LIGHT AND HEAT
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THE NATURE OF LIGHT
1.1The propagation of light
Light spreads in everything made of matter and also in the void or vacuum. It can travel at very high speed, approximately 3000,000 kms/sec. Despite this enormous speed, sunlight takes approximately 8 minutes to reach the Earth.
Light comes from light sources. We can see this in the formation of shadows (dark silhouettes in the shape of the objects). Light rays travel from light sources to objects, are reflected in them and reach our eyes.
1.2The interaction of light with objects
We can distinguish three types of objects or materials, depending on their behaviour with light [see]:
- Transparent objects [see]: they allow light to pass through them and make it possible to clearly see what is behind them. For example: air, the glass in a window, clean water, etc.
- Translucent objects [see]: they allow light to pass through them but do not make it possible to clearly see what is behind them. For example: frosted glass, cloudy water, fine fabric, etc.
- Opaque objects [see]: these do not allow light to pass through them. For example: a wooden door or a wall.
When an opaque object is put in the way of a ray of light, it stops it from spreading. The light does not reach behind the object and creates a dark area, which is called a shadow.
If the light source is smaller than the object, the shadow has clear lines and two areas can be clearly distinguished: the illuminated areas, where the light from the light source reaches, and the dark area, where the light does not reach.
However, in the majority of cases, the light source is large, that is, it is bigger in size than the object and the borders of the shadow are unclear. This area is called half-light. Only the light from certain points of the light source reaches it. The larger the light source is in relation to the object, the greater the area of half-light and the smaller the shadow.
We can see this by using a torch to illuminate opaque objects of different sizes and observing the shadows and areas of penumbra that they form.
In nature there are numerous phenomena related to light, shadow and half-light, such as day and night, eclipses, etc.
For example, as light is spread in a straight line, when the Moon positions itself between the Sun and the Earth, it projects its shadow onto our planet, which is what causes solar eclipses. In contrast, if the Earth is located between the Sun and the Moon, a lunar eclipse is created, that is, it is within the shadow cone projected by Earth and we cannot see it. In both cases, shadows and half-light are created.
THE PERCPTION OF LIGHT
Human beings perceive light through sight. When an object is illuminated, it reflects the light it receives. These rays penetrate our eyes and reach the retina. This is where the image of the object is formed. Inside the retina are cells called photoreceptors (cones and rods) which are responsible for transforming this light stimulus into nerve impulses that are transmitted to the brain thanks to the optic nerve. When the brain receives an image from each eye, each from a slightly different angle, we can perceive the distance at which we see things.
THE BREAKDOWN OF WHITE LIGHT
One of the most beautiful phenomena in nature is the rainbow, which is produced when sunlight crosses the small drops of water that are suspended in the air after rainfall. The bands of colour that we can see (red, orange, yellow, green, blue, indigo and violet), form the visible spectrum [see].
This same effect can be seen when you pass a ray of white light through a triangular glass prism. The English scientist, Isaac Newton (1642-1727) was the first to carry out this experiment and obtained the same bands of colour as the rainbow. With this he demonstrated that white light is a mixture of different colours.
The combination of colours that form white light is called the visible spectrum. Sunlight is white light, that is, it is the mixture of all the colours of the visible spectrum.
Objects absorb part of the rays that comprise white light and reflect others that reach our eyes. Why do we see red objects? Because the object absorbs all the light rays that it receives except red light rays, which it reflects. These rays reach our eyes and are converted into nervous impulses that the brain interprets, giving us the sensation of the colour red.
Green plants absorb all of the colours of white plant and only reflect green, which is the colour that reaches our eyes.
For this reason, an object that reflects all the white light that it receives will be seen as the colour white, while an object that absorbs it all, or when it does not reflect anything, we will see as the colour black.
When a beam of white light is broken down, we can see seven different colours. Three of these are called primary colours: green, blue and red.
The superimposition of the light of two primary colours makes it possible to obtain secondary colours: for example, yellow is the result of the combination of red and green, magenta is obtained from red and blue, while cyan is a combination of blue and green.
