Class 8 CBSE Science Notes of Light

Notes of Class 8th Science Chapter 16 – Light

What is light?

Light is a form of energy that is needed to see things around us.

What makes things visible?

It is light which makes things visible to us. Light enables us to see things from which it comes or from which it is reflected.

When light coming from an object enters our eyes then we can see that object.

What are luminous objects?

The objects which emit their own light are called luminous objects. The sun, other stars, lighted electric bulb, lighted electric bulb, glowing tube light, torch, fire and flame of a burning candle are all luminous objects.

What are non-luminous objects?

The objects which do not emit their own light are called non-luminous objects. They cannot make their own light. Examples are the moon, earth, other planets, table, chair, book, trees, plants, flowers, human beings, fan, bed, mirror, floor and roads are non-luminous objects.

The non-luminous objects are also called illuminated objects.

What is reflection of light?

The process of sending back light rays which fall on the surface of an object, is called reflection of light.

What is incident ray?

The ray of light which falls on the mirror surface is called incident ray.

What is point of incidence?

The point at which the incident ray strikes the mirror is called the point of incidence.

What is reflected ray?

The ray of light which is sent back by the mirror is called the reflected ray.

What is normal?

The “normal” is a line drawn at right angles to the mirror surface at the point of incidence.

What is angle of incidence?

The angle between the incident ray and normal is called the angle of incidence.

What is angle of reflection?

The angle between incident ray and normal is called the angle of incidence.

What are laws of reflection of light?

The angle of reflection is always equal to the angle of incidence.

The incident ray, the reflected ray and the normal (at the point of incidence) all lie on the same plane.

What will happen is a ray of light falls normally or perpendicularly on the surface of a plane mirror?

When a ray of light is incident normally or perpendicularly on a plane mirror, it means that it is travelling along the ‘normal’ to the mirror surface. The angle of incidence for such a ray of light is zero, so according to the second law of reflection, the angle of reflection should also be zero.

This means that the reflected ray will also travel back from the mirror along the normal.

Thus, a ray of light which is incident normally or perpendicularly on a mirror is reflected back along the same path.

What is regular reflection of light?

In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction.

Regular reflection of light occurs from smooth surfaces like that of a plane mirror or highly polished metal surfaces.

A plane mirror produces regular reflection of light.

What is diffused reflection or irregular reflection of light?

In diffused reflection of light, a parallel beam of incident light is reflected in different directions. In diffuse reflection, the parallel incident rays do not remain parallel after reflection, they are scattered in different directions.

The diffused reflection is also known as irregular reflection or scattering. The diffuse reflection of light takes place from rough surfaces like that of paper, cardboard, chalk, table, chair, walls and unpolished metal objects.

The surfaces of most of the objects are rough or uneven to some extent. So, most of the objects around us cause diffuse reflection of light and scatter the light falling on them in all directions.

What are the characteristics of image formed by a plane mirror?

The characteristics of image formed by a plane mirror are following:

i). Virtual or unreal (cannot be obtained on the screen).

ii). Behind the mirror.

iii). At the same distance behind the mirror as the object in front of it.

iv). Same size as the object.

v). Erect.

vi). Laterally inverted (right side of object appear on the left side and vice versa).

What is a periscope?

A periscope is a long, tubular device through which a person can see objects that are out of the direct line of sight. A periscope gives us a higher view than the normal.

A periscope work on the reflection of light from two plane mirrors arranged parallel to one another. Each plane mirror, however, makes an angle of 45 degree with the side of the tube.

What are the uses of periscopes?

Some of the uses of periscopes are given below:

i). A periscope is used to see over the heads of a crowd (say, as in a football match).

ii). A periscope is used by soldiers sitting in a trench or bunker to observe the enemy activities outside (over the ground).

iii). A periscope is used by a navy officer sitting in a submarine to see ships over the surface of water in the sea even though the submarine itself may be submerged under water.

When are multiple images formed?

When two plane mirrors are kept inclined at an angle, they can form multiple images of an object. This is because the image of object formed in one plane mirror acts as an object for the other plane mirror.

What is the formula to calculate the number of images formed when two plane mirrors are inclined at certain angle say x degree to each other?

Formula to calculate the number of formed images when two plane mirrors are inclined toe ach other at x degree is:

How many images will be formed between two plane mirrors inclined at an angle of 90 degree with each other?

Using the formula for Number of images formed when two plane mirrors are inclined at x degree

and putting x as 90 degree in above, we get number of images as 3.

What happens to the images formed when the angle between the two plane mirror decreases?

The number of images formed between two plane mirrors inclined to each other increases as the angle between the mirrors decreases.

What is kaleidoscope?

