LIGHT AND SIGHT
Match the idioms below with their definitions
Idioms Definitions
• according to one's lights
• see the light (of day)
• in the light of
• stand in one's own light
• light up
• strike a light
• to make a flame, as with a match
• as one's opinions, information, or standards may direct
• with knowledge of; considering
• to make or become light; to make or become bright, cheerful, etc.;
informal to begin smoking (a cigar, etc.)
• to come into existence; to come to public view; to understand
• to harm one's reputation by acting unwisely
I. Read the text about light and answer the questions that follow
to propagate - přenášet, šířit (se)
to hold sway - (to be master, to rule) vládnout, kralovat
1. What is the dual nature of light?
2. What is the amplitude of the light?
3. What does colour of light depend on in the classical interpretation?
4. Describe photons.
5. What do colour and brightness depend on in the quantum theory of light?
What is Light?
Light is simply a name for a range of electromagnetic radiation that can be detected by the human eye. What is
electromagnetic radiation, then?
Electromagnetic radiation has a dual nature as bom
particles and waves. One way to look at it is as
changing electric and magnetic fields which propagate
through space, forming an electromagnetic wave.
This wave has amplitude, which is the brightness of the
light, wavelength, which is the color of the light, and an
angle at which it is vibrating, called polarization. This
was the classical interpretation, crystallized in
Maxwell's equations. which held sway until Planck,
Einstein and others came along with quantum theory. In
terms of the modern quantum theory, electromagnetic
radiation consists of particles called photons, which are packets ("quanta") of energy which move at the speed of light.
In this particle view of light, the brightness of the light is the number of photons, the color of the light is the energy
contained in each photon, and four numbers (X, Y, Z and T) are the polarization.
Which interpretation is correct? Both of them, actually. It turns out electromagnetic radiation can have both wave-like
and particle-like properties as demonstrated in experiments such as the dual slit experiment.
Source: Discovering Light! ThinkQuest [cit. 28. 3. 2012]. Available at <http://library.thinkquest.org/27356/p_index.htm>
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2 Refraction of light
Read the text and fill in the gaps with these words.
bend incidence light normal optical effect refracted refraction transparent
When a pencil is half submerged in a beaker of water, the pencil should appear to bend at the point it enters the water.
This 1_____________________ is due to refraction. When light passes from one 2_____________________ medium
to another, such as from air to water, its speed changes. This change in speed causes the light to
3_____________________. When light travels from a less dense medium, such as air, to a denser medium, such as
glass or water, 4_____________________ bends towards the normal but when it travels from a denser medium to a
less dense medium it bends away from the 5_____________________. The angle between the inddent ray and the
normal is called the angle of 6_____________________ and the angle between the 7_____________________ ray
and the normal is called the angle of 8_____________________.
3 Fluorescent lamps
Read the text and underline the correct words.
The fluorescent lamp is a sealed 1 (cylindrical / circular) glass tube that contains mercury at 2 (high / low) pressure
and electrodes at either ends of the tube. Sometimes a small amount of inert gas, such as 3 (boron / argon), is placed
in the tube. A phosphor powder coats the inside of the glass. When the lamp is turned on, the current flows through
the electrical circuit to the electrodes, which shoot 4 (protons / electrons) into the mercury vapour causing it to emit a
bluish light and 5 (extraviolet/ ultraviolet) radiation. We are unable to see ultraviolet radiation but when it hits the
phosphor powder coating the glass tube, it causes the coating to emit 6 (visible/violent) light. These lamps do not
produce much heat incandescent light bulbs glow because the 7 (filament/element) is heated to a very high
temperature. So a lot of energy is wasted as heat. Therefore 8 (luminous / fluorescent) lamps are more economical
than conventional 9 (candescent / incandescent) light bulbs, even though the initial installation cost of the fluorescent
lamp is 10 (lower / higher).
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B Working with sentences
4 Real and apparent depth
Match the beginning and endings of the sentences.
Beginnings Endings
1. When observing an object in water from
2. This optical effect
3. An observer sees the object when light rays leave
the object and travel
4. However, as the light rays leave the water and
enter the air, they are bent
5. This means that although the eye receives the light
rays from the object, they appear
6. The angle of refraction is so large that all of the
ray has been refracted
7. This is called
a. a back into the denser medium as though it had
been
b. reflected, b is also due to refraction.
c. above it seems closer to the surface than it actually
is.
d. to the surface of the water and enter the observer's
eyes,
e. 'total internal reflection',
f. away from the normal.
g. to have come from a point above the location of
the actual object.
