Facts that Matter - Light | Science Class 8 PDF Download

Facts that Matter

• We can see an object only when light from an object enters our eyes. The light may have been emitted by the object or may have been reflected by the object.
• A mirror changes the direction of light that falls on it.
• Actually a narrow beam of light is made of several rays. For simplicity, we use the term ray for a narrow beam of light.
• Reflection: T he mirror surface or a shiny surface scatters back a beam of light falling on it. This scattering back of light by mirror or shiny surface is known as reflection.
• The ray of light which falls on the mirror is called incident ray. The point at which incident ray falls on the surface of a mirror is called point of incidence.   
• The ray that comes back from the surface of a mirror after reflection is known as the reflected ray.
• Normal: A  perpendicular (a line making an angle of 90º) at the point of incidence (where the incident ray strikes the mirror) is known as normal to the reflecting surface at point.
• The angle between the incident ray and the normal is called the angle of incidence (∠i).

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

• Laws of reflection 
  (i) The a ngle of incidence (∠i) is always equal to the angle of reflection (∠r), i.e. ∠i = ∠r.
  (ii) The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same plane.   
• The image formed by a plane mirror: The image formed by a plane mirror is erect.

It is virtual (image cannot be obtained on screen) and is of the same size as the object.
The image is at the same distance behind the mirror as the object is in front of it.

• Lateral inversion: The left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image. This is known as lateral inversion.

• Irregular and regular reflection: When rays of light fall on uneven shiny surface, the reflected rays are scattered in all directions, as shown in Fig. 16.3. In this case reflected rays are not parallel. Such a reflection is known as diffused or irregular reflection. It is caused due to irregularities of the reflecting surface.

When rays of light fall on smooth shiny surface, they are reflected in a particular direction as shown in Fig. 16.4 and are parallel. Such a reflection is called regular reflection. Images are formed by regular reflection.   

• Nearly everything we see around is seen due to reflected light. The objects which shine in the light of other object are called illuminated objects.

The objects which emit their own light are known as luminous objects.

• Periscope: T he periscope makes use of two plane mirrors placed in ‘z’ shaped box at 45º angle as shown in Fig. 16.5.

Due to reflection from mirror 1 and reflection from mirror 2 one is able to see objects which are not visible directly. Periscopes are used in submarines, tanks and also by soldiers in bunker to see things outside.

• Kaleidoscope — It is based on the principle of multiple reflections. It consists of three plane mirror strips arranged at 60º angle to each other in a tube (hard cardboard tube).

— One end of the tube is closed with the two circular glass discs
—the inner being transparent glass and outer being of ground glass.
— Glass pieces or bangle pieces of various colours are kept between the two discs.
— The other end is covered with transparent glass disc through which one can view the glass pieces.
— Due to reflection in mirrors hexagonal patterns of coloured bangles can be seen.
— As the tube is slowly rotated, new floral designs with different colour combinations can be seen.
— An interested feature of a kaleidoscope is that you will never see the same pattern again.
— Designers of fabrics, wallpapers and artists use kaleidoscope to get idea for new patterns. 

• Dispersion: Splitting of light into its constituent colours is called dispersion. The sunlight is referred to as white light that consists of seven colours.
• Structure of the eye : 
  Cornea: The outer coat of the eye is white. Its transparent front part is called cornea.
  Iris: A  dark muscular structure present behind cornea is called iris. The colour of the iris determines the colour of the eye.
  Pupil: It is a small opening in the iris. The size of the pupil is controlled by the iris. Thus, iris controls the amount of light entering into the eye.
  Eye Lens: It is a double convex lens situated behind the iris. The eye lens has the capacity to change its focal length. So that it can focus the images of objects at different distances on the retina of the eye.

Retina: It is the innermost layer of the eyeball on which eye lens focuses the image. Retina consists of:
— Nerve cells: Visual sensations are felt by nerve cells.
— Cone cells: They are sensitive to bright light. Cone cells sense colours.
— Rod cells: They are sensitive to dim light.
Optic Nerves: Sensations felt by the nerve cells of the retina are transmitted to the brain through the optic nerves.

Blind Spot and Yellow Spot: At the junction of the optic nerve and the retina, there are no rods and cones. So no vision is possible at that spot. This spot is called blind spot.
Yellow spot is located at the centre of the retina. It has maximum concentration of light sensitive cells.

• The impression of at image persists for about 1/16 of a second on the retina even after we have stopped seeing the object. If still images of a moving object are flashed on the eye at a rate faster than 16 second, then the eye perceives this object as moving.   
• Range of normal vision: The distance between infinity and 25 cm is called the range of normal vision. The most comfortable distance at which one can read with a normal eye is about 25 cm. 
• Cataract: It  is the eye disease in which eye lens becomes opaque and eye sight becomes foggy. The disease is treated by removing the opaque lens and inserting a new artificial lens.   
• Lack of vitamin A in diet may cause many eye diseases such as night blindness.
• Braille: L ouis Braille developed a system for visually challenged persons and published it in 1821. Braille system has 63 dot patterns or characters. These patterns when embossed on Braille sheets help visually challenged persons to recognise words by touching.
• Owl can see very well in the night. It has a large cornea and a large pupil to allow more light in its eyes. Its retina has a large number of rods but only a few cones. These features enable an owl to see in the night. During the day, the large pupil allows so much light in the eye that the owl cannot see objects.