Optic: Reflection of Light
1.0Light
Light is a form of energy which excites our sense of sight.
Sources of Light
During the day, the primary source of light is the Sun and the secondary source is the brightness of the sky. Other common sources are flames, electric bulbs, tube lights (fluorescent tubes), compact fluorescent lamps (CFLs) and light emitting diodes (LEDs).
Light travels in a straight line in vacuum or in a homogeneous transparent medium.
The Nature of Light
Light consists of electromagnetic waves (non-mechanical waves) which do not contain any particles and they do not require any material medium for their propagation.
● The speed of light depends on the nature of medium. In vacuum or free space, they travel fastest and their speed is 3 ×108 m/s. In air, the speed is almost same (marginally less), whereas, in glass or water, it reduces considerably.
● The wavelength range of visible light is about 4 × 10–7 m to 7 × 10–7 m.
● Though light does not contain particles, but it carries energy and momentum. Light is supposed to have massless particles or units called ‘photons’.
● Light travels in a straight line in vacuum or in a homogeneous transparent medium.
● Light consists of electromagnetic waves which are transverse in nature.
Objects
Luminous objects : The objects which emit their own light are called ‘luminous objects’.
Examples : Sun and other stars, lamp, bulb, tube light, candle flame, etc.
Non-luminous objects : The objects which do not emit their own light but only reflect or scatter the light which falls on them are called ‘non-luminous’ objects (or illuminated objects).
Examples : Table, chair, animals, plants, planets, satellites, moon, etc.
When the light falls on a non-luminous object, it reflects a part of the light towards us. When this light reaches our eyes, we could see such an object. A luminous object emits its own light. When this light reaches our eyes, we could see a luminous objects. Thus, for you to see an object, it must reflect or emit some light that reaches your eyes.
Opaque objects : An object is said to be ‘opaque’ if it does not allow any light to pass through it. An opaque object reflects a part of the light falling on it and remaining part is absorbed by it.
Examples : Table, Wood, Wall etc.
Transparent objects : An object is said to be ‘transparent’ if it transmits most of the light falling on it. Only a very small part of light may be absorbed or reflected from it. We can see clearly an object present behind the transparent object. e.g. a plain glass slab is a transparent object (or medium).
Translucent objects : An object is said to be ‘translucent’ if it transmits a part of the light falling on it that is not sufficient to see clearly an object present behind it.
Examples : Fog, stained glass, oil paper, tissue paper etc.
For a highly polished surface or mirror, most of the light falling on it gets reflected. For a transparent medium like glass or water, most of the light falling on it gets transmitted (refracted).
When light falls on a surface or a boundary separating two media, usually a part is reflected, a part is transmitted (refracted) and a part is absorbed.
Some basic terms
● Ray of light : A ray of light is the direction in which light travels.
● Beam of light : A bundle of light rays is called beam of light (or light beam).
● Convergent beam : A beam of light in which all the rays move towards a single point is called convergent beam [see figure (a)].
● Divergent beam : A beam of light in which all the rays emerge out from a single point is called divergent beam [see figure (b)].
● Parallel beam : A beam of light in which all the rays are parallel to each other is called parallel beam [see figure (c)].
2.0Reflection of Light
Reflection of light is the process in which light rays meeting the boundary between two media ‘bounce back’, to stay in the first medium (see figure).
The process of sending back of light rays which fall on the surface of an object is called reflection of light.
On reflection of light from a surface, the speed, wavelength and frequency of light do not change. This is because the light stays in the same medium. But, amplitude and intensity of reflected ray are slightly less than that of incident ray as some part of energy is absorbed at the surface.
Laws of Reflection
First law
The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.
Second law
The angle of incidence is equal to the angle of reflection.
The laws of reflection hold good for all kinds of waves.
These laws are applicable to plane as well as curved surfaces.
Some basic terms
Incident ray : The ray of light which falls on the mirror surface is called incident ray.
Reflected ray : The ray of light which is sent back by the mirror is called reflected ray.
Point of incidence : The point at which the incident ray falls on the mirror is called point of incidence.