1____ 2____ 3____ 4____ 5____ 6____ 7____
C Working with texts
5 How does light travel?
Read the text and then fill in the gaps on the diagram with suitable words.
Light travels in a straight line, which is why we cannot see around corners. In diagrams light is generally represented
as straight lines with arrowheads pointing in the direction in which the light travels. A single ray of light cannot be
seen but when they come together they become visible. These lines are called rays and a collection of equally spaced
rays is called a beam of light. There are different types of beams. Rays which do not meet are known as a parallel
beam of light. Those rays which start from a source and spread out from this point are called a diverging beam and
rays which come together to a point are known as a converging beam.
A single ray of light a___________________ but when they come together to
form beams they become b__________________.
Types of beam
c _____________________ d _____________________ e _____________________
of light – rays spreading out od the source – rays coming together
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6 The effect of different types of material on light
Read the text and then fill in the gaps on the diagram.
Opaque materials and objects block the passage of light, that is, they do not allow any light to pass through them. So it
is impossible to see through them to objects on the opposite side. However, not all materials are opaque. Transparent
materials allow light to pass through completely. As a result it is easy to see things clearly on the other side of a
transparent object. Clear glass and plastic wrap are examples of materials that are transparent. Translucent materials
on the other hand only allow some light to pass through them, so it is difficult to see objects on the other side clearly.
Examples of materials that are translucent are tracing paper, grease paper and frosted glass.
a____________ materials allow light
to pass through b____________,
making it c____________ to see
through them.
d____________ objects only allow
e_______________ to pass through,
making it f_______________ to see
through them clearly.
g_____________________ objects
h___________ allow i____________
to pass through, making it
j_____________ to see through them.
7 The filament lamp
Read the text and label the diagram.
A filament lamp is an incandescent source of
light, that is, it gives off light because it is very
hot. The filament light bulb has a very thin
tungsten filament coil, which is connected to
electrical foot contacts via two stiff pieces of
support wire. The support wires are enclosed in
insulation at the base of the glass mount. The
filament is enclosed in a glass case or bulb
filled with an inert gas such as argon, and is
fixed on a glass mount so that it sits in the
middle of the bulb. When the bulb is connected
to a power supply, an electric current passes
through the tungsten filament. This makes the
filament heat up to 2500 °C, at which point the
tungsten glows white hot and emits light and
heat. The heat is wasted energy since it
produces no illumination. At these high
temperatures the metal tends to evaporate and
become condensed on the sides of the bulb,
gradually darkening it. The inert gas in the bulb
slows down this process and prevents oxidation
of the tungsten which would happen if air was
used to fill the bulb. Some bulbs have a screw
thread to allow them to be securely fixed into a
lamp.
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8 Slide projector and camera
Read the text and label the diagram with these words.
concave mirror condensing lenses convex lens (2×) diaphragm film image
image inverted right way up light source object shutters slide inserted upside down
A slide projector uses an internal light source to illuminate a transparent slide. Light from the light source is collected
by a large, double lens called the condensing lens, which then makes the light rays converge towards the slide. The
slide is placed upside down in front of the light source and at a suitable distance behind the convex lens. The light
passes through the transparency of the slide and then through the lens which refracts the light rays and converges
them onto a screen, to give a large inverted image. However, the image will occur right side up if the slide (object)
was placed in the projector upside down. A camera is basically a lightproof box, which contains a light-sensitive film
behind a convex lens. When a photograph is taken, a shutter behind the diaphragm and lens opens and closes quickly.
During the brief period that the shutter is open, light enters the camera from the object. The lens first converges the
light rays causing them to cross and so produce a small inverted image on the light-sensitive film. The film is later
developed into a permanent picture. The illustration shows how an image is formed on the film inside a camera.
Ex 2–8; source: „Science“; Keith Kelly; 2007
IX Watch the video and then answer the questions below
(source: www.voutube.com)
1. What are rods and cones, as referred to in the recording?
2. What is cornea?
3. How is the shape of the lens controlled?
4. When does the ciliary body contract?
5. Where are rods and cones located?
6. What is the light energy turned into in rods and cones?
7. What is the function of the optic nerve?
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