Energy and Light

Learning outcomes

Recall From Year 7:

Recall different forms of energy and classify them as active or potential
Recall the difference between energy transfers and transformations
Recall the concept of the law of conservation of energy and how it relates to energy change problems

By the end of this unit you should be able to

distinguish between heat energy and temperature
Say that light is a type of energy related to other forms of radiant energy such as X-rays and radio
Say that light travels in straight lines in empty space, but can refract , reflect or be absorbed when it hits something.
Use the idea that light travels in straight lines to explain patterns of shadow, or lines of sight
Draw a diagram to show patterns of refraction and reflection in mirrors, prisms and lenses. Range: plane mirror, concave mirror and lens, convex mirror or lens, rectangular prism and triangular prism
Use the terms concave, convex, converging, diverging and plane to describe lenses and mirrors Range: given their shape, or information about the way they cause light rays to reflect or refract
Say that lenses and mirrors make pictures called images, and say what type of image is formed in different types of lenses or mirrors Range: real and virtual image, enlarged reduced or same size, upright or inverted; in converging lens or mirror or plane mirror
Explain the causes of imperfect vision and how it can often be corrected in various ways by using lenses or modifying the shape of the eye Range: short and long-sightedness, colour blindness
Say that light comes in a spectrum of colours, and our eyes see these and other colours which are mixtures of them
Use the theory of colour to explain observations such as why a red handkerchief looks black when viewed in blue light
Say visible spectrum is a small part of a much wider spectrum which is called the ‘electromagnetic spectrum’ and includes radio, microwaves, heat, ultraviolet, x-rays and gamma rays
Give some examples of how light is used in technology and relate these to the science ideas about light
Describe the anatomy of the eye and explain how light is refracted in our eyes to produce images on our light detector (retina) so that we can see Range: cornea, lens, iris, pupil, retina, optic nerve

Energy

We use the word energy a lot, but even scientists can find it hard to define what energy is in a way easily understandable to non-scientists. Energy is something that can make things change. It doesn't weigh anything and you can't touch it, but we see it all around us. Scientists define energy as the 'ability to do work'. Work in science means a force moving something through a distance.

There are different sorts of energy and different ways of classifying it. One way is to talk about active energy and stored energy.

Active energy

Active energy can be further subdivided into kinetic (or movement) energy and radiant energy:

Potential Energy

Stored energy is called potential energy. Energy can be stored in almost anything that can cause a force, such as elastic, gravity or electric or magnetic forces. It can also be stored in chemical bonds and in the forces that hold together the atomic nucleus.

Einstein's equation E=mc2 basically says that matter is also a very concentrated form of stored energy. Straight after the big bang, there was only energy. As the Universe cooled down, some of that energy changed into the atoms that were the starting material for stars and everything else.

One kilogram of matter is equal to 90 petajoules of energy; that is about equivalent to three years production from all NZ gas fields.

One of the most important laws of science is called the Law of Conservation of Energy: Energy cannot be created or destroyed, it can only be moved around or changed into another sort of energy.

Energy transfer and energy tranformation

Energy can be transferred from one object to another.

For example, when you tee off on a golf course, kinetic energy is transferred from the club to the ball. In an energy transfer, the type of energy remains the same.

Energy can also be transformed from one type to another.

For example, when you throw an object ball straight up into the air kinetic energy is transformed into gravitational potential energy.Whenever an energy transfer OR transformation takes place, the total amount of energy always stays the same. The unit of energy is called the joule, so the total joules has to stay the same. However, during most energy transformations some energy is changed to heat. If heat is not the energy type we want, we call the energy that is changed into the sort we want useful energy and the sort changed into heat waste energy.

Extra for experts: Although energy can't be destroyed, heat tends to be the 'end state' for a chain of energy changes.Heat energy can only be useful (do useful work) when there is a difference in temperature. For example, if I have hot rocks underground I can use them to do work - such as making electricity in a geothermal power station. However, I can only do this by cooling the rocks down which warms the atmosphere up. Once they reach the same temperature I can't get any more useful work out of the heat energy, even though the total amount of heat has not changed. Since heat ALWAYS naturally flows from hot to cold, that's the end of it. The energy hasn't been destroyed, but its ability to do useful work has. This is part of what is known as the Law of Entropy.

Heat energy

The end product for most energy changes is heat energy. For example: if you go running, your food energy becomes kinetic energy (only a tiny fraction of it) but then your energy is used to overcome friction and become heat. Your muscles become warm while you do it. This warmth is transferred to the air around you and warms up the atmosphere - very slightly.

