Reflection of Light

Have you ever thought why we are able to see objects in the day or when there is light but not able to see them in the dark or during night when lights are switched off?

It is because of the reflection of light, that we see any object.

Reflection: The bouncing back of rays of light from a polished and shiny surface is called reflection. It is similar to bouncing back of a squash ball after hitting the wall or any hard surface. Reflection is one of the unique properties of light

 

Laws of Reflection of light:

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

The angle of incidence is denoted by 'i' and angle of reflection is denoted by 'r'. The law of reflection is applicable to all types of reflecting surface.

Spherical Mirror

Mirrors having curved reflecting surface are called spherical mirrors. A spherical mirror is a part of a sphere.

Types of Spherical Mirror:

Concave Mirror: Spherical mirror with reflecting surface curved inwards is called concave mirror.

Convex Mirror: Spherical mirror with reflecting surface curved outwards is called convex mirror.

Important terms in the case of spherical mirror:

 

Pole: The centre of reflecting surface of a spherical mirror is known as Pole. Pole lies on the surface of spherical mirror. Pole is generally represented by ‘P’.

Centre of Curvature: 

The centre of sphere; of which the reflecting surface of a spherical mirror is a part; is called the centre of curvature of the spherical mirror. Centre of curvature is not a part of spherical mirror rather it lies outside the mirror. Centre of curvature is denoted by letter ‘C’.

In the case of concave mirror centre of curvature lies in front of the reflecting surface. On the other hand, centre of curvature lies behind the reflecting surface in the case of convex mirror.

 

Radius of Curvature:

 The radius of sphere; of which the reflecting surface of a spherical mirror is a part; is called the Radius of Curvature of the spherical mirror. The radius of curvature of a spherical mirror is denoted by letter ‘R’.

Similar to centre of curvature, radius of curvature lies in front of concave mirror and lies behind the convex mirror and is not a part of the mirror as it lies outside the mirror.

Aperture: The diameter of reflecting surface of a spherical mirror is called aperture.

Principal Axis:

 Imaginary line passing through the centre of curvature and pole of a spherical mirror is called the Principal Axis.

Focus or Principal Focus: 

Point on principal axis at which parallel rays; coming from infinity; converges after reflection is called the Focus or Principal Focus of the spherical mirror. Focus is represented by letter ‘F’.

 

In the case of a concave mirror, parallel rays; coming from infinity; converge after reflection in front of the mirror. Thus, the focus lies in front of a concave mirror.

In the case of a convex mirror, parallel rays; coming from infinity; appear to be diverging from behind the mirror. Thus, the focus lies behind the convex mirror.

Focal length: 

The distance from pole to focus is called focal length. Focal length is denoted by letter ‘f’. Focal length is equal to half of the radius of curvature.

Reflection from spherical mirror:

Reflection of Rays parallel to Principal Axis:

In the case of concave mirror: A Ray parallel to principal axis passes through the principal focus after reflection from a concave mirror.

Similarly, all parallel rays to the principal axis pass through the principal focus after reflection from a concave mirror. Since a concave mirror converge the parallel rays after reflection, thus a concave mirror is also known as converging mirror.

In the case of convex mirror: A ray parallel to principal axis appears to diverge from the principal focus after reflecting from the surface of a convex mirror.

Similarly, all rays parallel to the principal axis of a convex mirror appear to diverge or coming from principal focus after reflection from a convex mirror. Since a convex mirror diverges the parallel rays after reflection, thus it is also known as diverging mirror.

Refraction:

The change of direction of light because of change of medium is known as Refraction or Refraction of Light. The ray of light changes its direction or phenomenon of refraction takes place because of difference in speed in different media.

The light travels at faster speed in rare medium and at slower speed in denser medium. The nature of media is taken as relative. For example, air is a rarer medium than water or glass.

When ray of light enters from a rarer medium into a denser medium, it bends towards normal at the point of incidence. On the contrary, when ray of light enters into a rarer medium from a denser medium it bends away from the normal.

Ray emerging after the denser medium goes in the same direction and parallel to the incident ray.

The angle between the incident ray and normal is called Angle of Incidence and it is denoted as ‘i’. The angle between the refracted ray and normal is called the Angle of Refraction. Angle of refraction is denoted by ‘r’.

Laws of Refraction:

  1. The incident ray, refracted ray and normal to the interface of given two transparent media, all lie in same plane.
  2. The ratio of sine of angle of incidence and sine of angle of refraction is always constant for the light of given colour and for the pair of given media.

The Second Law of Refraction is also known as Snell’s Law of Refraction.

The constant is called refractive index of the second medium in relation to the first medium.

Spherical Lens

Lens is an optical device which converges or diverges the rays of light before transmitting. A lens has similar shape to lentils and genus of lentil is called Lens, thus a lens got its name after the shape and name of genus of lentils. A lens is made by combining at least one part of sphere made of transparent material, generally glass.

 Most of the lenses are made by the combination of parts of transparent sphere. Concave and Convex lens are most commonly use spherical lens.

Convex lens is the most commonly used lens in our day to day life.

Convex lens:

A lens having two spherical surfaces bulging outwards is called Convex Lens. It is also known as biconvex lens because of two spherical surface bulging outwards.

Fig: Spherical Lens

Concave lens:

A lens having two spherical surfaces bulging inwards is called Concave Lens. It is also known as biconcave lens because of two spherical surface bulging inwards.

Important terms for spherical lens:

Centre of curvature: The centre of sphere of part of which a lens is formed is called the centre of curvature of the lens. Since concave and convex lenses are formed by the combination of two parts of spheres, therefore they have two centres of curvature.

One centre of curvature is usually denoted by C1 and second is denoted by C2.

