Las estrategias y la planificación sectorial

The ray model of light explains two of the most important aspects of light propagation: Reflection and refraction. In a homogeneous medium, light travels along a straight path. When a light wave strikes a smooth interface separating two transparent materials (such as air and glass or water and glass), the wave is generally partly reflected and partly refracted (transmitted) into the second material (Fig. 1.65). The phenomenon of change in path of light as it goes from one medium to another is called refraction.

Fig. 1.64: The incident ray, reflected ray and the normal to the reflecting surface lie in the same plane

Laws of reflection and refraction

1. The incident, reflected and refracted rays and the normal to the surface all lie in the same plane (Fig. 1.64). The plane of the three rays is perpendicular to the plane of the boundary surface between the two materials.

2. The angle of reflection is equal to the angle of incidence for all wavelengths and any pair of materials. This relation, together with the observation that the incident and reflected rays and the normal all lie in the same plane is called the law of reflection. 3. For monochromatic light and for a given pair of materials, a and b, on opposite sides of

the interface, the ratio of the sines of the angles where both angles are measured from the normal to the surface, is equal to the inverse ratio of the two indexes of refraction:

This experimental result, together with the observation that the incident and refracted rays and the normal all lie in the same plane, is called the law of refraction or Snell’s law, after the Dutch Scientist Willebrord Snell.

Characteristic of the image formed by a plane mirror:

1. Image is as far as behind the mirror, as the object is in front of the mirror. 2. The size of the image is same as that of the object.

3. The image formed is virtual in nature. 4. The image formed is erect in nature.

5. The image formed is laterally inverted. The lateral inversion means that the right side of the object appears as the left side of the image and vice versa.

The portion of a reflecting surface, which forms part of a sphere is called a spherical mirror. The spherical mirrors are of two types:

Concave spherical mirror: A spherical mirror whose reflecting surface is toward the centre of the sphere of which mirror forms a part is called concave spherical mirror.

Convex spherical mirror: A spherical mirror whose reflecting surface is away from the centre of the sphere of which mirror forms a part is called convex spherical mirror.

Pole: The centre of spherical mirror is called its pole.

Principal axis: The line joining the pole and the centre of curvature of the mirror is called the principal axis of the mirror.

Centre of curvature: The centre of sphere of which mirror forms a part is called the centre of curvature of the mirror.

Radius of curvature: The radius of sphere of which mirror forms a part is called the radius of curvature of the mirror.

Aperture: The diameter of the mirror is called aperture of the mirror.

Principal focus: The point at which a narrow beam of light incident on the mirror parallel to its principal axis after reflection from the mirror meets or appears to come from is called principal focus of the mirror.

Focal length: The distance between the pole and the principal focus of the mirror is called the focal length of the mirror.

Applications of plane or curved mirrors:

1. Concave mirrors are used for dressing up or used as make up mirrors. It is because a person keeps his body or face between pole and focus of the concave mirror, a highly magnified image of his body or face is formed.

2. Concave mirrors are used by dental surgeons for examining dental cavities. 3. Concave mirrors are used by ophthalmologists for examining the eye.

4. Concave mirrors are used as reflectors in cinema projectors, magic lanterns, etc. 5. Concave mirrors are used to make reflecting type astronomical telescope of large aperture. 6. Concave parabolic mirrors are used in search lights.

7. Convex mirrors are used in vehicles as drivers mirror. The driver of the vehicle can get a clear and much wider field of view of the objects behind him.

8. Convex mirrors are used as a safety feature at sharp turns or dangerous corners of the road. These are also used to prevent shop lifting activities in the market.

dispersion

Ordinary white light is a superposition of waves with wavelengths extending throughout the visible spectrum. The speed of light in vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths. Therefore, the index of refraction of a material depends on wavelength. The dependence of wave speed and index of refraction on wavelength is called dispersion. The phenomenon of splitting up of white light into its constituent colors is called dispersion of light.

If a beam of white light is made to fall on one face of a prism, the light emerging from the other face of the prism consists of seven colors namely violet, indigo, blue, green, yellow,

orange and red. The deviation suffered by the violet color is maximum, while that by the red

is minimum. The band of seven colors produced at the screen is called spectrum (Fig. 1.66).