Normal : A line perpendicular to the surface of mirror passing through the point of incidence is called normal.
Angle of incidence : The angle made by incident ray with the normal at the point of incidence is called angle of incidence.
Angle of reflection : The angle made by reflected ray with the normal at the point of incidence is called angle of reflection.
Reflection from Plane Mirrors
A mirror is a highly polished surface used to reflect the light falling on it. Mirrors are usually made by depositing a thin layer of silver metal on one side of a plane glass sheet.
Some basic terms
Object : Anything which gives out light rays either of its own or due to reflection is called an object.
Point object : An object whose dimensions are negligibly small is called point object.
Extended object : An object whose dimensions are quite large is called extended object.
Image : An image of an object is formed when light rays coming from the object meet or appear to meet at a point after reflection from a mirror or refraction from a lens.
Real image : A real image is formed when the light rays actually meet at a point and which can be obtained on a screen. It is always inverted.
Virtual image : A virtual image is formed when the rays do not actually meet at a point but they appear to meet at a point. Such images cannot be obtained on the screen. It is always erect or upright.
Image formed by a plane mirror
The properties of image formed by a plane mirrors are :
(1) The image is virtual and erect.
(2) The distance of image from mirror is equal to distance of object from mirror.
(3) The size of image is exactly equal to the size of object.
(4) The image is laterally inverted.
Lateral inversion
When an asymmetric object is placed in front of a plane mirror, then the right side of the object appears to be the left side of image and the left side of the object appears to be the right side of its image. This change of sides of an object seen in the image is called left - right inversion or lateral inversion. The image is inverted side ways, thus, also called ‘side ways inversion’ .
3.0Angle of Deviation (δ)
The angle through which the incident ray of light is deviated from its original path when it is reflected from a mirror is called angle of deviation.
The angle between incident ray and reflected ray is called ‘angle of deviation’.
In general, if a ray is reflected (or refracted) many times, then the angle between the initial ray (incident ray) and the final ray is called ‘angle of deviation’.
Angle of Deviation for Reflection from a Single Mirror
Let an incident ray 1 falls on a plane mirror at an angle of incidence x, then by the law of reflection, the angle of reflection is also x (see figure).
Now, we can find angle of deviation (δ) from the figure,
x + x + δ = 180° (angle of a straight line is 180°)
or 2x + δ = 180°
or δ = 180° – 2x
Rotation of Incident Ray
For a fixed mirror, if the incident ray be rotated by an angle ‘θ’, the reflected ray turns by an angle ‘θ’ in opposite direction. For example, if the incident ray is rotated by an angle 30° clockwise, the reflected ray is rotated by angle 30° anticlockwise. ∠ B’OB = θ
Rotation of Plane Mirror
For a fixed incident light ray, if the mirror be rotated by an angle ‘θ’, the reflected ray turns by an angle ‘2θ’ in the same direction. For example, if the mirror is rotated by an angle 15° clockwise, the reflected ray is rotated by an angle 30° clockwise. ∠ BOB’ = 2θ
Multiple reflection of light by two plane mirrors inclined with each other :
First find
(a) If m is an even number, then for symmetric as well as asymmetric position, the number of images, n = m – 1
(b) If m is an odd number, then for symmetric position, n = m – 1 and for asymmetric position, n = m
(c) If m is a fraction and the object is symmetrically placed then, number of images, n = nearest even integer
If θ = 90°, then, n = 3 for any position of the object placed between the two mirrors.
If θ = 0°, the number of images, n = infinite i.e., a very large number of images are formed.
● If an object moves with speed ‘u’ towards a fixed mirror, the image also moves towards the mirror with speed ‘u’. The speed of the image relative to the object is ‘2u’. Similarly, if the mirror moves with speed ‘u’ towards or away from a fixed object, the image appears to move towards or away from object with speed ‘2u’.
● If the mirror moves away or towards an object by a distance ‘d’, then the image moves away or towards the object by a distance ‘2d’.
4.0Also Read
Frequently Asked Questions
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