Light

We’ve been doing a bit of work on the subject of Light this week. So, what exactly is light?

Light doesn’t just consist of the light we can see. Light is all around us even when it’s dark. The different parts of light can are all part of the electromagnetic spectrum.

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 1025 hertz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus. This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelength) end. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. Gamma rays and X-rays are classified as ionizing radiation as their photons have enough energy to ionize or remove electrons form an atom. These are in the upper end of the electromagnetic spectrum and have very high frequencies(in the range of 100 billion billion hertz) and very short wavelengths (1 million millionth of a metre).

Radiation in this range has high energy. It has enough energy to strip electrons from an atom or, in the case of very high-energy radiation, break up the nucleus of the atom.

Each ionisation releases energy that is absorbed by material surrounding the ionised atom. Ionising radiation deposits a large amount of energy into a small area. In fact, the energy from one ionisation is more than enough energy to disrupt the chemical bond between two carbon atoms. 

There are three main kinds of ionising radiation:

  • alpha particles, which include two protons and two neutrons
  • beta particles, which are essentially electrons
  • gamma rays and x-rays, which are pure energy (photons).

Alpha particles and beta particles are not part of the electromagnetic spectrum; they are energetic particles as opposed to pure energy bundles (photons).

Light can be created by making an electron oscillate, which creates an oscillating magnetic field and an oscillating electric field, which is an electromagnetic wave, or light.

The part of the electromagnetic spectrum we can see is Visible Light. Human eyes usually respond to wavelengths from 380 to 740 nanometers.

Different colours in the visible light spectrum have different wavelengths, with violet having the shortest wavelength and therefore the most energy and red having the longest wavelength and the least energy.

In 1666 Isaac newton proved that white light was made of the colours of the rainbow by shining light through a prism. When the white light was refracted it split into colours of the rainbow. He then passed it back through another prism where it reconstituted into white light again. He worked out that colour is a property of the light wave hitting it and not of the object itself. Can also demonstrate this by spinning a colour wheel, which will show the colours blending into white light.

Here’s how to make a colour wheel, otherwise known as Newton’s disc or a reverse rainbow!
White light passed through a prism defracts into the colours of the rainbow.

Although humans can only see the visible light spectrum , birds, bees and some animals can see ultra violet light, which makes plants particularly attractive to them.

Plants look even more beautiful to bees as they detect UV light

So how do we detect colour? The retina in our eyes contains cells called rods and cones that are sensitive to different colours of light.

And have you ever noticed you can’t see colour in the dark or in very dim light? That’s because the sensing rods and cones in your retina are both sensitive to light. The rods allow us to see in very dim light but cannot detect colour. The cones allow us to see colour but don’t work in dim light!

Newton observed that colour is not inherent in objects. Rather, the surface of an object reflects some colour and absorbs all the others. We perceive only the reflected colours. The colour an object appears depends on the colours of light it reflects.

E.g. A red book only reflects red light.

Here’s a great video explaining this

Vid by Edmund Scientific

Because we can’t see in different wavelengths, it’s usual to have instruments that can. An example of this is telescopes. Telescopes in different wavelengths allow us to see things in space that we wouldn’t usually be able to. This is a pic of the sun taken with telescopes of different wavelengths, each one allowing us to see different aspects of the sun.

So obviously we’re all now thinking how cool it would be if we could see in different wavelengths! Well, if we could see radio waves, we’d constantly be bombarded with light from all directions, as our smartphones, pcs, TVs and satellites in space all emit radio waves!

X-ray vision sounds cool though right? Well yeah, if you like looking at people’s bones.

Microwaves? We’d have light shooting at us from all directions as we see the afterglow from the big bang! Our brains would have a hard time processing that!


Cosmic Microwave background http://wmap.gsfc.nasa.gov/media/101080

If you’re a teacher and looking at Light with your class, book one of our light workshops!

http://www.labrascals.co.uk/

Have a great weekend!

Katie, Carol, Anita and Sarah

The Lab Rascals Team xx

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