LIGHT
IMPORTANT TERMS
Reflection Reflection is the phenomenon of bouncing back of light to the same medium, after striking a reflecting surface.
Incident Ray A ray coming from an object and striking any surface is called incident ray
Reflected Ray The ray that comes back from a surface after reflection is known as reflected ray.
Normal Normal is the line drawn perpendicular to surface at the point where incident ray meets the surface.
Angle of Incidence The angle between Normal and Incident ray is known as angle of Incidence.
Angle of Reflection The angle between normal and reflected ray is known as angle of reflection.
Lateral Inversion The property of mirror to show ‘left as right’ and vice versa is known as lateral inversion.
Regular Reflection Reflection from smooth surface like mirror, is known as regular reflection wherein all reflected rays go parallel to each other.
Diffused Reflection Reflection from rough surface like wall, is known as diffused reflection wherein all the reflected rays are not parallel to each other.
Luminous Objects The objects that emit their own light are called as luminous objects.
Non Luminous Objects The Objects which do not emit their own light and can be seen only in the presence of luminous objects.
Dispersion The phenomenon of splitting of white light into seven colors is known as dispersion.
Spoectrum The band of seven colors obtained, when white light splits up is called Spectrum
Cataract Sometimes the eye lens becomes opaque and cloudy. This cause a condition called cataract.
Myopia A person suffering from myopia cannot see far off objects clearly, but can see nearby objects with clarity. Also called Short-sightedness.
Hypermetropia A person suffering from hypermetropia cannot see nearby objects clearly, but can see far away objects with clarity. Also called Long-sightedness.
LIGHT
The sense of sight is one of the most important senses, through which we see mountains, rivers, trees, plants, animals, birds flying and everything around us. However, we cannot see the objects in dark, which means that presence of light enables us to see the things. Light strikes the objects, enters our eyes and then we are able to see objects.
REFLECTION
When a ray of light strikes a mirror or any polished surface, it goes back to same medium. This phenomenon is called reflection of light.
Incident Ray- The ray which comes from object and strikes a surface is called as incident ray.
Reflected Ray- The ray of light which gets reflected by surface after striking it, is called as reflected ray.
Normal- Normal is a line making an angle of 90 degree to the line representing the mirror at the point where the incident ray strikes the mirror.
Angle of Incidence- The angle between normal and incident ray is known as angle of incidence.
Angle of Reflection- The angle between normal and the reflected ray is known as angle of reflection.
LAWS OF REFLECTION
First Law- According to first law of reflection, the incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.
Second Law- According to second law of reflection, the angle of incidence is always equal to the angle of reflection.
IMAGE FORMED BY PLANE MIRROR
The two laws of reflection remains same regardless the shape of the mirror.
TYPES OF REFLECTION
We know light bounces off all kind of surfaces, but we can see clearly the images of objects in highly polished surfaces. This is because the polished surface reflects parallel beam of light in one direction whereas the rough surface reflects light in different directions.
Diffused Reflection- The reflection from rough surface is known as diffused reflection.
Regular Reflection- The reflection from polished surface is known as regular reflection.
TYPES OF OBJECTS
Moon and planets are non-luminous objects; we can see moon and planets because they reflects the light rays of sun.
MULTIPLE REFLECTIONS
We know that plane mirror forms a single image of an object, of the same size, upright and laterally inverted. But we can obtain number of images by using two mirrors, depending on the angle between two mirrors. For example if we place two mirrors at 90 degree to each other, we will get three images. As we decrease the angle between the mirrors, the number of images will increase. We obtain number of images by using formula,
Number of images N= (360/a) – 1, where “a” is the angle between two plane mirrors.
When two mirrors are placed parallel to each other, the number of images we obtain will be infinite.
KALEIDOSCOPE
Kaleidoscope is an optical instrument, typically a cylinder with mirrors containing loose, colored objects such as beads or pebbles and bits of glass. When a viewer looks into one end, light entering the other end creates a colorful pattern, due to repeated reflection in mirror. An interesting feature of kaleidoscope is that you will never see the same pattern again. Designers of wallpapers and fabrics artists use kaleidoscope in colored paper.
REFRACTION
Refraction is the phenomenon of change in direction of path of light ray when it passes from one transparent medium to another. Instead of mirror, if we had a transparent substance, like a thick piece of glass, the light ray will pass through it. When a ray of light passes from one transparent medium to another, it generally changes its direction. White light consists of different colors (VIBGYOR). When white light passes from one transparent medium to another at an angle, the different colors of light gets deflected by different angles. This is the reason why white light gets splits into its different colors when it passes through a prism. This phenomenon of splitting of white light into its different colors is known as DISPERSION. The band of colors produced when white light is split up is called a SPECTRUM. A rainbow is an example of a spectrum.
HUMAN EYE
Human eye is very small, about 2.5cm in length. The eyelids act as the shutters of the eye and protect them from injury. Human eye is made up of following parts:-
When the light rays reach the lens of the eye, passing through the pupil, they bend and an inverted image is formed on retina. The rods and cones convert the image into an electrical impulse, which is taken by the optic nerve to the brain. The brain interprets the impulse and we comprehend what we have seen.
PERSISTENCE OF VISION
The impression of an image does not vanish immediately from the retina. It persists there for about 1/16th of a second. This phenomenon is known as persistence of vision. So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives that those objects as moving.
DISEASES OF EYE
We can see clearly if the image is formed on the retina. If the image is formed behind or in front of retina, we cannot see the image clearly. For a person with normal eyes, it is easy to see both near and far off objects clearly. The minimum distance at which eye can see objects distinctly varies with age. The most comfortable distance at which one can read with normal eyes is about 25 cm.
Alternative Technology Available for the Blind
Some person can be visually handicapped. They have very limited vision to see things. Some person cannot see at all since their birth. Some person may lose their eyesight because of a disease.
The most popular resource for visually challenged persons is Braille. Louis Braille, who himself was a visually challenged person, developed a system for visually challenged persons and published it in 1821. Braille system has 63 dot patterns or characters. Each character represents a letter, a combination of letters, a common word or a grammatical sign.
Other such additional resources can enable them to develop their capabilities further. Resources can be of two types: Non-optical aids and optical aids.
Non-optical aids include visual aids, tactual aids (using the sense of touch), and auditory aids (using sense of hearing) and Electronic aids.
Optical aids include bifocal lenses, contact lenses, magnifiers and telescope aids.
Nutrition and Eye Health
Eye health has a very strong relationship with the food we eat. Eating a diet deficient in vitamin A over a long period of time may result in a condition called Xeropthalmia (night blindness). By consuming food rich in vitamin A, such as carrot, broccoli, tomatoes, egg yolk and butter, it can be reversed.
It is necessary that one takes proper care of your eyes. If there is any problem one should go to an eye specialist.
INTERESTING FACTS