Light Online Test 9th Science Lesson 6 Questions in English

Light is a form of energy which travels as electromagnetic waves. The branch of physics that deals with the properties and applications of light is called optics. In our day to day life we use number of optical instruments. Microscopes are inevitable in science laboratories. Telescopes, binoculars, cameras and projectors are used in educational, scientific and entertainment fields.

Light falling on any polished surface such as a mirror, is reflected. This reflection of light on polished surfaces follows certain laws and you have studied about them in your lower classes.

Consider a plane mirror MM′ as shown in Figure. Let AO be the light ray incident on the plane mirror at O. The ray AO is called incident ray. The plane mirror reflects the incident ray along OB. The ray OB is called reflected ray. Draw a line ON at O perpendicular to MM′. This line ON is called normal.

Laws of reflection are given as:

* The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same  

   plane.

* The angle of incidence is equal to angle of reflection.

The word lateral comes from the Latin word latus which means side. Lateral inversion means sidewise inversion. It is the apparent inversion of left and right that occurs in a plane mirror. Mirrors do not actually reverse left and right and they do not reverse up and down also. What actually mirrors do is reverse inside out.

If the light rays coming from an object actually meet, after reflection, the image formed will be a real image and it is always inverted. A real image can be produced on a screen. When the light rays coming from an object do not actually meet, but appear to meet when produced backwards, that image will be virtual image. The virtual image is always erect and cannot be caught on a screen.

To see your entire body in a mirror, the mirror should be atleast half of your height.
Height of the mirror= Your height/2.

We studied about laws of reflection. These laws are applicable to all types of reflecting surfaces including curved surfaces. In your earlier classes, you have studied that there are many types of curved mirrors, such as spherical and parabolic mirrors. The most commonly used type of curved mirror is spherical mirror.

In curved mirrors, the reflecting surface can be considered to form a part of the surface of a sphere. Such mirrors whose reflecting surfaces are spherical are called spherical mirrors. In some spherical mirrors the reflecting surface is curved inwards, that is, it faces towards the centre of the sphere. They are called concave mirrors. In some other mirrors, the reflecting surface is curved outward. They are called convex mirror.

The parallel rays of sun light could be focused at a point using a concave mirror. Now let us place a lighted candle and a white screen in front of the concave mirror. Adjust the position of the screen. Move the screen front and back. Note the size of the image and its shape. You can see a small and inverted image.

As you bring the object closer to the mirror the image becomes bigger. Try to locate the image when you bring the candle very close to the mirror. An erect magnified image of the candle is seen. In some positions of the object an image is obtained on the screen. However, at some positions of the object no image is obtained. It is clear that the behaviour of the concave mirror is much more complicated than the plane mirror.

With the use of geometrical techniques, we can simplify and understand the behaviour of the image formed by a concave mirror. In the case of plane mirror, we used only two rays to understand how to get full image of a person. But, for understanding the nature of image formed by a spherical mirror we need to look at four specific rules.

To find the position and nature of the image formed by a spherical mirror, we need to know the following rules.

Rule 1: A ray passing through the centre of curvature is reflected back along its own path

Rule 2: A ray parallel to the principal axis passes through or appears to be coming from the principal focus (in case of convex mirror) after reflection

Rule 3: A ray passing through the focus gets reflected and travels parallel to the principal axis

Rule 4: A ray incident at the pole of the mirror gets reflected along a path such that the angle of incidence is equal to the angle of reflection

When the object is far away (at infinity), the rays of light reaching the concave mirror are parallel to each other.

Position of the Image: The image is formed at the principal focus F.

Nature of the Image: It is real, inverted and highly diminished in size

When the object is beyond the centre of curvature. Position of the image: Between the principal focus F and centre of curvature C. Nature of the image: Real, inverted and smaller than object.

1. When the object is at the centre of curvature.
Position of the image: The image is at the centre of curvature itself.
Nature of the image: It is real, inverted and same size as the object.
2. When the object is in between the centre of curvature C and principal focus F.
Position of the image: The image is beyond C
Nature of the image: Image is real, inverted and magnified.
3. When the object is at the principal focus F.
Nature of the image: No image can be captured on the screen nor any virtual image can be seen.
4. When the object is in between the focus F and the pole P.
Position of the image: The image is behind the mirror.
Nature of the image: It is virtual, erect and magnified

We follow a set of sign conventions called the cartesian sign convention to measure distances in ray diagram. In this convention, the pole (P) of the mirror is taken as the origin. The principal axis is taken as the X-axis of the coordinate system.

* The object is always placed on the left side of the mirror.

* All distances are measured from the pole of the mirror.

* Distances measured in the direction of incident light are taken as positive and those measured in the opposite direction is taken as negative.