Focus: Point at which parallel rays of light converge in a concave lens and parallel rays of light diverge from the point is called Focus or Principal Focus of the lens.

Similar to centres of curvature; convex and concave lenses have two Foci. These are represented as F1 and F2.

Principal Axis: Imaginary line that passes through the centres of curvature of a lens is called Principal Focus.

Optical centre: The central point of a lens is called its Optical Centre. A ray passes through optical centre of a lens without any deviation.

Radius of curvature: The distance between optical centre and centre of curvature is called the radius of curvature, which is generally denoted by R.

Focal Length: The distance between optical centre and principal focus is called focal length of a lens. Focal length of a lens is half of the radius of curvature.

This is the case that the centre of curvature is generally denoted by 2F for a lens instead of C.

Magnification:

The ratio of height of image and that of object or ratio of distance of image and distance of object gives magnification. It is generally denoted by ‘m’.

The positive (+) sign of magnification shows that image is erect and virtual while a negative (-) sign of magnification shows that image is real and inverted.

Power of lens:

A convex lens with short focal length converges the light rays with greater degree nearer to principal focus and a concave lens with short focal length diverges the light rays with greater degree nearer to principal focus.

The degree of divergence or convergence of ray of light by a lens is expressed in terms of the power of lens. Degree of convergence and divergence depends upon the focal length of a lens. The power of a lens is denoted by ‘P’. The power of a lens is reciprocal of the focal length.

The SI unit of Power of lens is dioptre and it is denoted by ‘D’.

Power of a lens is expressed in dioptre when the focal length is expressed in metre. Thus, a lens having 1 metre of focal length has power equal to 1 dioptre.

Therefore, 1 D = 1 m−1

A convex lens has power in positive and a concave lens has power in negative.

Human Eye

Structure of Human Eye:

The human eye is a spherical structure which fits in the eye socket in the skull bone. There are following main parts in the human eye.

  1. Pupil: Pupil is the round black spot in front of eye. It regulates the amount of light entering the eyes. Pupil works like aperture of a camera. In case of dim light, the pupil dilate to allow more light to enter the eyes. In case of strong light, the pupil constrict allowing less light to enter.
  2. Iris: Iris is made of muscles. They control the size of the opening of pupil.
  3. Lens: Lens lies just behind the pupil. Lens becomes thin to increase its focal length. This enables us to see distant objects clearly. To focus on nearer objects, lens becomes thick to decrease its focal length. But there is a limit. The minimum distance of clear vision is 25 cm. Below this distance, we cannot see things clearly.
  4. Retina: Retina works like a screen or camera film. Retina is full of light and colour sensitive cells. These cells, upon receiving image send electrical signals to the brain, which processes this information to make a mental image of what we see. The photoreceptor cells in the eye are of two types, viz. rod cells and cone cells. The rod cells are sensitive to dim light. The cone cells are sensitive to bright light and colour.

Benefits of two eyes: One eye is having a field of vision of about 150 degrees. Both the eyes enable us to see up to a field of 180 degrees. Moreover, as two different images get juxtaposed in the brain, so we are able to see a three-dimensional view of the world.

Power of Accommodation of Human Eye: The human eye can clearly see a nearby object as well as an object on infinity. This ability of the human eye is called the power of accommodation of human eye.

Malfunctions of Eyes:

Cataract:

 In old age the cornea becomes cloudy. This reduces the vision in old age. Cataract can be cured by eye surgery. Sometimes, artificial lens is also transplanted during cataract surgery. This is called Intra Ocular Lens Transplantation.

Myopia: 

Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect may arise due to

  1. excessive curvature of the eye lens, or
  2. elongation of the eyeball.

Correction of Myopia:

 This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.

Hypermetropia:

 Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point (25 cm). Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a nearby object are focused at a point behind the retina. This defect arises either because

  1. the focal length of the eye lens is too long, or
  2. the eyeball has become too small.

Correction of Hypermetropia:

 This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.

Refraction of Light through a Prism:

Prism is a transparent optical element which refracts light. An optical object to be defined as prism must have at least two faces with an angle between them. Triangular prism is the most common type of prism. It has a triangular base and rectangular sides.

When a ray of light enters the prism, it bends towards the normal; because light is entering from a rarer medium to a denser medium. Similarly, when the light emerges from the prism, it follows the laws of refraction of light. Due to the angle of the prism and due to different wavelengths of different components of white light; the emergent ray gets segregated into different colours. Finally, a colourful band of seven colours is obtained. This phenomenon is called dispersion of white light by the prism.

Scattering of Light

When light hits a particle, it scatters in different directions. Refraction happens because of non-uniformities of particles of a medium. Many interesting phenomena can be observed because of scattering of light. Some of them are given here.

Tyndall Effect

The optical effect because of scattering of light from the particles of colloid or suspension is called Tyndall Effect. For Tyndall effect to be possible, the size of particles should be less than or equal to the wavelength of the visible spectrum. So, the size of particles should be between 40 and 900 nanometers. Tyndall effect is responsible for many natural phenomena. The white beam of light which appears to come through the ventilation or through a slit in the door is because of Tyndall Effect and the dust particles in the air cause the scattering of light in this case. The white beam appears because scattering of light makes the dust particles visible in the light.

Why is the colour of the clear Sky Blue?

We know that the wavelength of red colour is more than that of blue colour. The size of particles in air is smaller than the wavelength of visible light. Hence, these particles scatter the light of shorter wavelength more effectively than light of longer wavelength. The blue end of the visible spectrum has shorter wavelength than the red end. Due to this, blue colour is scattered more strongly in the atmosphere; compared to the red colour. This is the reason sky appears blue. Since red colour is scattered the least hence it is used in traffic lights for showing the danger signal.