The kaleidoscope is an instrument or toy containing inclined plane mirrors which produce multiple reflections of coloured glass pieces or coloured plastic pieces and create beautiful patterns.

The kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 60 degree to one another forming a hollow prism and fitted in a cardboard tube.

How many colours the white sunlight consists of?

The white sunlight actually consists of seven colours mixed together.

What is dispersion?

The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light.

The seven colours of the spectrum of white light are: Violet, Indigo, Blue, Green, Yellow, Orange and Red.

Which natural phenomenon shows the dispersion of light?

Rainbow is the natural phenomenon showing the dispersion of light. The tiny rain drops function as a prism.

What are the main parts of the human eye?

The main parts of the human ey are: Cornea, Iris, Pupil, Ciliary muscles, Eye lens, Retina and Optic nerve.

How does human eye work?

When we look at an object, the light rays coming from the object enter the pupil of the eye and falls on the eye-lens.

The eye-lens is a convex lens. It converge the light rays and produces a real and inverted image of the object on the retina. The retina has a large number of light sensitive cells. When the image of the object falls on the retina, then the light sensitive cells generate electric signals. The retina sends these electrical signals to the brain through the optic nerve and we are able to see the object.

What are rods and cones in an eye?

Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light. Rods are the most important for vision in dim light as during night.

Rod cells of the retina, however, do not provide information about the colour of the object.

Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light or normal light. Cones also cause the sensation of colour of objects in our eyes.

Thus, cones present in the retina of the eye are responsible for colour vision. Cones do not function in dim light.

What is blind spot?

Blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye.

Prove the existence of blind spot in human eye.

The existence of blind spot in the human eye can be demonstrated by following the given steps.

i). Take a drawing sheet and draw a thick cross and a circular dot on this sheet of paper about 7 cm apart as shown in the above figure.

ii). Hold the sheet of paper at an arm’s length from your eyes.

iii). Now close your left eye and look at the cross on the paper with your right eye. You will also see the black dot.

iv). Bring the sheet of paper towards you slowly.

v). At a certain distance, the dot disappears because its image has fallen on the blind spot of your right eye.

The disappearance of dot from the view shows that there is a point on the retina of the eye which cannot send message to the brain when the image of dot falls on it. This point is the blind spot. The black dot disappears and cannot be seen by the eye because its image falls on the blind spot of the right eye.

The same activity can be done by now closing the right eye. Keep looking at the dot mark with left eye and bring the sheet of paper from arm’s length towards you slowly. At a certain distance the cross disappears because its image has fallen on the blind spot of your left eye.

What is persistence of vision?

The image of an object seen by our eyes persists or remains on the retina for about 1/16th of a second even after the object has disappeared from our view. This happens because the stimulated light-sensitive cells of the retina take a little time to some back to their original state.

The ability of an eye to continue to see the image of an object for a very short duration even after the object has disappeared from view is called persistence of vision.

Due to which phenomenon we are able to see movie pictures in a cinema hall?

It is due to the phenomenon of persistence of vision that we are able to see move pictures in a cinema hall.

This is because if the still pictures of a moving object are flashed on our eyes at a rate faster than 16 pictures per second then the eyes perceive this object as moving.

In a cinema hall the still pictures are projected on the screen at the rate of 24 pictures per second.

How can the phenomenon of persistence of vision be demonstrated? (Pic to be pasted)

Take a piece of cardboard and a pencil as shown in the part “a” of the figure.

Make the sketch of the bird on one side of the cardboard and the sketch of the cage on its back side (see figure “b” and “c”).

Make two small, horizontal slits or cuts in the piece of the cardboard, one near its top and the other near its bottom.

Insert the pencil through the two cuts on the cardboard as shown in the “d” part of the figure.

Hold the lower end of pencil between the palms of your hands.

Move your palms forwards and backwards quickly so as to spin the cardboard rapidly.

Look at the cardboard when it is spinning rapidly.

It will be seen that the bird is in the cage. (Figure “d”)

This happens due to the phenomenon of persistence of vision.

What is the range of vision of normal human eye?

The range of vision of a normal human eye is from infinity to about 25 centimetres. The nearest point up to which human eye can see an object clearly without any strain is called the near point of the eye. The near point of a normal human eye is at a distance of 25 centimetres from the eye.

How can human eye see distant objects as well as near objects?

A normal eye can see the distant objects as well as the nearby objects by focusing the images of distant object as well as nearby objects on its retina by changing the thickness or converging power of the eye lens.

A thin eye-lens has less converging power whereas a thick eye-lens has greater converging power

Explain myopia

Myopia is that defect of eye (or defect of vision) due to which a person cannot see the distant objects clearly though he can see the nearby objects clearly.

