UNIT 5. LIGHT

Presentation by Liannette Bellido Cintrón

Natural Sciences2˚ ESO

I.E.S. FRANCISCO RODRÍGUEZ MARÍNFEBRARY 2011

WHAT IS LIGHT?• Light is a type of radiation that spreads in

waves. The waves that can spread in a vacuum are called electromagnetic waves. Light is electromagnetic radiation.

• Visible light is the light which allows you to see the colour and shapes of objects. Light can come from a natural source or an artificial source.

Electromagnetic waves are classified according to frequency as shown in the diagram below:

Light is the radiation of the electromagnetic spectrum that we can capture with our eyes.

SPEED OF LIGHT• Light travels in a straight line. It travels in waves and does

not require a medium.• The speed of light however, does depend on the medium.• The speed of light in a vacuum and in the air is quite

similar, about 300,000 Km/s.• It takes light just eight minutes to travel from the Sun to

the Earth.

Nothing in nature can go faster than the speed of light.

Properties of lightThe light has three distinctive properties:

• Propagates in a straight line.

• It reflects when it reaches a reflecting surface.

• Changes direction as it passes from one medium to

another (is refracted).

SHADOWSSHADOWS• Something which proves the linear propagation of

light is the formation of shadows. • An opaque object placed in front of a source of light

will create a shadow.• A shadow is the dark area where there is little or no

light.• The shadow will have the same shape as the object

creating it, this is because light travels in straight lines.

• If a source of light, big or small, is very far from an object it produces sharp shadows.

• The less sharp area around the shadow is called shade or penumbra

Shadow

Shadows and Penumbras

SOLAR ECLIPSE

A solar eclipse occurs when the Moon passes between the Sun and the Earth so that the Sun is fully or partially covered. This can only happen during a new Moon, when the Sun and Moon are aligned to the Earth. At least two and up to five solar eclipses can occur each year on Earth, with between zero and two of them being total eclipses.

TYPES OF SOLAR ECLIPSESfa

There are four types of solar eclipses:

A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark silhouette of the Moon.

An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than the Sun’s size. Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon.

A hybrid eclipse (also called annular/total eclipse) transitions between a total and annular eclipse. At some points on the surface of the Earth it is visible as a total eclipse, whereas at others it is annular. Hybrid eclipses are comparatively rare.

A partial eclipse occurs when the Sun and Moon are not exactly in line and the Moon only partially obscures the Sun.

LUNAR ECLIPSE• A lunar eclipse occurs when the moon passes behind

the Earth such that the Earth blocks the sun’s rays from striking the moon.

• This can occur only when the Sun, Earth and Moon are aligned exactly, or very closely so, with the Earth in the middle.

• That means there is always a full moon the night of a lunar eclipse.

SOLAR ECLIPSE vs LUNAR ECLIPSE

• Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth.

• A lunar eclipse lasts for a few hours. A total solar eclipse lasts for only a few minutes at any given place.

• The reflection of light is represented by two rays: the one that arrives at the surface, the incident ray, and the one that “bounces” from the reflective surface, the reflected ray.

• If you draw a line perpendicular to the surface (this line is called called normal), the incident ray makes an angle with that line, which is called the angle of incidence.

Reflection

normal

The laws of reflection are as follows:

1.The incident ray, the reflected ray and the normal are in the same plane perpendicular to the reflective surface. 2.The angle of incidence is equal to the angle of reflection.

Normal

Why do we see objects?We can see objects around us because light the reflected in

them reaches our eyes.

There are two types of light reflection:

Light RefractionThe refraction of light is the change of direction experienced by light rays passing from one medium to another in which they propagate with different speed. For example, when passing from air to water, the light is refracted.

The fundamental laws of refraction are:

• The refracted ray, the incident and the normal are in the same plane.

• The refracted ray gets closer to the normal as it passes from a medium in which it spreads faster to another that it spreads slower. On the other hand it moves away from the normal when it passes to a medium that it spreads faster.

The dispersion of light, a demonstration of refraction

White light is a mixture of colors: if a beam of white light passes through a dispersant medium, for example, a prism, the colors are separated because they have different refractive indices.

LENSES• A LENS is an optical device with perfect or approximate

axial symmetry which transmits and refracts light, converging or diverging the beam.

• Are used for very different purposes: glasses, magnifying glasses, binoculars, camera lenses, telescopes, etc

• A simple lens consists of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis;

• Lenses are typically made of glass or transparent plastic.

Types of simple lenses

• Converging Lenses: They are thicker in the middle than at the edges. The rays refracted by them converge at a point called the focus.

• Diverging lenses: They are thicker at the edges than the center. Refracted rays do not converge at a point, but separate.

TRASMISION OF LIGHT TROUGH OBJECTS

Objects can be transparent, translucent or opaque. This depends on how light travels through them:

• TRANSPARENT OBJECTS they allow light to travel through them. Air, water, glass.

• TRANSLUCENT OBJECTS they absorb some light and they allow some to travel through. Onionskin paper and frosted glass.

• OPAQUE OBJECTS they absorb most of the light they receive. They don’t allow light to travel through. A dark piece of paper, a piece of wood, a piece of metal.

PRIMARY COLOURS OF LIGHT

Red, blue, and green are the primary colors of light. Mixing these colors can produce all of the colors of the spectrum.

The secondary colours of light are yellow, magenta and cyan.

By mixing:Red + green yellowRed+ Blue magentaGreen + Blue cyan

 

TRANSMISSION OF COLOUR BY TRANSPARENT AND COLOURED OBJECTS

• When white light passes through a coloured crystal, only a part of this white light is transmitted. It depends on the object’s colour.

If we see an object through a coloured crystal, the colour we see the object depends on its original colour and on the crystal’s colour too.

An “emmetropic” eye is one that is able to focus on both far and near objects. But there are some defects associated with the vision:

Defects of vision

Myopia : A problem with the focusing ability of the eye. It is when the eye is not able to focus properly on objects in the distance. It happens when the cornea becomes a different shape (too curved) from normal or when the eye is longer than normal and so causes light entering the eye to focus on a point within the eye before the retina. This causes the eye and brain to see the image of the distant object as a blurred image. Diverging lens are used to correct the condition.

Hypermetropia: The eyeball is shorter than normal and the image of nearby objects is formed behind the retina. People with the condition have problems focusing nearby objects. It can be corrected using converging lenses.

Astigmatism: a very common defect is due to deformities in the curvature of the cornea. The vision is not clear.