Students can go through AP Board 7th Class Science Notes 7th Lesson Reproduction in Plants to understand and remember the concept easily.
AP Board 7th Class Science Notes 8th Lesson Wonders of Light
→ Light is a form of energy.
→ Objects that emit light are known as sources of light.
Ex : Sun, candle, tube light.
→ Sources like sun, stars emit light on their own. Such type of sources are called natural sources of light.
→ Sources that release light artificially are called man made sources of light or artificial sources of light.
→ Light is not a single ray, but a bundle of rays.
Ex : bulb, torch light, candle.
→ This bundle of light rays are called beam of light rays. These are three types :
- Parallel beam of light rays
- Converging beam of light rays
- Diverging beam of light rays.
→ Light rays which travel parallel to each other are called Parallel beam of light rays.
→ “Light rays which travel from different directions to meet at a point are called as Con-verging beam of light rays”.
→ “Light rays which travel from a source moving in different directions are called as Diverging beam of light rays.”
→ The process of bouncing back of light rays into the same medium after falling on a smooth or rough surface from the light source is called “reflection of light”.
→ Objects are visible only when light falls on the objects and bounces back to the eye.
→ The process of bouncing back of light rays in the same medium after hitting the surface of an object is called reflection.
→ The Light rays that fall on the objects are called incident rays. The light rays that bounce back from the objects are called reflected rays.
→ Reflection from a smooth and shiny surface is called regular reflection. Clear images are formed in case of regular reflection.
→ Reflection from an irregular or uneven surface is called irregular reflection or diffused reflection.
→ Images are not clear or sometimes cannot form the images at all in case of irregular reflection.
→ There are two laws of reflection :
- Angle of incidence is equal to Angle of reflection.
- The incident ray, reflected ray and normal to the surface are present in the same plane.
Incident and reflected rays are on either side of normal.
→ Characteristics of image by plane mirror:
- Object distance is equal to image distance.
- Size of the object is equal to size of the image.
- The image formed is always virtual and erect.
- Laterally inverted image is formed, (left and right alternates)
→ The distance of the object from the mirror is called Object distance.
→ The distance of the image from the mirror is called Image distance.
→ The image which we get on screen is called real image.
→ The image which cannot get it on screen is called virtual image.
→ All the plane mirrors form virtual and erect image.
→ Formula for number of images formed between two mirrors .
→ Number of images (n) = 360°/θ -1, where 0 is the angle between the mirrors.
→ Periscope is an instrument used in submarines to see the objects or persons above the water level.
→ The mirrors which are the parts of spheres are called spherical mirrors.
→ Spherical mirrors are of two types :
- Convex mirrors (Reflecting surface bent outward)
- Concave mirrors (Reflecting surface bent inward)
→ A concave mirror forms real and virtual images, erect and inverted images, smaller, same size and bigger images depending on the position of object in front of it.
→ A convex mirror always forms virtual, erect, smaller image irrespective of the position of the object.
→ Concave mirrors are used by ENT doctors as Head Mirrors,
→ Dentists also use concave mirrors to get a bigger image of the teeth.
→ Eye specialist using a special instrument called Ophthalmoscope. It is fitted with a concave mirror having a small hole near its center.
→ The surface of reflection in a torch light or the headlights of vehicles is concave.
→ Convex Mirrors are used as Rear-view mirrors and also used at the junctions of roads.
→ A piece of glass or any other transparent material with curved sides is called a lens.
→ Lens which is thick in center and thin at the edges is called convex lens.
→ Lens which is thin in the centre and thick at the edges is called concave lens.
→ Yellow light has been proven effective in protecting retina than blue light which causes damage to retina.
→ Natural sources of light : Sources which release’light energy on their own are called natural sources of light.
→ Manmade sources of light : Sources which need the human involvement to release light energy are called manmade sources of light.
→ Reflection of light : The process of bouncing back of light rays into the same medium after falling on a smooth or rough surface from the light source is called “reflection of light”.
→ Incident ray : The light rays that fall on the objects are called incident rays.
→ Reflected ray : The light rays that bounce back from the objects are called reflected rays.
→ Regular reflection : Reflection from a smooth and shiny surface is called regular reflection.
→ Diffused reflection : Reflection from an irregular or uneven surface is called irregular reflection or diffused reflection.
→ Angle of incidence (I) : The angle made by incident ray with the normal is called angle of incidence (i).
→ Angle of reflection (r) : The angle made by the reflected ray with the normal is called the angle of reflection (r).
→ Normal : Perpendicular line to the surface of the mirror at incident point is called normal to the surface.
→ Concave mirror : Concave mirrors is a spherical mirror with reflecting surface bent inward.
→ Convex mirror : Convex mirrors is a spherical mirror with reflecting surface bent outward.
→ Real image : The image which can get on a screen is called real image.
→ Virtual image : The image which cannot get it on screen is called virtual image.
→ Lens : A piece of glass or any other transparent material with curved sides is called a lens.
→ Dispersion : Splitting of white light into seven colours is called Dispersion.
→ Safe browsing : Opening of authorized websites in computers is called safe browsing.
→ Convex lens : Lens which is thick in center and thin at the edges is called convex lens.
→ Concave lens : Lens which is thin in the centre and thick at the edges is called concave lens.