It occurs because either the eyeball is too long, or because the cornea is too curved.

How is myopia or short sightedness corrected?

Myopia is corrected by using spectacles containing concave lenses (diverging lenses) of suitable power. The image of distant object is then formed on the retina and hence can be seen clearly.

What is hypermetropia?

Hypermetropia is that defect of eye (or defect of vision) due to which a person cannot see the nearby objects clearly though he can see the distant objects clearly.

It is also called long sightedness.

What is the cause of the hypermetropia/long sightedness?

Hypermetropia is caused either due to low converging power of eye-lens or due to eye ball being too short.

What happens to the image formation in an eye suffering from hypermetropia/long sightedness?

In an eye suffering from hypermetropia, the image of the object is formed behind the retina due to which the person cannot see clearly.

How is hypermetropia corrected?

Hypermetropia is corrected by using spectacles containing convex lenses (converging lenses) of suitable power. The image of nearby object is then formed on the retina and hence the person can see it clearly.

Correction of long sightedness

What is cataract?

Cataract is the medical condition in which the lens of eye of a person becomes progressively cloudy resulting in blurred vision. It happens due to the formation of a membrane over the eye-lens.

What happens to vision of the person suffering from cataract?

Cataract decreases the vision of the eye gradually and can ebem lead to total loss of vision of the eye.

How can vision be restored in a person suffering from cataract?

The vision of the person can be restored after getting surgery done on the eye having cataract. The opaque lens is removed from the eye of the person by surgical operation and a new artificial lens is inserted in its place.

How should one take care of his/her eyes?

Some of the precautions to protect our eyes and maintain healthy eyesight are as follows:

1. Wash your eyes at least twice a day with clean water.

2.  Do not read or write in dim light because it puts strain on the eyes and may cause headache.

3. Never look directly at the sun or other powerful lights.

4. Always read by keeping the book at normal distance for distinct vision (25 cm). Do not read by bringing the book too close to your eyes or by keeping it too far from the eyes.

5. Raise your eyes from time to time while reading, writing or watching television so as to relax eyes.

6. Protect eyes from injuries or foreign bodies like dust particles and insects etc. If something (like a dust particle or insect etc.) gets into the eyes, splash the eyes with a lot of clean water. Do not rub the eyes with hands to prevent injury to the eyes.

7. In case of an injury or any other problem to the eyes, consult an eye-specialist doctor.

8. Eat food items that are rich in Vitamin A such as carrots, cod-liver oil (Fish liver oil), green vegetables (such as spinach), cabbage, broccoli, eggs, milk, butter, curd, cheese and fruits such as papaya and mango.

How can crab see an enemy from behind?

The eyes of a crab are quite small but they enable the crab to look around. Due to this a crab can see an enemy even it comes from behind.

How can butterflies see in front and sides but at the back as well?

Butterflies have large eyes which appear to be made up of thousands of little eyes. Thus, butterflies are able to see not only in the front and sides but at the back as well.

Why can a night bird (owl) see very well in the night (dim light) but not during the day (when there is bright light)?

The owl has big eyes having large cornea and a large pupil to allow more light in the eye.

The owl’s eyes have a large number of rods in the retina (which are sensitive to dim light) and only a few cones (which are sensitive to bright light).

The day light birds (such as kite and eagle) can see very well during the day (in bright light) but not in the night (when there is dim light). The eyes of day light birds (such as kite and eagle) are small and have more cones in retina (which are sensitive to bright light) but fewer rods (which are sensitive to dim light).

What is meant by visually challenged persons?

Those persons who are “unable to see” are known as visually challenged persons.

What is Braille?

Braille is a written language for the visually challenged persons in which characters (letters and numbers etc.) are represented by patterns of raised dots. The visually challenged persons recognise the words written on a Braille sheet by touching the pattern of dots which are raised slightly to make it easier to touch.

Who developed Braille?

The Braille was developed by Louis Braille who was himself a visually challenged person.

What helps visually challenged persons in taking notes, reading and writing?

Braille writer slate and stylus help the visually challenged persons in taking notes, reading and writing.

Give examples of auditory aids used by visually challenged persons.

Examples of auditory aids used by visually challenged persons are cassettes, tape recorders, talking books and talking calculators.

What are the visual aids used by visually challenged persons?

Closed-circuit televisions (equipment that enlarges and contrasts textual items), glasses, artificial eyes and laser eye surgery are the visual aids used for visually challenged persons.

Write names of some great visually challenged persons and their achievements.

Hellen Keller (an American lecturer and author), Ravindra Jain (famous lyricist, singer and music composer obtained his Sangeet Prabhakar degree from Allahabad University) and Diwakar (a very good singer) are some of the visually challenged persons who did great achievements.