LIGHT- Short Notes

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• To understand light you have to know that what we call light is what is visible to us.Visible light is the light that humans can see. Other animals can see different types of light. Dogs can see only shades of gray and some insects can see light from the ultraviolet part of the spectrum.
• As far as we know, all types of light move at one speed when in a vacuum. The speed of light in a vacuum is 299,792,458 meters per second.
• Any medium through which light can travel is an optical medium. If this medium is such that light travels with equal speed in all directions, then the medium is called a homogeneous medium. The homogeneous media through which light can pass easily, are called transperant media. The media through which light cannot pass, are called opaque media. Again the media through which light can pass partly, are called translucent media.
• LIGHT TRAVELS ALONG A STRAIGHT LINE.
• Light is reflected from all surfaces. Regular reflection takes place when light is incident on smooth, polished and regular surfaces.
• After striking the surface, the ray of light is reflected in another direction. The light ray, which strikes any surface,is called the incident ray. The ray that comes back from the surface after reflection is known as the reflected ray.
• The angle between the normal and incident ray is called the angle of incidence . The angle between the normal and the reflected ray is known as the angle of reflection.
Two laws of reflection are:

  1. The angle of incidence is equal to the angle of reflection.
  2. Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.

• When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection. On the other hand reflection from a smooth surface like that of a mirror is called regular reflection.
• When rays of light coming from a point of source, after reflection or refraction, actually meet at another point or appear to diverge from another point, the second point is called the image of the first point. Images may be of two types, viz., (i) real and (ii) virtual.
• An image which can be obtained on a screen is called a real image. An image which cannot be obtained on a screen is called a virtual image.
• The image formed by a plane mirror is erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.
• The reflecting surface of a spherical mirror may be curved inwards or outwards. A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.
• A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.
• The centre of the reflecting surface of a spherical mirror is a point called the pole. It lies on the surface of the mirror. The pole is usually represented by the letter P.
• The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is called the centre of curvature of the spherical mirror. It is represented by the letter C. Please note that the centre of curvature is not a part of the mirror. It lies outside its reflecting surface. The centre of curvature of a concave mirror lies in front of it. However, it lies behind the mirror in case of a convex mirror.
• The radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature of the mirror. It is represented by the letter R. You may note that the distance PC is equal tothe radius of curvature.
• Imagine a straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.
• Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light. They are often used as shaving mirrors to see a larger image of the face. The dentists use concave mirrors to see large images of the teeth of patients. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
• Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving. Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

• Lenses are widely used in spectacles, telescopes and microscopes.Those lenses which feel thicker in the middle than at the edges are convex lenses. Those which feel thinner in the middle than at the edges are concave lenses. Notice that the lenses are transparent and light can pass through them.
• A convex lens converges (bends inward) the light generally falling on it. Therefore, it is called a converging lens. On the other hand, a concave lens diverges (bends outward) the light and is called a diverging lens.
• A convex lens can forms real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass.
• A concave lens always forms erect, virtual and smaller image than the object.
• The two surfaces of the lens are parts of two spheres. The straight line joining obtained by joining two centres of the spheres is called Principal axis. Generally we use lenses whose surfaces have equal curvature. In such lenses, if we take a point on the principal axis inside the lens equidistant from the two surfaces, the point is called the optical centre of the lens.
• If a beam of parallel rays, travelling parallel to the principal axis of a convex lens, are refracted by the lens, the rays become converging and intersect each other at a particular point of the axis. The point is called the focus of the convex lens. The focal length of a lens is the distance between the optical centre and the focus of the lens.
• The power of a lens is a measure of the degree of convergence( in the case of a convex lens) or divergence ( in the case of a concave lens). It is defined as the reciprocal of its focal length expressed in meters. The S.I. Unit of power of a lens is dioptre, the symbol being D. Thus, 1 dioptre is the power of a lens whose focal length is 1 metre. 1D = 1m–

  1. You may note that the power of a convex lens is positive and that of a concave lens is negative.
    • The phenomenon due to which a ray of light deviates from its path , at the surface of seperation of two media, when the ray of light is travelling from one optical medium to another optical medium is called refraction of light. When a ray of light travels from an optically rare medium to an optically denser medium.
    • When a ray of light travels from an optically denser medium to an optically rare medium, it bends away from the normal at the surface of seperation of two media.
    • When a ray of light strikes the surface of seperation of two media normally, it does not deviate from its original path. Some indexes of refraction are diamond (2.419), glass (1.523), and water (1.33).
    • Total internal reflection is the phenomenon which involves the reflection of all the incident light off the boundary. Total internal reflection only takes place when both of the following two conditions are met: (i) the light is in the more dense medium and approaching the less dense medium., and (ii) the angle of incidence is greater than the so-called critical angle. Total internal reflection will not take place unless the incident light is traveling within the more optically dense medium towards the less optically dense medium.
  2. • Dispersion of Light: It is the phenomenon of splitting of a beam of white light into its constituent colors on passing through prism. The order of colors from the lower end are violet, indigo, blue, green, yellow, orange and red. At one end of the band, there is red and at the other violet. The sequence of colours can be best remembered by the word VIBGYOR’ which is formed by taking the initial letter of each colour.
  3. • A laser is just a really powerful beam of light. Laser isn’t a word but an acronym. It stands for LIGHT AMPLIFICATION by STIMULATED EMISSION of RADIATION.

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