📝 Light - Reflection

Introduction

Light is a form of invisible energy that produces the sensation of sight. We can see the objects in the presence of light, although light itself is invisible. The objects which emit light are known as sources of light or luminous objects. When light emitted by the source of light falls on objects like chairs, tables, books, etc, some of the incident light is returned back (or reflected) by the object. The light returned back or reflected by the object enters our eyes. Hence, the object is seen by us. Light enables us to see the objects which do not emit their own light known as non-luminous objects.

When sunlight falls on a solid object like a building, a stone, or a blackboard, etc. a shadow is formed behind the solid object (or an opaque object). This shows that light travels in a straight line. Light traveling in a straight line is represented by rays of light. A ray of light is a path followed by light energy in a transparent medium. A group of parallel rays of light emitted by the source of light is called a beam of light.

Ray of light

Beam of light

Reflection of light

The process of returning (or bouncing) back) the light to the same medium after striking a surface is called reflection of light. The surface which reflects the light is known as a reflector.

Reflection of light

θ = ∠i = ∠r

Perpendicular has drawn to the reflecting surface is known as normal to the reflecting surface.
Any ray of light falling on a reflecting surface is known as an incident ray of light.
Any ray of light which is reflected back by a reflecting surface is known as a reflected ray of light.
The angle between the incident ray and normal to the point of incidence on the reflective surface is known as an incident angle.
The angle between the reflected ray and the normal to the point of incidence on the reflecting surface is known as the angle of reflection.

Reflectance: The ratio of the amount of light reflected from a surface to the amount of light falling on the surface is called reflectance.

Reflectance = Amount of light reflected from a surface/ Amount of light falling on the surface.

Laws of Reflection

The angle of incidence is equal to the angle of reflection. ∠i =∠r
Incident ray, reflected ray, and normal to the reflecting the surface at the point of incidence lie in the same plane.

Reflection of light

θ = ∠i = ∠r

Images are of two types

Real image
Virtual image

1. Real image, when a beam of light from an object actually meets at a point after reflection, then the image of the object formed at that point is known as a real image.

A real image can be obtained on a screen.

2. Virtual image, when a beam of light from an object does not meet at a point but appears to diverge from it after reflection, then the image of the object at that point is known as a virtual image.

A virtual image can not be obtained on a screen.

Curved or spherical mirrors

A curved or spherical mirror is the reflecting part of a hollow spherical mirror.

Types of spherical mirrors

Concave mirror
Convex mirror

1. Concave mirror is the part of a hollow sphere whose outer surface (i.e., bulging surface) is silvered, and the inner surface act as a reflecting surface.

2. Convex mirror is the part of a hollow sphere whose outer surface (i.e., bulging surface) acts as a reflecting surface and the inner surface is silvered as shown in fig(b)

Convergence and divergence of light

●If a parallel beam of light after reflection meets at a point, then the process is known as the convergence of light. Concave mirror ➠ Converge the light.

● If a parallel beam of light after reflection diverges, then the process is known as divergence of light. Convex mirror ➠ Diverge (Spread out) the light

(a) Concave mirror, (b) Convex mirror

Important terms related to spherical mirrors

➤ Pole: The centre of the reflecting surface of a spherical mirror is a point called the pole. It lies on the surface of the mirror. The pole is usually represented by the letter P.

➤ Centre of curvature: 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 is represented by the letter C.

➤ 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 curvature of the mirror. It is represented by the letter R. The distance PC is equal to the radius of curvature (Fig. a & b).

➤ Principal axis: Imagine a straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis. Remember that the principal axis is normal to the mirror at its pole.

➤ Principal focus: A point on the principal axis of a spherical mirror where the rays of light, parallel to the principal axis meet or appear to meet after reflection, is called the principal focus. The principal focus is represented by the letter F.

➤ Focal length: The distance between the pole and the principal focus of a spherical mirror is called the focal length. It is represented by the letter `f`.

➤ Aperture: Reflecting surface of a spherical mirror is called its aperture. In (Fig. a & b), distance MN represents the aperture.

Relationship between the radius of curvature R, and focal length `f`, of a spherical mirror

For spherical mirrors of small apertures, the radius of curvature is found to be equal to twice the focal length. We put this as R = 2`f`. This implies that the principal focus of a spherical mirror lies midway between the pole and centre of curvature.

Representation of Images Formed by Spherical Mirrors Using Ray Diagrams

Consider an extended object, of finite size, placed in front of a spherical mirror. Each small portion of the extended object acts as a point source. An infinite number of rays originate from each of these points. It is more convenient to consider only two rays, for the sake of clarity of the ray diagram.

The intersection of at least two reflected rays gives the position of an image of the object.

1. A ray parallel to the principal axis, after reflection, will pass through the principal focus in the case of a concave mirror or appear to diverge from the principal focus in the case of a convex mirror.

2. A ray passing through the principal focus of a concave mirror or a ray that is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.

3. A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path. The light rays come back along the same path because the incident rays fall on the mirror along the normal to the reflecting surface.

4. A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror, is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis.

Image formation by Concave Mirror

Image formation by a concave mirror for different positions of the object

Ray diagrams for the image formation by a concave mirror

Uses of concave mirrors

1. Concave mirrors are commonly used in torches, searchlights, and vehicles headlights to get powerful parallel beams of light.

2. They are often used as shaving mirrors to see a larger image of the face.

3. Dentists use concave mirrors to see large images of the teeth of patients.

4. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Image formation by a Convex Mirror

Nature, position, and relative size of the image formed by a convex mirror

Formation of the image by a convex mirror

Uses of convex mirrors

1.Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view a much larger area than would be possible with a plane mirror.

2. Convex mirrors are used in street lights to diverge light over a large area.

3. Convex mirrors are used in big shopping stores to watch the activities of the customers. This is because the field of view of a convex mirror is large and even a single convex mirror can monitor the activities of the customers in the large areas of the stores.

Complete notes will be upload soon

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Summary

◆ Light seems to travel in straight lines.

◆ Mirrors and lenses form images of objects. Images can be either real or virtual, depending on the position of the object.

◆ The reflecting surfaces, of all types, obey the laws of reflection. The refracting surfaces obey the laws of refraction.

◆ New Cartesian Sign Conventions are followed for spherical mirrors and lenses.

◆ The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object. `( m = h^'/h)`

◆ Mirror formula, `1/v + 1/u = 1/f` , gives the relationship between the object-distance `(u)`, image-distance `(v)`, and focal length `(f)` of a spherical mirror.

◆ The focal length of a spherical mirror is equal to half its radius of curvature. `(f = R/2)`

Light - Reflection Class 10