Light is a form of energy detected by a light, which depending on certain factors can appear in the form of a wave or a stream of particles. As we have learned previously, sound needs a medium to travel through, but this is not the case with light, it can travel from the sun through space to reach earth.
Having previously covered longitudinal and transverse waves regarding sound, light falls into the second category, where the waves resemble a ripple in water. The vibrations occur at right angles to the direction of movement from the source which created the wave. Waves of light form what is referred to as a radio wave and when the eye recognises this wave it takes the part of the radio receiver. Light which is visible to humans tend to occur between 400 THz to 750 THz (Terahertz are Hertz multiplied by 10 to the power of 12). The range of frequencies appear in a vast range of colours, typically low frequencies appear red and high frequencies appear violet with various other stages in between. The colour white appears as a mixture of many different colours.
Velocity of Light
In a vacuum, light has a constant speed of 300,000 kilometres per second, with it being slighty less and air and down to approximately 200,000 in glass. The halt in speed in glass means it is useful for lenses. Light travels about a million times faster in air than speed does.
To travel a 6m length, you can measure the time taken by the following formula.
Time taken = Wavelength/Velocity
= 6 / (300000000)
= 20 ns
The frequency of a light wave is the number of complete cycles per second, which is independent from the medium it is travelling through. The formula for working out Velocity is to multiply the frequency by the wavelength. It is the same formula discussed at length in lecture one.
A 500 THz lightwave has a wavelength in air.
Wavelength = Velocity (300 x10 to the power of 6) / Frequency (500 x10 to the power of 12)
= 600 mm
The below chart shows the frequencies at which colours are prevalent, so we can assume this wave will be orange. It would still appear the same if the medium to change from air to glass, as like previously mentioned, the frequency is independent to the medium.
White light, originating from the sun, it's a combination of all of the colours in the visible spectrum. Some colours have more presence than others and this varies greatly from source to source.
The above diagram shows a Visible Light Spectrum, showing which kind of waves have which levels of frequency from the low frequency radios and microwaves, moving towards the higher frequency X-rays and gamma rays. Below is another diagram which gives an indication on frequency for each independent colour on the spectrum. The variation in colours of visible light have different frequencies.
The majority of light sources don't radiate single colour light, often referred to as monochromatic. However, a rare source for this is the yellow street light (sodium), giving off the effect of total colour-blindness, aided by the incorporation of shades of grey.
The brightness of light is measured by units called candela and human. These are more prevalently used by scientists rather than photographers, who use light exposure meters measured against a scientific standard. Object brightness is an extremely biased quality and is very much dependent on reflectance, colour and surroundings as well as the possible different state of the onlooking eye.
Environmental Effects On Light:
Both the atmosphere and surrounding objects affect light ways by the following means:
- Transmission
- Reflection
- Absorption
- Scattering
- Refraction
A large fraction of the incident light is transmitted by transparent or translucent objects. For example, the reflection of the surface surroundings are what is revealed in glass, were there none it would be invisible. Reflecting glass on to an object can make the shape appear differently.
Light intensity varies:
The intensity of light received from a source varies inversely as the square of the distance R from the source as 1/R squared. This means that a light reflected from an object will have one twenty-fifth of the intensity at a distance of five metres from observation, than it would from one metre.
Editing the colours and the contrast of an image can affect the perception of distance. For example as the amount of blue in an image is increased and the amount of contrast is decreased, an image will appear further and further away.
On the occurrence of energy travelling between mediums, typically some is passed while some is reflected. Reflections from a flat boundary appear like a mirror, but reflections from a curved surface tends to be focused to a point, line or area.
Specular reflection normally takes a mirror like form, whereas diffuse light occurs as a result of light scattering in various directions, adjacent objects typically colour each other through their reflective light, despite occasionally casting shadows on each other. An example of diffuse light would be the sun giving a piece of clothing a brighter illusion.
It is often asked why the sky is blue, when the air around us appears to have no visibility. Various factors in the air - molecules, vapour and dust particles - scatter sunlight throughout the air and short wavelength light is far more common than long wavelength light and as previously described, blue light occurs at short wavelengths, with long wavelength light going more towards red light. The sky appears more red upon the sun setter as the dust and vapour are travelling a longer path, through denser air, which compromises much of the blue colour, which appears to be scattered away. As a result of this, it becomes evident how a user can simulate a sunset effect on an image, as the image will be comprised of blue, green and red colours, it makes sense to increase the red at the expense of the other two, which will be further aided by being reduced, to reach the desired effect.
Refraction is when light gets bent being passed between mediums and occurs due to the fact that the speed of light varies in different material. When light takes an angled approach at hitting a boundary, it must decrease or increase in speed before it passes through and reaches the new material.
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