NCERT Class 8 Science Chapter 16 - Light Notes

Chapter 16 - Light Notes

1. Light is a form of energy which enables us to see objects from which it comes or from which it is reflected. Light helps us to see objects. We cannot see objects in the dark. An object becomes visible when light emitted or reflected by it reaches our eyes.

2. Properties of Light:
• Light travels in a straight line.
• The speed of light is faster than sound. Light travels at a speed of 3 x 10⁸ m/s.
• Reflection of light.

3. The objects which emit light on their own are known as luminous objects. Example: The sun, lamp, candle, etc.

4. The objects which do not emit light on their own are known as non-luminous objects. Example: chair, table, window, etc.

5. Materials which don't allow light to pass through them completely are called opaque objects. Example: Wood etc

6. Materials which allow light to pass through them partially are called translucent objects. Example: Butter Paper etc

7. Materials which allow light to pass through them completely are called transparent objects. Example: Glass etc

8. An image that can be formed on the screen is called real image.

9. An image that can’t be formed obtained on the screen is called virtual image.

10. An erect image is the image that appears right side up.

11. The laws of reflection are
• The angle of incidence is equal to angle of reflection.
• The incident ray, reflected ray and the normal all lie in the same plane.

12. The incident ray is the ray of light striking the surface.

13. The reflected ray of light which is returned back into the same medium after the incident ray strikes the surface.

14. Reflection happens when light rays fall on an object their direction changes and they turn back.

15. Lateral inversion is an image formed by mirror in which the left of the object appears on the right and right appears on the left.

16. Normal is an imaginary line that is perpendicular to the reflecting surface.

17. The angle between the normal and the incident ray is known as the angle of incidence (∠i).

18. The angle between the normal and the reflected ray is known as the angle of reflection (∠r).

19. When parallel rays of light fall on a smooth surface, they are reflected back as parallel rays only. This is called regular reflection.

20. When parallel rays of light fall upon an irregular surface, they are reflected back into all the directions. This is called diffused or irregular reflection.

21. The characteristics of the image formed by a plane mirror are:
• Image distance and object distance are equal.
• Size of the objects and images are equal.
• The image formed is erect and virtual.

22. When 2 or more mirrors are placed at some angle to each other, we get to see multiple images.

23. A Periscope is an instrument that is used to look at objects that are not in the line of sight of the observer. The Periscope works on the phenomena of multiple reflections of light. Inside the periscope, are the two mirrors which are inclined at 45°. They are generally used in submarines, tanks etc.

24. Kaleidoscope is a device that works on the phenomena of multiple reflections with the help of multiple mirrors. It has a cylindrical structure with three plane mirrors inside it. It contains several pieces of beads, broken glass, colourful bangles, pebbles etc. The observer can look into the Kaleidoscope from one end and the light enters into the Kaleidoscope from the other end. When the light falls upon these objects in the Kaleidoscope, multiple reflections of these objects are created in the three Mirrors. As a result many beautiful and colourful patterns are formed. Since the reflection of light is not always the same the observer never sees a similar pattern in the kaleidoscope.

25. The white light consists of seven colour namely Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR). This arrangement of 7 colours of a white light is known as Spectrum. The phenomena of the splitting of light into its constituents is called the dispersion of light.

26. 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.

27. Cornea - It is a transparent covering present on the outer side of the eyes. It consists of 6 layers. Tear fluid covers cornea and protects the eye.

28. Iris - It is a dark coloured muscular structure present in the centre of the cornea. The Iris consists of a small opening call Pupil through which the light enters into the eye. The Iris has colourful pigments like grey, blue, black, green, brown etc. The amount of light that enters the eye is controlled by the Iris.

29. Lens - It is like a sphere of liquid present inside the eye. The eye lens is located behind the pupil which helps in the formation of an image in the eye.

30. Retina - Retina is a layer located behind the lens at the back of the eye. The eye lens focuses the light that enters into the eye over the retina. The retina consists of different nerve cells that perform different tasks. There are sensory cells that detect the light. They then transform that information into an electrical impulse. These electrical impulses are sent to the brain via the optic nerve. Two types of vision cells in the retina are:

31. The rods are sensitive to dim light.

32. The cones are sensitive to bright light and colour.

33. Blind spot - There is a point in the eye located at the junction of the retina and the optic nerve where no sensory cells are present. This spot is therefore called the blind spot as it does not support any vision.

34. The ciliary muscle helps the eye and to focus image of object on the retina.

35. The optic nerves carry the sensation to the brain.

36. Defects of Vision
The normal human eye can see nearby as well as distant objects clearly. The minimum distance at which objects can be seen clearly is 25 cm.

Some persons can see nearby objects clearly but cannot see distant objects clearly. Some persons can see distant objects clearly but cannot see nearby objects clearly. These defects can be corrected by using suitable corrective lenses.

Sometimes in old age people cannot see clearly because the eye lens becomes cloudy. This defect is called cataract. It can be corrected by removing the eye lens and replacing it with an artificial lens.

37. Care of the eyes
• If there is any problem of the eyes have a checkup and use suitable spectacles if advised.
• Too little or too much light is bad for eyes.
• Don't look at the sun or very bright light directly.
• If dust particles enter the eye don't rub the eyes. Wash your eyes with clean water.
• Always read at a normal distance. Don't read the keeping the book to close to the eyes or keeping it too far.
• Deficiency of Vitamin A causes night-blindness. So take food rich in Vitamin A.

38. Raw carrots, broccoli and green vegetables (such as spinach) and cod liver oil are rich in vitamin A. Eggs. milk, curd, cheese, butter and fruits such as papaya and mango are also rich in vitamin A.

39. Braille System (Louis Braille)
Braille system is a system used by visually challenged persons (blind persons) to read and write. It has 63 dot patterns which represents alphabets, common words or signs. The dots are arranged in cells of two vertical rows of three dots each. The dots are raised slightly so that the words can be recognised by touching.

40. Louis Braille developed a system for visually challenged persons and published it in 1821. The present Braille system was adopted in 1932.

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