CHAPTER 16 LIGHT 1 What makes things visible

CHAPTER - 16 LIGHT

1) What makes things visible : 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) Reflection of light : When light falls on a polished or shiny surface it changes its direction. This is called reflection of light.

The ray of light which strikes the surface is called incident ray. The ray which comes back after reflection is called reflected ray. The perpendicular line drawn at the point of incidence is called normal. The angle between the normal and incident ray is called angle of incidence ( i ). The angle between the normal and reflected ray is called angle of reflection( r ). The angle of incidence is always equal to the angle of reflection.

3) Laws of reflection of light : The laws of reflection of light are : i) The angle of incidence is equal to the angle of reflection. ii) The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. Incident rays Normal i r Reflected rays

4) Image formed by a plane mirror : i) The image is erect. ii) The image is same size as the object. iii) The image is at the same distance from the mirror as the object is in front of it. iv) The image is virtual (cannot be obtained on a screen). v) The image is laterally inverted.

5) Regular and diffused (irregular) reflection : i) Regular reflection : - When parallel rays of light fall on a smooth surface, the reflected rays are parallel. This is called regular reflection. Regular reflection forms images of objects. ii) Irregular reflection : - When parallel rays of light fall on a rough surface, the reflected rays are not parallel. This is called diffused or irregular reflection. Diffused reflection does not form images of objects. Regular reflection Irregular reflection (Diffused reflection)

6. Multiple images : When two mirrors are kept inclined at an angle, they can together form multiple images. This is because the image formed by one mirror forms the object for the other mirror. Angle between the mirrors Number of images 450 7 600 5 900 3 1200 2 1800 1 00 – Parallel to each other Infinite number

7) Kaleidoscope : A kaleidoscope is a device used to see multiple images of objects. It has three inclined mirrors at an angle of 600 to each other. This arrangement is usually kept in a cylindrical box and has a few pieces of coloured glasses kept at one end. We can see beautiful patterns and designs due to multiple images formed by the mirrors.

8) Dispersion of light : Sunlight or white light is a mixture of seven colours. The splitting up of sunlight or white light into its colours is called dispersion of light. When a beam of sunlight is passed through a glass prism it is split up into the seven rainbow colours. The colours are violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Rainbow is caused due to the dispersion of sunlight by water drops in the atmosphere.

9) The human eye : -

9) The human eye : The human eye is the sense organ which helps us to see objects around us. The eye ball is almost spherical in shape. Light enters the eye through a transparent membrane called cornea. Behind the cornea is a muscular structure called iris which has an opening called pupil. The pupil controls the amount of light entering the eye. Behind the pupil is the eye lens. It is a convex lens which focuses the image of the object on the retina. The ciliary muscles helps the eye lens and to focus the image of objects on the retina. The retina has two types of cells called rods and cones. The rods are sensitive to bright light and colours. The cones are sensitive to dim light. The optic nerve carry the sensations to the brain.

10) 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.

11) Care of the eyes : i) If there is any problem of the eyes, have a checkup and use suitable spectacles if advised. ii) Too little and too much light is bad for the eyes. iii) Do not look at the sun or very bright light directly. iv) If dust particles enter the eye do not rub the eyes. Wash your eyes with clean water. v) Always read at a normal distance. Do not read by keeping the book too close to the eyes or keeping it or too far. vi) Deficiency of Vitamin A causes night blindness. So take food rich in Vitamin A.

12) 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.