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16. Light

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)


16.4           Laws of Reflection

The two laws of reflection are –

  1. The angle of incidence is equal to the angle of reflection.
  2. 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    Regular Reflection

Reflection from a smooth surface like that of a mirror is called Regular Reflection. Images are formed by regular reflection.

DIAGRAM    Diffused Reflection

When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as Diffused or Irregular reflection.


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
  • Rods

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

  1. If advised, use suitable spectacles.
  2. Too little or too much light is bad for eyes.
  3. Do not look at the sun or a powerful light directly.
  4. Never sub your eyes if any dust particles go into your eyes. Wash your eyes with clean water.
  5. Wash your eyes frequently with clean water.
  6. Always read at the normal distance for vision. Do not read by bringing the book too close or by keeping it too far.
  7. 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.