UD 7. LIGHT.
1. WHAT IS LIGHT?
LIGHT is a type of electromagnetic radiation.
All electromagnetic waves can travel through a vacuum at a
speed of 300 000 km/s: this is the speed of light in vacuum
and is represented by the letter c.
1. WHAT IS LIGHT?
Electromagnetic waves are classified according to their frequency as
shown on the electromagnetic spectrum below. Light is the visible
radiation of the electromagnetic spectrum. (VIDEO)
2. PROPERTIES OF LIGHT.
PROPERTIES
•1. LIGHT TRAVELS IN A STRAIGHT LINE.
•2. LIGHT IS REFLECTED.
•3. LIGHT IS REFRACTED.
2. PROPERTIES OF LIGHT.
•1. LIGHT TRAVELS IN A STRAIGHT LINE.
Light always travels in a straight line. It produces shadows
when its path is completely blocked by an obstacle.
Penumbras are formed when the light is partially blocked.
2. PROPERTIES OF LIGHT.
•1. LIGHT TRAVELS IN A STRAIGHT LINE.
2. PROPERTIES OF LIGHT.
•1. LIGHT TRAVELS IN A STRAIGHT LINE.
ECLIPSES.
An eclipse occurs when the Sun or the Moon becomes partly
or completely dark because of the position of the Sun, Moon
and Earth in relation to each other.
SHADOWS AND PENUMBRAS ARE PRODUCED DURING
ECLIPSES.
2. PROPERTIES OF LIGHT.
•1. LIGHT TRAVELS IN A STRAIGHT LINE.
ECLIPSE OF THE SUN.
The moon moves between the Sun and the Earth, the area of
shadow is very small because the Moon is small.
•At the place on the Earth´s surface where the shadow is formed, the
day gets darker and darker (total eclipse of the Sun).
•In
In the area of penumbra in the Earth
Earth´ss surface, we see a partial eclipse
of the Sun.
2. PROPERTIES OF LIGHT.
•1. LIGHT TRAVELS IN A STRAIGHT LINE.
ECLIPSE OF THE MOON.
The earth moves between the Sun and the Moon. Because
the Earth is bigger than the Moon, the areas of shadow and
penumbra formed by the Earth on the Moon are bigger than
the Moon´s diameter. So, a lunar eclipse can last around three
hours.
2. PROPERTIES OF LIGHT.
•2. LIGHT IS REFLECTED.
Light reflection is the change in direction of a light ray when it hits
the surface of an object.
2. PROPERTIES OF LIGHT.
•2. LIGHT IS REFLECTED.
There are two types of light reflection.
SPECULAR REFLECTION.
-Perfectly smooth surface.
-All the reflected rays leave the surface in the same direction.
-Mirrors.
2. PROPERTIES OF LIGHT.
•2. LIGHT IS REFLECTED..
DIFFUSE REFLECTION occurs when surfaces aren´t smooth, so
the rays are reflected in many different direction. We can see
objects and their shapes because of the diffuse reflection of light
from their surfaces.
2. PROPERTIES OF LIGHT.
•2. LIGHT IS REFLECTED.
There are two types of light reflection.
DIFFUSE REFLECTION.
2. PROPERTIES OF LIGHT.
2. PROPERTIES OF LIGHT.
•3. LIGHT IS REFRACTED.
There are two types of light reflection.
Light refraction is the change in direction of light rays when they
pass from one medium to another in which they travel at a
different speed.
THE REFRACTED RAY MOVES
CLOSER TO THE NORMAL
WHEN IT PASSES FROM A
MEDIUM IN WHICH IT TRAVELS
MORE QUICKLY TO ANOTHER
MEDIUM IN WHICH IT TRAVELS
MORE SLOWLY.
2. PROPERTIES OF LIGHT.
•3. LIGHT IS REFRACTED.
The relationship between the speed of light in
a vacuum and its speed in a transparent
medium is called index of refraction.
n = c/v
If yellow light travels at 170 455 km/s in sapphire, what is the refractive index of
sapphire for yellow light?
3. LIGHT AND MATTER: THE COLOURS OF THINGS.
Depending on how they behave in light, objects can be
transparent, opaque or translucent.
TRANSPARENT OBJECTS.
If light hits the surface of a transparent object at a right angle, It
travels through in a straight line and comes out the other side with
no change of direction.
GLASS, AIR, WATER.
3. LIGHT AND MATTER: THE COLOURS OF THINGS.
Depending on how they behave in light, objects can be
transparent, opaque or translucent.
OPAQUE OBJECTS.
Light cannot pass through them. That is why we can´t se images
through opaque objects.
WOOD, METAL…
3. LIGHT AND MATTER: THE COLOURS OF THINGS.
Depending on how they behave in light, objects can be
transparent, opaque or translucent.
TRANSLUCENT OBJECTS.
These absorb or reflect light and some light can travel through
them, but it is sent in different directions.
PAPER…
3. LIGHT AND MATTER: THE COLOURS OF THINGS.
3.2 THE COLOUR OF OBJECTS.
The colour of a transparent object depends on which colours
it transmits and which it absorbs.
EXAMPLE: A piece of glass is red when it absorbs all of the other
colours except red. Red is the only colour that this piece of glass
can transmit.
3.2 THE COLOUR OF OBJECTS.
The colour of an opaque object depends on which colours it
reflects and which it absorbs.
EXAMPLE: so, an object is red if it only reflects red. An object is
white when it reflects all of the colours and an object is black
when it absorbs all of the colours and doesn´t reflect any of them.
4. THE EYE AND THE SIGHT.
4. THE EYE AND THE SIGHT.
Rods cannot distinguish colours, but they are sensitive to
low intensity light and so they work well when there isn´t very
much light: rods allow us to see at night. But because rods
cannot distinguish colours, we can´s see colours at night.
DEVELOPMENT OF BASIC COMPETENCES
Page 97.