* All distances measured perpendicular to and above the principal axis are considered to be positive.

* All distances measured perpendicular to and below the principal axis are considered to be negative.

The expression relating the distance of the object (u), distance of the image (v) and the focal length (f) of a spherical mirror is called the mirror equation. It is given as:

Magnification produced by a spherical mirror gives how many times the image of an object is magnified with respect to the object size. It can be defined as the ratio of the height of the image (hi) to the height of the object (ho). A negative sign in the value of magnification indicates that the image is real. A positive sign in the value of magnification indicates that the image is virtual.

In dentist’s head mirror, a parallel beam of light is made to fall on the concave mirror. This mirror focuses the light beam on a small area of the body (such as teeth, throat etc.). When a makeup mirror is held near the face, an upright and magnified image is seen. Here, our face will be seen magnified (concave mirror is used). Concave mirrors are also used as reflectors in torches, head lights in vehicles and search lights to get powerful beams of light. Large concave mirrors are used in solar heater.

Stellar objects are at an infinite distance. Therefore, the image formed by a concave mirror would be diminished, and inverted. Yet, astronomical telescopes use concave mirrors.

Convex mirrors are used as rear-view mirrors in vehicles. It always forms a virtual, erect, small-sized image of the object. As the vehicles approach the driver from behind, the size of the image increases. When the vehicles are moving away from the driver, then image size decreases.

Convex mirrors are installed on public roads as traffic safety device. They are used in acute bends of narrow roads such as hairpin bends in mountain passes where direct view of oncoming vehicles is restricted. It is also used in blind spots in shops.

In early seventeenth century, the Italian scientist Galileo Galilee (1564‒1642) tried to measure the speed of light. In 1665, the Danish astronomer Ole Roemer first estimated the speed of light by observing one of the twelve moons of the planet Jupiter. He estimated the speed of light to be about 220,000 km per second. In 1849, the first land-based estimate was made by Armand Fizeau. Today the speed of light in vacuum is known to be almost exactly 300,000 km per second.

The bending of light rays when they pass obliquely from one medium to another medium is called refraction of light. Light rays get deviated from their original path while entering from one transparent medium to another medium of different optical density. This deviation (change in direction) in the path of light is due to the change in velocity of light in the different medium.

The velocity of light depends on the nature of the medium in which it travels. Velocity of light is more in a rarer medium (low optical density) than in a denser medium (high optical density).

When a ray of light travels from optically rarer medium to optically denser medium, it bends towards the normal. When a ray of light travels from an optically denser medium to an optically rarer medium it bends away from the normal. A ray of light incident normally on a denser medium, goes without any deviation.

The refractive index has no unit as it is the ratio of two similar quantities. The refractive index of a medium is also defined in terms of speed of light in different media.

Laws of refraction, also known as Snell’s law of refraction are given below as:
• 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
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a light of given colour and for the given pair of media.

When light travels from denser medium into a rarer medium, it gets refracted away from the normal. While the angle of incidence in the denser medium increases the angle of refraction also increases and it reaches a maximum value of r = 90º for a particular value. This angle of incidence is called critical angle. The angle of incidence at which the angle of refraction is 90º is called the critical angle. At this angle, the refracted ray grazes the surface of separation between the two media. When the angle of incidence exceeds the value of critical angle, the refracted ray is not possible. Since r > 90º the ray is totally reflected back to the same medium. This is called as total internal reflection.

In order to achieve Total Internal Reflection following conditions must be met:
• Light must travel from denser medium to rarer medium. (Example: From water to air).
• The angle of incidence inside the denser medium must be greater than that of the critical angle

Mirage is an effect of Total Internal Reflection. On hot summer days, patch of water may be on the road. This is an illusion. In summer, the air near the ground becomes hotter than the air at higher levels. Hotter air is less dense, and has smaller refractive index than the cooler air. Thus, a ray of light bends away from the normal and undergoes total internal reflection. Total internal reflection is the main cause for the spectacular brilliance of diamonds and twinkling of stars.

Optical fibres are bundles of high-quality composite glass/quartz fibres. Each fibre consists of a core and cladding. The refractive index of the material of the core is higher than that of the cladding. Optical fibres work on the phenomenon of total internal reflection. When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated total internal reflection along the length of the fibre and finally comes out at the other end.

Optical fibres are extensively used for transmitting audio and video signals through long distances. Moreover, due to their flexible nature, optical fibres enable physicians to look and work inside the body through tiny incisions without having to perform surgery.

Optical fibres are bundles of high-quality composite glass/quartz fibres. An Indian-born physicist Narinder Kapany is regarded as the Father of Fibre Optics.