VIII-SCIENCE & TECHNOLOGY CHAPTER-16:LIGHT

VIII-SCIENCE & TECHNOLOGY
CHAPTER-16:LIGHT

FROM 06-12-2010 TO 15-12-2010


Meaning of reflection-Incident and reflected ray/angle-laws of
reflection-mirror-image formation in plane mirror-lateral inversion-real
and virtual,erected and inverted,diminished and magnified-parts of the
eye-care of the eye

The light ray, which strikes any surface-incident ray.
The ray that comes back from the surface-reflected ray.
Draw a line making an angle of 90º to the line representing the mirror at the point
where the incident ray strikes the mirror. This line is known as the normal to the
reflecting surface at that point

The angle between the normal and incident ray- angle of incidence
The angle between the normal and the reflected ray-angle of reflection
Angle of incidence is always equal to the angle of reflection.

The incident ray, the normal at the point of incidence and the reflected ray all
lie in the same plane
Formation of image by a plane mirror

The reflected rays do not actually meet but only appear to do so-virtual image
image formed by a mirror the left of the object appears on the right and the right
appears on the left- lateral inversion
When all the parallel rays reflected from a plane surface are not parallel, the
reflection is known as diffused or irregular reflection
Take two plane mirrors. Set them at right angles to each other with their edges
touching

Place a coin in between the mirrors.

Hinge the mirrors using the adhesive tape at different angles, say 45º, 60º, 120º, 180º
etc. Place some object in between them. Number of images=(360°/θ) -1
Note down the number of images of the object in each case.
Set the two mirrors parallel to each other. Find out how many images of a candle
placed between them are formed
Number of images formed by mirrors placed at an angle to one another is used in a
kaleidoscope to make numerous beautiful patterns.
Parts of the eye
Eye has a roughly spherical shape.

Outer coat of the eye is white. It is tough so that it can protect the interior of the
eye from accidents. Its transparent front part is called cornea
Behind the cornea, a dark muscular structure called iris.
In the iris, there is a small opening called the pupil.
The size of the pupil is controlled by the iris.
The iris is the part of that eye which gives it its distinctive colour. a person has green
eyes, refer actually to the colour of the iris.

Iris controls the amount of light entering into the eye.
Behind the pupil of the eye is a lens which is thicker in the centre.
The lens focuses light on the back of the eye, on a layer called retina
Retina contains several nerve cells. Sensations felt by the nerve cells are then
transmitted to the brain through the optic nerve.
Cones, which are sensitive to bright light and Rods, which are sensitive to dim light.

Besides, cones sense colour
At the junction of the optic nerve and the retina, there are no sensory cells, so no
vision is possible at that spot. This is called the blind spot.
The impression of an image does not vanish immediately from the retina. It persists
there for about 1/16th of a second.
Number of separate pictures in proper sequence. They are made to move across the
eye usually at the rate of 24 pictures per second (faster than 16 per second). So, we
see a moving picture.

Eyes with eyelids to protect from any object entering the eye. Eyelids also shut out
light when not required.
Can see distant objects as well near objects
Minimum distance at which the eye can see objects distinctly varies with age
Most comfortable distance at which one can read with a normal eye is about 25 cm.
Some persons can see near objects clearly but cannot see distant objects so clearly.
Some persons cannot see near objects clearly but they can see distant objects quite
well.

With suitable corrective lenses, these defects of the eye can be corrected.
in old age, eyesight becomes foggy-due to the eye lens becoming cloudy-cataract-loss
of vision, sometimes extremely severe.-opaque lens is removed and a new artificial lens
is inserted.
Too little or too much light is bad for eyes. Insufficient light causes eyestrain and
headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can
injure the retina.

Do not look at the sun or a powerful light directly.
Never rub eyes. If particles of dust go into eyes, wash eyes with clean water.
Wash eyes frequently with clean water.
Always read at the normal distance for vision. Do not read by bringing book too close
to eyes or keeping it too far.

Lack of vitamin A in foodstuff is responsible for many eye troubles.
Limited vision to see things-cannot see at all since birth-may lose their eyesight
because of a disease-try to identify things by touching and listening to voices
Louis Braille visually challenged person, developed a system for visually challenged
persons and published in 1821.

Braille system has 63 dot patterns or characters. Each character represents a letter,
a combination of letters, a common word or a grammatical sign.
Dots are arranged in cells of two vertical rows of three dots each.
These patterns when embossed on Braille sheets help visually challenged to recognise
words by touching. To make them easier to touch, the dots are raised slightly