If the Sun were to stop shining, its heat would be radiated away into space as infra-red (heat) radiation, until eventually the atmosphere froze and changed to a solid layer of ices the same temperature as deepest space (-269°C or 4 kelvin).

Heat energy in matter consists of vibrations in the particles. It is therefore a special kind of kinetic energy. In gases, the freely-moving particles move faster at higher temperatures. It is gases that help us work out that there must be a temperature called absolute zero, where the particles stop moving. This happens at -273.4 degrees celsius. The SI unit for temperature, the kelvin, starts at this temperature and has units the same size as degrees celsius. The freezing point of water is therefore 273.4 kelvin, which we write as 273.4 K. You can't have negative temperatures in kelvin, because you can't have less than no heat. Note that temperatures in kelvin have no degree symbol.

Heat energy can be transferred by conduction, convection or radiation.

Conduction is where heat is passed from on particle to the next.

This happens most efficiently in solids. Metals are the best heat conductors, but they vary a lot. Copper is one of the best metal conductors; silver and gold are even better. Iron and aluminium are good conductors and used in cookware. Titanium is an example of a metal which is a poor heat conductor.

Many non-metals are poor conductors. Ones with void spaces, such as polystyrene foam, conduct heat so poorly they are termed insulators.

Convection happens in liquids and gases in a gravity field. Most substances expand when heated, which makes them less dense so they rise. Convection drives most of our weather when air is heated in warmer places then moves around in response to the spin of the Earth. Rising air produces rain, descending air is dry and produces clear skies. Zones of descending air in the diagram on the right are the 'desert belts'. This is why Australia is so dry.

Radiation: the vibrating electrons in warm matter cause electromagnetic ripples.

These ripples radiate away as radiant heat energy. The hotter an object, the more energy these waves have. At cooler temperatures we feel 'heat radiation' which is known as infra-red light. At higher temperatures our eyes can see it as light.

Temperature is dependent on the amount of heat energy something has. If two things are the same temperature, here will be no heat energy transferred from one to the other. Otherwise, heat energy ALWAYS flows from hot to cold.

Heat energy can be stored. For example, when you cool down a bottle of water the temperature drops, but when it reaches zero that stops. Heat is still being removed, but it is stored heat. As this stored heat is removed, the water changes to ice. The temperature will only continue to drop once all the water has changed to ice.

In the same way, when you turn on the kettle the temperature rises as electrical energy changes to heat energy and this is transferred into the water by conduction and convection. Once it reaches boiling point the temperature stops rising. The water starts changing into gas, with the extra heat energy (at the same temperature) stored in the heat potential energy of the gas.

Stored heat energy in a change of state is called latent heat energy.

Light Energy

What is light?

Light is a type of radiant energy; we can tell this because strong light can make you hot.

The light we see is part of the electromagnetic spectrum. All parts of the electromagnetic spectrum are made of waves of electricity and magnetism, caused by moving electrons. These waves spread out in space as ripples (waves) of electric and magnetic force fields..

The red dot only moves up and down; it takes 5 seconds which is the period. This means you get 1/5 of a wave per second arrive at the right of the picture. Therefore the frequency is 0.2 per second.

Waves are a form of moving energy, where the energy is constantly changed from one sort to another. For example, in ocean waves the energy is changing from gravitational potential energy to kinetic energy.

Waves have several properties: speed (how fast they move), wavelength (how far between the tops of the waves), frequency (how many waves go pas per second) period (how long it takes for one wave to go past) and amplitude (how tall the waves are).

In EM waves, the energy changes backwards and forwards between electric and magnetic force fields. The ripples move at the speed of light - 300,000 km per second.

The different colours of light we see are waves of different wavelength. The shorter the wavelength of an EM wave, the higher in energy it is.

The wavelengths our eyes can see are just a small part of the much larger set of waves of the electromagnetic (EM) spectrum. Above is a diagram showing the different parts of this spectrum.

Our eyes can see EM wavelengths from around 0.0007 mm (red) to 0.0004 mm (blue). This light we can see is called the visible spectrum. it is just a small part of a much bigger range in wave sizes. For example, a wave of 0.001 mm would be longer than we can see, and we call it infra red. Waves this size are used in remote controls and for night vision (thermal imaging). If they are powerful enough, we feel them as heat.

Waves shorter than 0.0004 mm make up ultra violet and other waves such as X-rays ad gamma rays. These are high-energy types of waves which can cause damage to living things because they can penetrate into cells. Here, they have enough energy to damage DNA.

All warm objects radiate electromagnetic waves. We see as 'light' the waves radiated by objects between 2000 and 10,000 degrees celsius. Cooler objects radiate heat as infra-red and really hot objects, such as very hot stars, radiate energy in the ultra-violet and above.

