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.

  1. Size- The image formed by the plane mirror is of same size as the size of the object. When we see ourselves in mirror, we do not find any change in size of our image.
  2. Upright- The plane mirror always forms upright image of any object. When we see ourselves in plane mirror we see our image standing upright, this upright image is also called as erect image.
  3. The distance of image as compared to that of object- In the plane mirror, the distance of the image from the mirror is equals to the distance of object from the mirror.
  4. Lateral Inversion- When we stand in front of mirror, our left arm appears as right arm of the image and vice versa. This happens because the reflected image undergoes left-right inversion, lateral inversion.

 

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

  1. Luminous Objects- Luminous objects are those objects which emit their own light, they are known as sources of light. Example - Sun, Stars and Light bulb etc.
  2. Non- luminous objects- Non luminous objects are those which do not emit their own light, they can be seen only when light from luminous objects fall on them. Example - Moon, Table and Chair etc.

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:-

 

  1. PUPIL- The inner aperture that we can see in the centre of the eye is known as pupil. It is like a hole through which light enters the eye.
  2. IRIS- It is colored part of the eye and is involved mainly in controlling the size of pupil. The iris increase and decrease the size of pupil and regulate the amount of light that enters through the pupil.
  3. SCLERA- The white part of the eye that we can see is known as sclera. It is filled with a clear watery fluid.
  4. CORNEA- It is a thin transparent tissue that covers the front of the eye.
  5. RETINA- Just behind the eyeball, there is a lining called the retina. It is retina that is sensitive to light and has receptors called rods and cones. The rods and cones respond to light and generate impulses.
  6. Rods are the cells which are sensitive to dim light.
  7. Cones are the cells which are sensitive to bright light.
  8. CILIARY MUSCLES- They controls the focal length of the eye lens.
  9. LENS- It is the transparent tissue between pupil and retina. The lens helps in focusing the light that passes through the pupil into eye. This helps in focusing the image on the retina, by bending the light rays.
  10. OPTIC NERVE- It connects the eye to brain and carries impulse to and from the brain.
  11. BLIND SPOT- The portion on retina where the nerves fibres enters the optic nerve. This portion does not have any rods and cones. The image falling on this portion of retina cannot be seen. This spot is known as Blind Spot.

 

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.

 

  1. Myopia- The formation of image in front of retina results in condition called myopia or short sightedness. A person suffering from myopia cannot see far off objects very clearly. This defect can be corrected by wearing spectacles with concave lenses.
  2. Hypermetropia- The formation of image behind the retina results in condition called hypermetropia or long sightedness. A person suffering from hypermetropia cannot see nearby objects very clearly. This defect can be rectifying by using spectacles with convex lenses.
  3. Cataract- Sometimes the lens of eye becomes opaque and cloudy. This condition is known as cataract. Cataract is very common disease all over the world and caused blurred or dimmed vision. In some cases it could be double vision or no vision at all. Minor cases of cataract can be rectifying by using corrective lens.

 

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.

 

  1. If advised, use suitable spectacles.
  2. Too much or too little light is bad for eyes. It can cause headache, eyestrain etc.
  3. Do not look at sun or powerful light directly.
  4. Never rub your eyes, if particles of dust go into your eyes, wash your eyes with clean water.
  5. Always read at normal distance for vision. Do not read by bringing the book too close to your eyes or keeping it too far.

 

INTERESTING FACTS

  • A night bird (owl) can see very well in the night but not during day. The owl has larger cornea and a large pupil to allow more light in the eyes. Also, it has on its retina, a large number of rods and only a few cones.
  • Some animals can see ultraviolet light that we cannot see. For example-Bees.
  • Space is dark because light is only visible when it has an object to strike and bounce off.
  • Light is made up of tiny particles called “photons” which contains lots of energy.
  • The sunlight that we see on earth actually left the sun 10 minutes before.
  • The human eye can distinguish about 10 million different colors.
  • Corneas are the only tissues that do not have blood.
  • Dogs cannot distinguish between red and green.
  • All babies are colour blind at birth.
  • An ostrich’s eye is bigger than its brain.