Mixing the primary colours of light PRIMARY COLOURS SECONDARY COLOURS Red Yellow (red and green) Green Magenta (red and blue) Blue Cyan (blue and green)
From the mixtures of lights of the three primary colours, we can obtain the rest of the colours in the visible spectrum of white light, and if they are combined in a balanced way, they make white. This composition of colours is known as the mixture of the light’s primary colours, as it is due to the mixture of colours produced by a light source, such as a television screen.
REFLECTION OF LIGHT
Reflection is a phenomenon that takes place when a ray of light falls on an object and bounces off it in another direction. We can say in this case that light is reflected. All bodies, except black bodies, reflect the light they receive to a greater or lesser extent; mirrors in particular reflect practically all the light and provide very clear images [see].
Mirrors are highly polished surfaces and are almost always made of plate glass or metal. There are different types of mirror and they reflect the light in different ways:
- Flat mirrors: this is what we usually use in our homes. When we place an object in front of it, the image we see has the following characteristics [see]:
- It has the same size as the object.
- It is virtual, because it looks as though it is inside the mirror and cannot be shown on a screen.
- It is straight, because it conserves the same position as the object, that is, the upper part of the image coincides with the upper part of the object.
- It is symmetrical, because it looks as though it is at the same distance from the mirror as the object.
- Curved mirrors: these have the shape of a hollow sphere. They can be concave if the curved surface is on the inside, or convex if it is on the outside:
- Concave mirrors: are what we use in the bathroom to see ourselves “enlarged”. They can be images that are larger or smaller than the object, straight or inverted (we would see our head at the bottom), depending on the distance of the object to the mirror. The hollow of a metal spoon works as a concave mirror if the surface is well polished. Try looking into it close up and far away and see what type of image you get.
- Convex mirrors: these are the rear view mirrors of vehicles, the ones on street corners with limited visibility and those used for surveillance in shopping centres. They always give virtual, straight images that are smaller than the object.
REFRACTION OF LIGHT
Lenses are usually made of glass, but they can also be made with other transparent materials. At least one of their sides is always curved. We can distinguish between two main types of lenses [see]:
- Convergent lenses: these are thicker in the centre and have a convex shape, that is, they are curved on the outside. They make the rays of light that pass through them converge so that they meet at one point. They are used in a lot of optical instruments such as magnifying glasses and to correct far-sightedness.
- Divergent lenses: these are thinner in the centre and have a concave shape, that is, they are curved on the inside. Rays of light are separated after passing through the lens. They are used to correct short-sightedness.
Human beings have optimum vision when all the parts of the eye work correctly. The eye has the function of concentrating and focusing the rays of light coming from an object inside the retina. The retina acts as a screen where the image forms.
Any problem in any of the parts of the eye prevents the correct formation of the image on the retina is known as a vision defect. This is why some people have to wear glasses to correct them.
Vision defects are usually associated with the incorrect working of the crystalline lens, which is the part of the eye that focuses the ray of light that goes from an object to the retina. It has the shape of a convex lens and is able to increase or decrease its thickness to focus on objects that are near or far.
The crystalline lens
The crystalline lens is a flexible lens that can be made thinner or thicker to focus on objects. This is known as accommodation.
Some of the most common vision problems are due to the incorrect formation of the image of objects in the retina due to a lack of focus. They are as follows:
- Short-sightedness (myopia): the rays that pass through the crystalline lens converge and form the image before reaching the retina. Far away objects look blurred but very close objects are very clearly focused.
- Far-sightedness (hypermytropia): this takes place when the crystalline lens is not sufficiently curved to focus on close objects. The rays converge and form the image behind the retina.
- Astigmatism: this is caused by a deformation in the cornea or by a defective curve of the crystalline lens. It produces a deformed image in the retina.
- Presbyopia: this usually appears in elderly people because as they become older, the crystalline lens becomes harder and loses its ability to vary its thickness (its accommodation ability is reduced, which makes it difficult to focus on objects.
- Cataracts: are characterised by the appearance of opaque areas in the crystalline lens, which prevents the passing of light and therefore damages vision. They normally appear in elderly people. They are corrected by the extraction of the crystalline lens and the use of convex lenses.