Samacheer Class 8 Science - Light

Given : Radius of curvature = 25 cm
To find: f = ?
f = \(\frac { R }{ 2}\) = \(\frac { 25 }{ 2 }\)
f = 12.5 cm

Concave mirrors have several important applications due to their ability to converge light and produce magnified images. One major application is in personal grooming, where concave mirrors are used while applying make-up or shaving because they provide a magnified, upright image of the face, allowing for precise application and better visibility of details. Another important application is in lighting devices such as torches, search lights, and vehicle headlights. In these devices, a light source is placed at the focal point of a concave mirror, and the mirror reflects and directs the light rays parallel to each other, producing a strong, focused beam of light that can travel a long distance. Convex mirrors also have practical applications in everyday life. The most common application is as rear-view mirrors in vehicles such as cars, motorcycles, and buses. Convex mirrors are preferred for this purpose because they provide an upright, virtual image and offer a wider field of view compared to flat mirrors, allowing drivers to see a larger area behind the vehicle and reducing blind spots. Another important application of convex mirrors is in the hallways and corridors of various buildings including hospitals, hotels, schools, shopping malls, and stores. These mirrors are usually mounted on walls or ceilings at locations where hallways make sharp turns or intersect. The wide field of view provided by convex mirrors helps people navigate safely around corners and prevents collisions by allowing them to see oncoming traffic or pedestrians from a distance.

The laws of reflection are fundamental principles that govern how light behaves when it strikes a reflecting surface. The first law of reflection states that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane. The normal is an imaginary line drawn perpendicular to the reflecting surface at the point where the light ray strikes it. This law establishes that reflection occurs in a two-dimensional plane rather than in three dimensions. The second law of reflection states that the angle of incidence is always equal to the angle of reflection. The angle of incidence is measured between the incident ray and the normal, while the angle of reflection is measured between the reflected ray and the normal. Both angles are measured from the normal, not from the surface itself. These two laws apply to all types of reflecting surfaces, whether they are plane mirrors, curved mirrors, or any other reflective surface. The laws of reflection are independent of the wavelength of light and the nature of the reflecting surface, making them universal principles of optics.

Given :
Angle of inclination = 45°
To find : Number of images formed = \(\frac { 360° }{ angle }\) – 1
= \(\frac { 360° }{ 45% }\) – 1
= 8 – 1
= 7 images

The refractive index of a medium is a dimensionless number that quantifies how much a medium slows down light compared to its speed in vacuum or air. It is defined as the ratio of the speed of light in air (or vacuum) to the speed of light in that particular medium. Mathematically, the refractive index n is expressed as n = c/v, where c is the speed of light in air or vacuum (approximately 3 × 10⁸ metres per second) and v is the speed of light in the medium. The refractive index indicates the amount of refraction that occurs when light enters a medium. A higher refractive index means that light travels more slowly in that medium and bends more when entering it from air. For example, the refractive index of water is approximately 1.33, meaning light travels 1.33 times slower in water than in air. The refractive index of glass is typically around 1.5, indicating that light travels even slower in glass than in water. The refractive index is always greater than 1 for any medium denser than air or vacuum. This property is crucial in understanding phenomena such as refraction, the formation of images by lenses, and the behaviour of light in optical instruments.

Refraction of light rays, as they travel from one medium to another medium, obeys two laws, which are known as Snell’s laws of refraction. They are:
1. The incident ray, the refracted ray and the normal at the point of intersection, all lie in the same plane.
2. The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is equal to the refractive index of the medium, which is a constant.
\(\frac { sin i }{ sin r}\) = µ

A ray of light, falling on a body having a shiny polished and smooth surface alone is bounced back. This bouncing back of the light rays as they fall on the smooth, shiny and polished surface is called reflection.
Regular reflection:
When a beam of light (collection of parallel rays) falls on a smooth surface, it gets reflected.
After reflection, the reflected rays will be parallel to each other. Here, the angle of incidence and the angle of reflection of each ray will be equal.
Hence, the law of reflection is obeyed in this case and thus a clear image is formed. This reflection is called ‘regular reflection’ or ‘specular reflection’.
Irregular reflection:
In the case of a body having a rough or irregular surface, each region of the surface is inclined at different angles.
When light falls on such a surface, the light rays are reflected at different angles.
In this case, the angle of incidence and the angle of reflection of each ray are not equal.
Hence, the law of reflection is not obeyed in this case and thus the image is not clear. Such a reflection is called ‘irregular reflection’ or ‘diffused reflection’.

Periscope:
It is an instrument used for viewing bodies or ships, which are over and around another body or a submarine.
It is based on the principle of the law of reflection of light.
It consists of a long outer case and inside this case mirrors or prisms are kept at each end, inclined at an angle of 45°.
Light coming from the distant body, falls on the mirror at the top end of the periscope and gets reflected vertically downward.
This light is reflected again by the second mirror kept at the bottom, so as to travel horizontally and reach the eye of the observer.
In some complex periscopes, optic fibre is used instead of mirrors for obtaining a higher resolution.
The distance between the mirrors also varies depending on the purpose of using

Dispersion is the phenomenon of splitting of white light into its seven constituent colours when it passes through a transparent medium such as a prism or water droplet. This occurs because white light is composed of light of different colours, each having a different wavelength. When white light enters a medium, light of different colours travels at different speeds within that medium. Since refraction of a light ray depends on its speed in the medium, each colour bends or refracts by a different amount. Colours with shorter wavelengths bend more than colours with longer wavelengths. This differential refraction causes the white light to separate into its component colours in the order of violet, indigo, blue, green, yellow, orange, and red, with violet bending the most and red bending the least. A natural example of dispersion is the formation of a rainbow, where water droplets in the atmosphere act as tiny prisms and disperse sunlight into its constituent colours. Dispersion demonstrates that white light is not a single colour but a mixture of all colours of the visible spectrum.

Given :
Speed of light in air c = 3 x 10 8 ms -1
Refractive index of a medium µ = 1.5
To find : Speed of light in medium v = ?
Formula :
µ = \(\frac { c }{ v}\)
1.5 = \(\frac{3×10^{8}}{v}\)
v =\(\frac{3×10^{8}}{1.5}\)
v = 2 x 10 8 ms -1
Speed of light in medium v = 2 x 10 8 ms -1