Light and other electromagnetic waves travel in straight lines. In empty space they travel at 300 000 km per second, but in other substances which they can pass through they travel more slowly. The change of speed when light travels from one transparent substance to another is called refraction. It can also cause light to bend.

The substance of space that light travels through is called the medium. Empty space is a medium, but light can also travel through other substances such as air, water and glass. Such substances are said to be transparent.

Some substances allow some light through but absorb or scatter much of it so you can't see clearly through them. Such materials are translucent.

The semi-opaque roofing panel lets some light through but reflects back more than the translucent one (which is why it appears whiter). The amount of transparency and translucency in different materials can be highly variable.

Opaque materials either reflect the incoming light, or absorb it and change it to heat. Coloured opaque materials reflect the colour they appear but absorb the other colours and change them to heat e.g. a red roof reflects red light and absorbs the other colours.

Colour

As mentioned above, our eyes are 'electromagnetic wave detectors' for EM waves with a wavelength from about 0.0004 mm to about 0.0007 mm. Within the range we see the different wavelengths as different colours. When we see 'pure' light (of one wavelength only) we see the following range of colours:

However, the light we see most of the time is not made of 'pure' waves of one wavelength only. Instead, it is a mixture of different wavelengths. For example, sunlight is a mixture of all the wavelengths above plus some we can't see such as infra-red (heat) and ultra-violet.

We see sunlight as the colour 'white'. In fact, our eyes have three different colour detectors. The red detector detects waves from about 740-570 nm; the green detector detects from about 590- 530 nm and the blue one from about 520-380 nm.

This means that if you see pure yellow light, both the red and green detectors can see it so your brain says 'yellow'. However, you can fool the brain by making light that is a mixture of red and green light, This is how your phone screen can appear yellow, even though it only has red, green and blue dots on it - the red and green dots are lit up but the blue ones aren't so you see yellow. When all three colour dots are lit up, the screen appears white.

Some birds have separate 'yellow' detectors and could tell the difference between 'screen yellow' and 'real yellow'. To them, we are colour-blind. We have trichroic vision, with three colours; some birds have tetrachroic vision with four colours but most mammals (other than primates) have only dichroic vision with just two colours. These animals can't tell the difference between red and green; this is also the most common form of colour-blindness in humans.

A simulation showing the difference between what a partly dichroic person would see versus a full colour image.

Below is a diagram showing how the main colours we see are created:

Red, green and blue are the additive colours. A computer screen can create cyan by lighting up the green and blue pixels - our eyes interpret this as cyan.

Paper reflects light, it doesn't give it off. This means that a different system is used: translucent inks which reflect some of wavelengths in the background white and absorb others. For example, cyan ink allows green and blue through but absorbs red.

Magenta ink allows red and blue through but absorbs green. You can make blue by layering magenta over cyan: blue is the only wavelength that can go through both. Full colour can by obtained using three colour inks: cyan, magenta and yellow.

Actual printers use black ink as well, which absorbs all colours. This is for two reasons. First, most inks don't make a very 'black' colour when all three colour inks are mixed (it tends to be a very dark brown). The second is that it is much cheaper to use just one ink for printing lots of black and white, such as text.

Our brains use a lot of additional information to 'interpret' colour and are easily fooled - the purpose of vision is to sense your environment, not to be a scientific measurer of light information. For example, in the picture on the left, squares A and B are actually the same colour despite what your brain is telling you.

Eyes

Your eyes are light detectors. They have evolved to gain information about your environment by forming images on the retina which are sent to the brain. Different animals have eyes that are specialised for different environments and functions e.g. fish eyes are relatively flat because this helps them focus underwater.

Light enters the eye through a transparent window called the cornea. This also does much of the focusing of the light.

Behind they cornea is a coloured muscle called the iris. This closes up a hole in the middle, called the pupil, in bright light so that damaging amounts of light don't enter the eye. It also opens right up in dim light to let more light in, at the expense of blurrier vision. Usually it as half open.

Behind the iris, the lens can change shape to let you go from distant to close-up vision by refocusing the light on the retina, The retina contains the light detecting cells, which send their signal to the brain through the optic nerve.

There are no retina cells just in front of the optic nerve, so this is your blind spot or optic disc.

The non-transparent part of the eye is surrounded by the sclera, which is usually white in humans. A thin, transparent layer over the sclera called the conjunctiva contains tiny blood vessels which can swell up when irritated, turning your eyes red (conjunctivitis).

