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Light and Its Ideosyncrasies

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Light and Its Ideosyncrasies LIGHT AND ITS IDEOSYNCRASIES CONTENTS - What is light? - wavelengths - How does it move - Bending light - Modifying movement - Filters - Blue sky thinking - Dawn and sunset - Apps and what to look for - Light trails - Bonfire night - Christmas lights WHAT IS LIGHT? Good question! “Visible light” is a small part of a range of emanations called ELECTROMAGNETIC RADIATION. It’s a wave made of pure energy but can sometimes behave like a particle. This is called particle-wave duality. Therefore, not only can light be thought of in terms of rays (waves) but also in terms of photons (packets of light that can be thought of as particles). WAVELENGTH As wavelength gets smaller (and therefore frequency increases) the penetration of EM radiation into matter increases. That’s one reason why gamma radiation is so dangerous. It’s hard to stop and has very high energy, enough to ionise atoms within the body, changing their properties and making you sick. Visible light just bounces off (reflection) or gets absorbed. For photographers, the wavelength is all about colour. Each colour has a different wavelength. White light (sunlight) is a mixture of all of the colours. POWER OF TEN NOTATION You’ll notice that the frequencies are written in powers of 10. 1024 - 102. The formal way of writing this is actually 1*102 which means 1*100=100. So 3*108 is 300,000,000. We might come back to this number in a minute. Negative powers refer to decimals. Thus, a nanometre (nm) is 10-9 metres, which means 0.000000001 of a metre, moving the “1” 9 decimal places backwards. HOW DOES LIGHT MOVE? Unlike sound, light does not need a medium in order to move, it can travel through a vacuum. It has a few properties related to movement:- RECTILINEAR PROPAGATION Light travels in straight lines. A single light ray also travels in a single plane. However, unless the beam is coherent (lasers etc), then a light source emits many wavelengths on multiple planes. SPEED OF LIGHT (c) The speed of light is constant (in an inertial rest frame!). It is the same for all EM radiation, and is 2.99792458 *108 ms-1 (metres per second), often rounded up to 3*108 ms-1. You cannot travel faster than the speed of light (c).So that is approximately 300,000 kilometres per second, or 200,000 miles per second (very approximately). And so a light-year is a distance of 3*108 * 365 (days) * 24(hours) * 60(minutes) * 60(seconds) = 9.4608*1015 metres. Nearly 6 trillion miles ( 6x1012 miles) BENDING LIGHT AND MODIFYING ITS MOVEMENT Can light be bent? Well the short answer is….NO. Light does not bend! It travels in straight lines. If someone says the light is bent they probably mean it has changed direction. However, it is possible to bend spacetime. Bends in spacetime are called gravity! So, the path of light will travel around the bend in spacetime. Only very big gravitational forces can noticeably bend the path of light, for example black holes. REFRACTION This is the change in the path of light. The angle of change depends on the wavelength, and this is how rainbows are formed, since the colours are all different wavelengths. DIFFRACTION Diffraction is what happens to light when it passes through a gap. The narrower the gap gets, the bigger the effect is. Narrow apertures in lenses lead to diffraction lessening the sharpness of a shot. REFLECTION This is light bouncing off a surface. There is a complication with reflection, however. Non-metallic surfaces polarize light, so any reflection from a surface like that will be in a single plane, or clustered tightly around a single plane. So, if you could remove that plane from the light entering the camera you could remove the reflection! This is how polarizing filters work. Source sunlight reflecting filter stops polarised light Surface Use on leaves, water, sky, anything that reflects, to saturate colours Light can also be diffused, absorbed and emitted POLARISING FILTERS Only circular polarising filters allow modern autofocus systems to work. Turn the outer ring to remove the plane of light that you don’t want. This means that you need to be careful which lens to buy the filter for, as it screws into the filter thread on the front of the lens BLUE SKY THINKING Why is the sky blue? At the blue end of the spectrum the atmosphere scatters EM radiation, protecting us and making the sky blue. Good job too! The shorter the wavelength the more deadly to life the radiation is. DAWN AND SUNSET At sunrise and sunset, the angle of the sun is low. This means there is more atmosphere for the light to travel through, the blue gets scattered more, and the remaining light looks increasingly golden, depending on conditions. The sky goes deep blue just after sunset and just before sunrise. This is called the blue hour (lasts about 10 mins). Blue hour PREDICTING CONDITIONS! Good luck! The British weather is fickle, and any prediction is approximate, but you can increase your chances… WEATHER APPS I use the BBC app and the one that comes with my phone: WHAT TO LOOK FOR For colourful sunsets and sunrises look for moon with cloud and sun with cloud. For backlighting and golden light look for clear skies. For morning dew look for clear colder nights followed by a clear sunrise. Sometimes the gaps between rain clouds can give very dramatic light. Sun and cloud…sunset…moon/stars and cloud dramatic sunsets Clear both before and after sunset orange/red light Clear cool nights…clear sunset dew or frost or mist Sunset orange sky……sunset…….red sky…….blue hour Sunrise blue hour…..red sky…..sunrise……pink/orange sky The Photographers’ Ephemeris (TPE) and Photopills Extremely useful apps. They can tell you the times of sunrise etc and the direction you can expect them in any given location superimposed on an HD map. There are also other options such as night sky, and hyperfocal distance . TPE photopills The Photographers’ Ephemeris has a free to use web based service https://www.photoephemeris.com/ But the mobile app is incredibly useful on location and in the planning stage. Both cost about £10 for the mobile app. TIDE TIMES I use “ayetides” (about £5.99), but there are probably free apps too. ORDINANCE SURVEY £2.99 per month buys access to all of the UK maps, including all of the different scales. All types of OS maps are available and change automatically as you zoom in and out. A single paper map is approx. £17.99 SPECIAL SITUATIONS LIGHT TRAILS - Pick your location with care. There should be background interest - Use a tripod - Aperture priority or manual exposure - 20 – 30 second shutter speed depending on background - F16 - 100 ISO - Control exposure exp comp = -1.66 - You may need an nd filter to get the longer shutter speed - Make sure you get the whole trail! No partials - Manual focus at the hyperfocal distance, or a third into the shot - Blue hour can add a lot - Try Burford hill - IS and long exposure noise reduction off FIREWORKS - Use a tripod - Manual focus at infinity or your AF will hunt, missing the shot - Bulb mode. B on the mode dial, or in manual go past 30 seconds shutter speed to B - 100 ISO - F16 to give dof and get rid of smoke - Open shutter at start of set - Close at the end of the set - Max duration about 15 seconds before things get confused - If there’s a lit background set your exposure time for that (use Av) - You will need a remote shutter release - IS and long exposure noise reduction off Night-time cityscapes and Christmas lights - - Tripod - Manual focus on back of camera - Aperture priority or manual - Exposure compensation -1.66 to -2 - F16 - ISO 100 - Shutter speed whatever as long as under 30 seconds - Watch for the brightest lights being blown out - Image stabilisers off! - Long exposure noise reduction turned on GET STUCK IN! .
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