Topic outline
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Outcomes
By the end of this course you will:
- Know what light is
- Understand the role that photoreceptors play in your ability to see
- Be able to identify different sources of light and provide examples of each
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Your sight is one of the five senses that most people are blessed to have (the other four being hearing, taste, touch and smell).
Have you ever considered how important light is in your everyday life? Have you sometimes wished that it was lighter when you walked home from a late soccer match and you've struggled to see where you're going?
In this course you will explore the concept of what light is, some of the different sources of light as well as being able to differentiate between luminous and non luminous bodies.
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What is light?
The reason you can see is because there is an interaction between light and your eye. When you look at an object, the light is reflected off that object and enters your eye:
Image: Needpix (CC)
Let's dig a little deeper into the photoreceptors.
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Photoreceptors
Your eye has two types of photoreceptors namely rods and cones:Rods are responsible for your night vision and are highly responsive to the stimulation of light. Rods are able to detect outlines, shapes, movement and contrasts but are not able to determine colour. You may have noticed that when the light is very low that you can see shapes but not always the colour of them?
Image: Wikimedia Commons (CC BY-SA)Cones are responsible for your colour vision but require a large amount of light to be stimulated.
Click on the link below to find out more interesting features of light.
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Luminous and non-luminous objects
Objects can be classified as either luminous or non luminous:
- A luminous object is one which has its own source of light energy e.g. sun, stars and even some animals and plants. You are able to see luminous objects in the dark.
- A non-luminous object is one which does not have its own source of light energy. Non-luminous objects reflect light from a luminous object. You cannot see non-luminous objects in the dark. Examples include the moon, planets, coal, cloth, houses and YOU! (Note that coal is non-luminous when it is not burning, when it is burning it is classified as a luminous object)
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The moon
Isn't it interesting that the moon is a non-luminous body. The moon does not emit its own light. We can only see the moon when it reflects the light from the sun.
Image: Wikimedia Commons (CC BY) -
Natural sources of light
Some of the natural sources of light include the following:
Click on the hyperlink below to find out more about these natural sources of light.
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Incandescent light sources
Certain objects can emit light when heated to a high temperature e.g. candles, firewood, electric lights and even gas lamps.
Electric lights emit light by the following process: the bulb filament converts electric energy to heat energy. The filament then becomes very hot and is able to then produce light.
Image: Pix (CC0) -
Luminescent light sources
Luminescent light sources are different to incandescent light sources since they do not produce heat when emitting light, they are non-incandescent.
There are different types of luminescence, some of them being:
Chemiluminescence: the emission of light is due to a chemical reaction which takes place e.g glow sticks and chemical experiments in the science lab
Image: Wikimedia Commons (CC BY)
Bioluminescence: some plants, insects and sea life are able to emit light via a chemical reaction which converts chemical energy into light energy e.g. bioluminescent clams, fire flies and glow worms. Note that these are natural sources of light and not artificial light
Image: Flickr (CC BY)
Electroluminescence occurs when light is emitted as a result of the interaction of an electric field with a solid. A common example of this being car dashboard displays and head lights.
Image: Wikimedia Commons (CC BY) -
Gas discharge light sources
Fluorescence: Florescent materials are able to absorb energy and then emit light e.g. fluorescent lights. The electricity causes the gas to emit ultraviolet radiation which stimulates the tube coating to emit light. When the energy is switched off, the lamp ceases to emit light.
Image: Wikimedia Commons (CC BY)
The main difference between fluorescence and phosphorescence is the time taken to emit light. Fluorescence is quicker to emit light and ceases to emit as soon as the energy source is cut. Phosphorescence will take longer to emit light and is able to continue to emit light even after there is a cut off of the energy supply e.g. safety signs, glowing paints.
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