Muscles move the eye and it is nourished and lubricated by liquid from the lacrymal or tear glands. This is especially important for the cornea, which has no blood vessels in it. Tears are also antiseptic and help keep your eye sterile. Excess tears drain away through the tear ducts to the inside of the nose. Eyelids help distribute tears across the eye by blinking, and shut automatically when the brain senses a threat to the eye (to protect them) or during sleep (to stop the eye drying out). Extra tears are produced when the eye is irritated in order to wash away any potential irritants.

It is common for the eyes to not focus perfectly so that light converges either in front or behind the retina. This can be corrected with lenses (glasses or contact lenses).

Vision correction

The cornea and lens don't always focus light perfectly on the retina. This is called myopia, or short sight, or hyperopis (long sight).

It is common for the amount of correction to be different in different directions, which is called astigmatism.

Since the cornea does most of the focusing of the light, altering the shape of the cornea with laser surgery has become a popular alternative to correction with lenses.

There is considerable evidence that lots of up-close focusing increases the risk of short sight. Short-sightedness is very rare in cultures with no indoor electric lighting. It is quite likely that the era of phone screens will increase the amount of short-sight in the population.

Most people develop a degree of long sight in later middle age and will need glasses for close up tasks such as reading.

Behaviour of Light

Vision is one of our most important senses. Therefore, the behaviour of light has important implications for the way we sense the world.

Light sources and light reflectors

Our eyes can't usually tell the difference between something that is giving off light and something that reflects it. Things that give off light are light sources but most things we see are light reflectors.

The Moon might 'glow' brightly, but it is really only reflecting light from the Sun.

Glow-worms are sources of light, even if only very faint. We call light produced by organisms bioluminescence.

If your window faces away from the Sun, chances are the room is only being lit by reflected sunlight from objects and clouds outside.

Most of our phone screens use light-source technology. These light sources can struggle to be seen in bright sunlight, and many researchers are working on reflective technologies that work well in outdoor light. E-paper, such as in a Kindle, is one of the more widely used to date, but its refresh rate is too low for video and it struggles with colour.

OLED is the brightest display technology currently in use - the pixels directly emit light but still struggle in sunlight.

E-paper screens reflect light; they are easily visible in bright light but they have very slow refresh rates and are usually black and white only.

Light travels in straight lines

A ray of light will travel in a straight line until it hits something. It will then reflect, or be absorbed (becoming heat) or refract (bend). In many situations it can do two ore even all three of these things.

The fact that light travels in straight lines causes shadows to form. The Latin word for shadow is 'umbra'.

Notice that the shadow gets bigger when the source is closer to the object casting the shadow.

Shadows are less 'fuzzy' if the light source is a smaller point. Larger light sources produce a fuzzy edge to the shadow called the penumbra.

Smaller light sources produce much sharper shadows than larger ones. This is why professional photographers use large reflectors for the camera flashes - to produce much softer shadows.

Light travelling in straight lines is also the explanation for a pinhole camera:

Each ray of light passing through a tiny hole can only come from one place, and can only go to one place on the screen.

You can turn a whole room into a pinhole camera

Reflection

Light that hits a surface can be reflected, absorbed or refracted. Coloured objects such as green leaves absorb some of the light wavelenths and reflect the colour that you see. Plants usually reflect the green light, particularly if they need to keep cool (plants in cold places often absorb it for extra heat).

This means that objects may appear black in light of the opposite colour:

In white light all colours are present in the light. The red apple reflects red and the green pear reflects green.

Red light contains no green. The red apple still reflects the red, but there is no green for the pear to reflect so it appears black.

Green light contains no red. The green pear still reflects the green light, but there is no red light for the apple to reflect.

Light can be reflected in two ways. Most things scatter reflected light in all directions., which is why we can see them. For example, snow reflects all wavelengths equally, which is why it appears white. It reflects very efficiently, which is why it is so bright. However, the reflected light is scattered in all directions.

The other way light can be reflected is in an organised way by a smoot surface. A mirror is an example of such a smooth surface. Light rays reflected from a mirror aren't scattered randomly. Instead, they obey the law of reflection:

The angle of reflection is equal to the angle of incidence.

The angles are measure from a perpendicular line called the normal, although the law still applies if you measure from the surface on a flat mirror. We use the normal to measure the angle from because the law will still work for curved mirrors.

The organised way in which the rays are reflected causes an illusion that they seem to have come from behind the mirror. We therefore see a picture in the mirror that we call an image.

Your brain thinks a mirror is like a window, and assumes that the light has come from behind the mirror.

Because there are really no rays of light behind the mirror, we call the 'imaginary' rays here virtual rays.

