This PSEB 8th Class Science Notes Chapter 16 Light will help you in revision during exams.
PSEB 8th Class Science Notes Chapter 16 Light
→ Light is a form of energy.
→ Light travels in a straight line.
→ Light enables us to see the objects around us.
→ Light reflected from the objects falls into our eyes and thus, we can see the objects.
→ Those objects, which emit their own light, are called luminous objects.
→ The objects, which do not emit light of their own, but reflect light falling on them, are called non-luminous objects.
→ Polished or shiny surfaces reflect light.
→ A mirror changes the direction of light, that falls on it.
→ The angle of incidence (∠i) is always equal to the angle of reflection (∠r).
→ The incident ray, normal at the point of incidence, and reflected ray all lie in the same plane.
→ A number of reflections can be achieved with the mirrors arranged at an angle.
→ When the light passes through a prism, it undergoes dispersion and white light splits up into seven colours.
→ The spectrum of sunlight consists of seven colours – Violet, Indigo, Blue, Green, Yellow, Orange, and Red.
→ These seven colours can be remembered by the term VIBGYOR.
→ Rainbow is a natural phenomenon of dispersion.
→ The human eye is a sensitive organ, which enables us to see the objects around us.
→ A human eye has a convex lens, whose focal length is adjusted by ciliary muscles.
→ Reflections can be parallel or regular and diffused.
→ Cones and rods are two types of nerve endings found in the blind spots of the eye.
→ Two types of resources for visually challenged persons are Non-optical aids and optical aids.
→ The Braille system is one of the most important and popular resources for visually challenged persons.
→ Reflection of Light: The change in direction of light by a shiny surface is called the reflection of light.
→ Screen: The white sheet or surface on which, the image is obtained.
→ Regular Reflection: Reflection takes place from a polished and regular surface.
→ Scattering of Light: Diffusing of light in all directions.
→ Diffused Reflection: The light rays reflected from the non-polished or irregular surface.
→ Incident Ray: A ray of light from a source of light falling on a given surface.
→ Kaleidoscope: An instrument based on multiple reflections used to create new designs.
→ Mirror: A smooth and shiny surface.
→ Normal: The perpendicular to the polished surface (mirror) at the point of incidence.
→ Source of Light: An object which emits light.
→ Real Image: The image formed, when incident rays after reflection actually meet at a point.
→ Virtual Image: The image formed, when incident rays after reflection do not actually meet but appear to meet at a point.
→ The angle of Incidence: The angle between the incident ray and the normal.
→ The angle of Reflection: The angle between the reflected ray and the normal.
→ Power of Accommodation is the ability of the eye to focus the distant objects as well as the nearby objects, at the retina by changing the focal length or converging power of its lens.
→ The least Distance of Distinct Vision is the minimum distance at which an object must be placed so that a normal eye may see the object clearly without any strain on the eye. It is about 25 cm for the normal eye.
→ Myopia or Short-Sightedness means a person can see only nearby objects clearly, but cannot see distant objects. This defect is removed by using spectacles with a concave lens.
→ Hypermetropia or Long-sightedness: A person suffering from this defect can see distant objects clearly but cannot see nearby objects. The defect is removed by using spectacles fitted with a convex lens.
→ Persistence of Vision: Impression of an image on the retina lasts for 0.1 sec after the removal of the object. This effect is known as the persistence of vision.
→ Dispersion of Light is the splitting of white or some other light into its constituents.
→ Perception of Colour:
- The human eye contains a large number of cell rods and cones which are sensitive to light.
- Rods respond to the intensity of light and cones respond to the color of the light.
- If cone cells are absent in the eye, then such a person is colour blind.