of the water
• sodium hydrogencarbonate - formula NaHCO₃ - is added to the water to supply carbon dioxide - a reactant in photosynthesis - to the plant
• the light intensity is proportional to distance - it will decrease as the distance away from the bulb increases - so light intensity for the investigation can be varied by changing the distance from the lamp to the plant

The bubbles produced over one minute periods are recorded.

Aim

To investigate the effect of light intensity on the rate of photosynthesis.

Method

Set up a boiling tube containing 45 cm³ of sodium hydrogencarbonate solution (1%). Allow the tube to stand for a few minutes and shake to disperse any air bubbles that might form.Cut a piece of the pondweed, Cabomba. The pondweed should…