class 8 Light Short notes science chapter 16

Introduction to Light

The world as we know is largely known through senses and the sense of sight is one of the most important senses. It is only when the light from an object enters our eyes that we see it. The light might have been either emitted by the object or reflected by it. Some of the important terminologies related to light are:

Light is the energy that enables us to see. Light is emitted from a source such as the Sun. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight.

What makes Thing Visible?

Our eyes alone do not allow us to see. Light from a source falls on an object and then bounces off onto our eyes and that is how we perceive it.

Reflection

Light gets reflected off surfaces. Any surface that is really well polished or shiny acts as a mirror. The phenomenon of light bouncing off surfaces is called reflection.

Laws of reflection

• The ray of light that strikes a reflecting surface is called as the incident ray and the ray that gets reflected back is called the reflected ray.
• The imaginary line that is perpendicular to the reflecting surface is called the normal.
• The angle of incidence ∠i is the angle between the incident ray and the normal. The angle of reflection ∠r is the angle between the reflected ray and the normal.
• Laws of Reflection: Angle of incidence ∠i is always equal to the Angle of reflection ∠r (∠i=∠r). The angle of incidence, the angle of reflection and the normal all lie on the same plane.

Regular and Diffused Reflection

Regular and diffused reflection

Reflection from a smooth surface like a mirror that follows the laws of reflection is called as regular reflection.

Reflection over a surface that has irregularities where it does not follow laws of reflection is called as irregular or diffused reflection

Image formation in a plane mirror

• The image formed by a plane mirror is always virtual, erect and object and image are equidistant from the mirror.
• The image formed in a plane mirror undergoes lateral inversion.

Difference between the real and virtual image

A real image is formed by the actual convergence of light rays. In optics, a virtual image is an image formed when the outgoing rays from a point on an object always diverge. The image appears to be located at the point of apparent divergence. Because the rays never really converge, a virtual image cannot be projected onto a screen.

Multiple reflections

• Two mirrors inclined to each other give multiple images, due to multiple reflections.
• When an object is kept between two parallel plane mirrors, infinite images are formed.

Calculating the total number of images

Number of images formed by plane mirrors is given by n=360∘θ−1.

Kaleidoscope:

• The idea of number of images formed by mirrors placed at an angle to one another is used in a kaleidoscope to make numerous beautiful patterns.
•  To make a kaleidoscope, get three rectangular mirror strips about 15 cm long and 4 cm wide each. Join them together to form a prism. Fix them in a circular cardboard tube or tube of a thick chart paper.

Human Eye

Human Eye

The human eye is a spherically shaped sense organ(diameter about 2.3 cm) that gives the sense of sight and helps us perceive colours and depth.

Functions of different organs in the human eye

• Light enters the eye through a thin membrane that forms a transparent bulge on the front of the eye, called as the cornea.
• Iris is a dark muscular diaphragm that controls the size of the pupil.
• The pupil is the small opening in the centre of the iris.
• The lens which is behind the pupil helps to adjust the focal length required to clearly focus on objects at various distances.
• The retina is the screen where the images get formed. It is a delicate membrane with a large number of photosensitive cells called rods and cones.
• Cones are sensitive to bright light and rods are sensitive to dim light.
• The optic nerve transmits the electrical signals from the eye to the brain.

Vision Defects:
Some of the vision defects are as follows:

1. Myopia: A person with myopia can see near object clearly while distant objects appear blurred. This disease is also called short sightedness or near sightedness. Spectacles with concave lenses can improve the vision in a myopic person. 

2. Hypermetropia: A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This disease is also called long sightedness. Spectacles with convex lenses can improve the vision in a hypermetropic person.

3. Cataract: During old age, the crystalline lens becomes milky and cloudy in a cataract patient and thus the person cannot see clearly. Cataract is treated by surgery. In cataract surgery, the cloudy lens is removed and an artificial lens is transplanted in the eye.

Visually Challenged Person:
(i) Some people including children face with disability of vision. This disability of vision can be partial or complete. Such persons are called visually challenged persons.
(ii) A visually challenged person usually show a marked development of other senses; like the sense of hearing and sense of touch. However, additional resources can enable them to develop their capabilities further. Resources can be of two types: Non-optical aids and optical aids.

1. Optical Aids: Optical aids help a person who is partially visually challenged. These aids enlarge an image or a text so that they could be visible. Bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids, magnifying devices and telescopic devices come under this category.

2. Non-optical Aids: Non-optical aids help a person who is completely visually challenged. Non-optical aids include visual aids (magnify words), tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids.
(i) Visual aids can magnify words; provide suitable intensity of light and material at proper distances.
(ii) Tactual aids, including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing.
(iii) Auditory aids include cassettes, tape recorders, talking books and other such devices. Electronic aids, such as talking calculators, are also available for performing many computational tasks.
(iv) Closed circuit television, also an electronic aid, enlarges printed material with suitable contrast and illumination.
(v) Now a day, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text.

Louis Braille:
The most popular resource for visually challenged persons is known as Braille. Braille code is used for common languages, mathematics and scientific notation. Louis Braille was born in 1809 in France. He became blind due to an accident in childhood. He developed the Braille script in 1824.

Braille Script: This system has of 63 raised dot pattern or characters. Each character or pattern represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. A reader needs to touch the Braille script to read it. Example of dot patterns used in Braille System:

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