This chapter talks about properties of light and how it helps us in seeing objects.
The sense of sight is one of the most important senses through which we see the world.
16.1 What makes things visible?
When light from an object enters our eyes, we see the object. The
light may have been emitted by the object or may have been reflected by
it.
16.2 Light Rays
16.2.1 Ray of Light
Actually the ray of light is a narrow beam of light which is made up of several rays.
16.2.2 Incident Ray
The light ray, which strikes any surface is called the Incident Ray.
16.2.3 Reflected Ray
The ray that comes back from the surface after reflection is known as the Reflected Ray.
16.3 Light Angles
16.2.4 Normal
The line making an angle of 90º to the point where the incident ray strikes the mirror.
16.2.5 Angle of Incidence
The angle between the normal & incident ray is called the angle of Incidence (<i)
16.2.6 Angle of Reflection
The angle between the normal & the reflected ray is known as the angle of reflection (<r)
DIAGRAM
16.4 Laws of Reflection
The two laws of reflection are –
- The angle of incidence is equal to the angle of reflection.
- Incident ray, reflected ray & the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.
16.5 Formation of Image
16.5.1 Lateral Inversion
In an image formed by a mirror, the left of the object appears on the
right & the right appears on the left. This is known as Lateral
Inversion.
16.5.2 Regular & Diffused Reflection
16.5.2.1 Regular Reflection
Reflection from a smooth surface like that of a mirror is called Regular Reflection. Images are formed by regular reflection.
DIAGRAM
16.5.2.2 Diffused Reflection
When all the parallel rays reflected from a plane surface are not
parallel, the reflection is known as Diffused or Irregular reflection.
DIAGRAM
16.6 Illuminated Objects & Luminous Objects
The objects which shine in the light of other objects are called Illuminated Objects. Example – Moon.
The objects which give their own light, are called as Luminous
Objects. We see them when their light falls on our eyes. Example – Sun,
Candle
16.7 Reflected Light can be Reflected Again
The most common example of this can be remembered as the one that we
see in the salon after a haircut. We can see the hair at the back of our
head when the hair dresser holds a mirror behind us & we are
sitting in front of another mirror. In this, two plane mirrors are used.
A periscope also makes use of reflected light being reflected again using two plane mirrors.
16.7.1 Multiple Images
When two mirrors are inclined to each other, multiple images are
formed. This is what makes a kaleidoscope. Beautiful patterns are formed
in a kaleidoscope of multiple reflections.
16.8 Sunlight – White or coloured
Sunlight is referred to as white light although it consists of seven
colours. Splitting of light into its colours is known as Dispersion of
light. Rainbow is a natural phenomenon showing dispersion.
16.9 Eyes – What’s inside it?
Eye is one of our most important sense organs. We can see things only when light coming from them enters our eyes.
16.9.1 Structure of Eye
The eye has a roughly spherical shape.
- The outer coat of the eye is white. It is tough, so that it can protect the interior of the eye from accidents.
- The transparent front part of the eye is called the cornea.
- Behind the cornea, is the dark muscular structure called the Iris.
- In the iris, there is a small a opening called the pupil. The size of the pupil is controlled by the Iris.
- Iris gives the distinctive colour to the eye.
- Iris controls the amount of light entering the eye.
- Lens of the eye focuses light on the back of the eye on a layer
called Retina. Retina contains nerve cells. Sensations felt by the nerve
cells are then transmitted to the brain through the optic nerve.
Two kinds of cells that are present in the eye are –
Cones: They are sensitive to bright light & sense colour.
Rods: They are sensitive to dim light.
Blind Spot: At the junction of the optic nerve & the retina,
there are no sensory cells. So, no vision is possible at this spot. So
this is called as Blind Spot.
16.9.2 How do we see objects?
The impression of an image does not vanish immediately from the retina. It persists there for about 1/6th of a second. This is how we see an object.
16.9.3 How do we see moving objects?
If still images of a moving object are flashed on the eye at a rate
faster than 16 per second, then the eye perceives this object as moving.
The movies that we see are actually a number of separate pictures in
proper sequence. They are made to move across the eye usually at a rate
of 24 pictures per second. So, we see a moving picture.
Eye, on its own is a wonderful instrument. It has eyelids which
prevent any object from entering the eye. Eyelids also shut out light
when not required. Eye can clearly see distant objects as well as nearby
objects. The minimum distance at which the eye can see objects
distinctly varies with age. The most comfortable distance at which one
can read with a normal eye is about 25 cm.
16.9.4 Diseases of Eye
Disease – Cataract
Sometimes, particularly in old age, eyesight becomes foggy as the eye
lens become cloudy. This condition is called as Cataract. In Cataract,
there is a loss of vision, sometimes severe. It is possible to treat
this defect. To treat this, the opaque lens is removed & a new
artificial lens is inserted in the eye. This is a surgical procedure,
however with advancement in medical technology, this has become a quick
and safe operation.
Taking Care of the Eyes
It is important to take proper care of the eyes. To do so, following are advised
- If advised, use suitable spectacles.
- Too little or too much light is bad for eyes.
- Do not look at the sun or a powerful light directly.
- Never sub your eyes if any dust particles go into your eyes. Wash your eyes with clean water.
- Wash your eyes frequently with clean water.
- Always read at the normal distance for vision. Do not read by bringing the book too close or by keeping it too far.
- Eat a well balanced diet. The diet should include vegetables rich in
Vit A like Raw carrots, Broccoli, spinach & cod liver oil. Fruits
such as Papaya & mango are also rich in Vit A.
16.9.5 Visually challenged persons can read & write
Visually handicapped people, develop their other senses more sharply.
However, additional resources can enable them to develop their
capabilities further.
16.9.6 Resources for visually handicapped
Resources can be of two types – Non-optical Aids & Optical Aids.
Non-Optical Aids include visual aids, tactual aids (using sense of
touch), auditory aids (using the sense of hearing) & electronic
Aids. Tactual aids, including Braille writer state & stylus help the
visually challenged people in taking notes, reading & writing.
Optical Aids include bifocal lenses, contact lenses, magnifiers &
telescopic aids. All these are used to rectify visual limitations.
Braille system – The most popular resource for
visually challenged persons is Braille which was developed by Louis
Braille, who himself was a visually challenged. This method depends upon
recognition by touching.