(General Science) PHYSICS - Optics: Nature of Light, Concave, Convex Mirror, Refractive Index & Spherical Lenses


Optics: Nature of Light

Earlier light was believed to be of wave nature only. At the beginning of the 20th century, it became known that the wave theory of light often becomes inadequate for treatment of the interaction of light with matter, and light often behaves somewhat like a stream of particles. This confusion about the true nature of light continued for some years till a modern quantum theory of light emerged in which light is neither a ‘wave’ nor a ‘particle’ – The new theory reconciles the particle properties of light with the wave nature.

Rectilinear propagation: the formation of shadows with sharp edges demonstrates the rectilinear propagation of light, i.e., the fact that light travels in straight lines. When an opaque sbstacle is placed between a source of light and a screen, a shadow of the obstacle is formed on the screen. (light from a small hole), the shadow obtained is a region of total darkness, called umbra. If an extended source of light, e.g.., a bulb, is used, the umbra is surrounded by a region of partial darkness, called penumbra.

Reflection of Light

A highly polished surface, such as a mirror, reflects most of the fight falling on it. Reflection is governed by following laws:

  • The angle of incidence is equal to the angle of reflection, and
  • The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Reflection of light can be two types of surfaces

  • Plane Mirrors
  • Spherical Mirrors

Plane Mirrors: Image formed by a plane mirror is

  • Virtual and erect.
  • The size of the image is equal to that of the object.
  • The image formed is as far behind the mirror as the object is in front of it.
  • The image is laterally inverted.

Kaleidoscope: the kaleidoscope is a toy in which multiple images are formed by two strips of plane minors placed at an angle of 60⁰

Inside a tube. Small, bright-coloured glass pieces are scattered on a ground – glass plate at the bottom of the tube. When viewed from the other end of the tube, beautiful symmetrical patterns, formed by the coloured galss pieces and their five images, are seen.

Spherical mirrors: the reflecting surface of such mirrors can be considered to form a part of the surface of a sphere. The reflecting surface of a spherical mirror may be curved inwards or outwards. A spherical mirror, whose reflecting surface mirror. A spherical mirror, whose reflecting surface is curved outwards, is called a convex mirror.

Important terms associated with mirrors-pole: The centre of the reflecting surface of a spherical. It lies on the surface of the mirror. The pole is usually represented by the letter P.

Centre: 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 has represented by the letter C in the figure. The centre of curvature is not a part of the mirror. it lies outside curvature of a concave mirror lies in front of it. However, it lies behind the mirror in case of a convex mirror.

Radius of curvature: The radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of caurvature of the mirror. It is represented by the letter R. in the fig, PC is the Radius of Curvature.

Principle Axis: The imaginary straight line passing through the pole and the centre of curvature of a spherical mirror is called the principal axis. It is normal to the mirror at its pole.

Focus: The point where the incident rays which are parallel to the principal axis converge to form an image or appear to be emerging from, in case of concave mirror, its called focus. in the fig, F is the focus.

Focal Length: The distance of focus from the position of the mirror gives the focal length of the mirror. In the fig, PF is the focal length.

Aperture: The reflecting surface of a spherical mirror is by and large spherical. The surface, then, has a circular outline. The diameter of the reflecting surface of spherical mirror is called its aperture.

For spherical mirrors of small apertures, the radius of curvature is found to be equal to twice on the focal length. spherical mirror lies midway between the pole and centre of curvature.

Uses of Concave Mirror

  • Concave mirror are commonly used inn 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.

Uses of Convex Mirror

Convex mirror are commonly used as rear-view (wing) mirror 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 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.

Refraction of Light

When a ray of light passes from one medium to another, it suffers a change in direction at the boundary separating the two media. This phenomenon is called refraction. The following are the laws of refraction of light.

(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

(ii) The ratio of sine of angle of incidence to the sine of angle refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

If I is the angle of incidence and r is the angle of refraction, than,

Sir i/Sin r = constant

This constant value is called the refractive index of the second medium with respect to the first

The refractive Index

The extent of the change in direction that takes place when a ray of light travels from one media to the other, is expressed in terms of the refractive index. The value of the refractive index for a given pair of media depends upon the speed of light in the two media. Consider a ray of light travelling from medium 1 into medium 2. Let V1 be the speed of light in medium 1 and V2 be the speed of light in medium 2. The refractive index of medium 2 with respect to medium 1 is given by the ration of the speed of light in medium 1 and the speed of light in medium 2. This is usually represented by the symbol n21. This can be expressed in an equation form as

Speed of light

If medium 1 is vacuum or air, then the refractive index of medium 2 is considered with respect to vacuum. This is called the absolute refractive index of the medium. It is simply represented as n2. If c is the speed of light in air and v is the speed of light in the medium, then, the refractive index of the medium n/m is given by

Speed of light

The absolute refractive index of a medium is simply called its refractive index. In comparing two media, the one with the larger refractive index is optically denser medium than the other. The other medium of lower refractive index is optically rarer. The speed of light is higher in a rarer medium to a denser medium slow down and bends towards the normal when it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.

Some effects of refraction

  • The bottom of a tank or a pond containing water appears to be raised.
  • When a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab.
  • A pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air and water.
  • An object kept in water in glass tumbler appears to be bigger than its actual size, when viewed from the sides.
  • A coin kept in a bowl and is not visible initially becomes visible on pouring water into the bowl.
  • The apparent random wavering or flickering of objects seen through a turbulent stream of hot air rising above a fire or a radiator due to refraction between the hot air just above the fire the surrounding cold air.
  • The twinkling of stars is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth due to gradually changing refractive index.
  • The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.

Spherical LensesRefraction by Spherical Lenses

A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens. A lens may have two spherical surface, bulging outwards. Such a lens is called a double convex lens. It is simply called a convex lens. It is thicker at the middle as compared to the edges. Convex lens converges light rays as shown in the figure. Hence convex lenses are called converging lenses. Similarly, a double concave lens is bo8unded by two spherical surfaces, curved inwards. It is thicker at the edges than at the middle. Such lenses diverge light rays as shown in figure.

Such lenses are called diverging lenses. A double concave lens is simply called a concave lens. a lens either a convex lens or a concave lens, has two spherical surface. Each of these surfaces forms a part of a sphere. The centres of these spheres are called centres of curvature of the lens. The centre of curvature of a lens is usually represented by the letter C. there are two centres of curvature, one for each curved surface. An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.


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