The image formed by virtual rays is called a virtual image.

Images in a plane mirror are the same size, the right way up but laterally inverted. Sometimes we can be fooled into thinking the image is upside down; the way to check is to see if writing is the right way around.

Curved mirrors

Mirrors that curve inwards are termed concave. They will reflect light inwards to a point called the focus. The distance from the centre of the mirror to the focus is called the focal length.

The flatter the mirror, the further from the mirror is the focal length, Concave mirrors can produce two sorts of image:

Objects close to the mirror will produce an image that is the right way up (erect), enlarged, virtual and laterally reversed.

Objects that are further away away will form images that are upside down (inverted). These images are not reversed - you could read the writing if you stood on your head. They may be bigger or smaller than the original.

These images are real. This means they are made of actual light rays. A solar cooker works because it forms a real image of the Sun in the place you cook the food:

The light rays are concentrated on the bottom of the saucepan to make a "real" image.

The mirror is large, so all the Sun's rays from this large area are focused into a small image. This makes the image very hot.

The same principal is used in a satellite dish to concentrate the radio waves onto the tranceiver.

Headlight reflectors use this principal in reverse. The light source is placed so the image is directed down onto the road and not into the eyes of oncoming drivers.

A mirror that curves outwards is termed convex.

These mirrors produce images which are the right way up, laterally reversed and reduced, or smaller in size than the objects. They tend to give a wider field of view, so they are used in places such as security mirrors and side mirrors for cars.

Most large telescopes use concave mirrors. The small real image is examined through a microscope.

Refraction

When light passes from one transparent substance into another it usually changes speed. Light travels fastest in empty space. It slows down about 1% in air, 25% in water, 33% in glass and 60% in diamond.

Light is made of waves. When waves change speed along a boundary that isn't parallel to the wave-top, the slower part of the wave falls behind:

This causes the waves to change direction. You can see in the diagram that when the waves slow down, they bend towards the boundary.

The arrows in the diagram represent light rays. Since the rays are perpendicular to the wave tops, the rays are bent towards the normal to the boundary.

You can see in the photo to the left that the ray of light bends inside the glass block. It bends towards the normal.

Because the two sides of the glass block are parallel, the light ray coming out the other side is parallel to the incoming ray. This is why the image you see through a window is not distorted (if the glass is smooth).

Different colours of light bend by different amounts. For glass and water, red bends the least and violet bends the most. This is called dispersion.

You don't see it in the rectangular block because the dispersion on the opposite faces is reversed and cancels out. But if the two faces are not parallel, as in the triangular prism to the right, the dispersion does not cancel out and white light gets split into the spectrum of its component colours.

When light passes from a denser medium to a less dense one it is refracted away from the normal. This means that there must be some angle where it is refracted at 90 degrees along the surface. This is called the critical angle.

Light rays that hit the surface at more than the critical angle are reflected back. This is called total internal reflection. Some light rays are always reflected at a boundary, which is called partial reflection.

Total internal reflection is the reason why, when you are underwater, the surface more than a certain distance from you looks like a mirror.

It is the principle behind optical fibres, which bounce the light around on the inside of a very thin glass fibre and turn it into a 'light pipe' that can carry a beam of light for many kilometres.

Rainbows are caused by a combination of total internal reflection and dispersion:

This is why rainbows always seem to be some distance away from you: all the raindrops at the right angle from your eyes - regardless of distance - will reflect the sun back to you. However, because of the curve of the raindrop surface, the red light is bent less than the violet so you see it at a smaller angle. All the drops at a particular angle make a cone in the sky which you see as a circle. Unless you are up in the air, you will normally only see a half-circle because it goes into the ground.

Lenses

A lens is a specially shaped piece of glass that refracts light to make an image. A lens that is thinner on the edges than the middle is called a convex lens. One that is thicker on the edges than the middle is called a concave lens.

Convex lenses bend light inwards into a focus i.e. they are converging lenses.

The point where the light rays come together is called the focus. If you focus sunlight with a convex lens you can use it as a 'burning glass'.

Close up objects form a virtual, erect (right way up) and enlarged image with a convex lens. This is why they are used for magnifying glasses.

Distant objects form a real, inverted (upside down) and reduced image. Combinations of convex lenses can be used as a telescope.

Projectors use a convex lens to make an enlarged real image of a slide or electronic screen.

Concave lenses bend light outwards i.e. they are diverging. Concave lenses always form a virtual, erect and reduced (smaller) image that is closer to the viewer. This is why they are used for short-sighted people, as they bring the image close up where the person can see it clearly.

Image through a concave lens

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