Class 10 Science Light Reflection Exam Notes

Class 10 Science Light Reflection Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

The Nature Of Light

Light is a form of energy (optical energy) which helps us in seeing objects by its presence.

(A) Theories about nature of light :

(i) Particle nature of light (Newton's corpuscular theory) :

 According to Newton light travels in space with a great speed as a stream of very small particles called corpuscles.This theory was failed to explain interference of light and diffraction of light. So wave theory of light was discovered.

(ii) Wave nature of light :

 Light waves are electromagnetic waves so there is no need of medium for the propagation of these waves.They can travel in vacuum also.The speed of these waves in air or in vacuum is maximum i.e., 3 × 10⁸ m/s.

Photoelectric effect was not explained with the help of wave theory, so Plank gave a new theory which was known as quantum theory of light.

(iii) Quantum theory of light :

When light falls on the surface of metals like caesium, potassium etc., electrons are given out. These electrons are called 'photo-electrons' and phenomenon is called 'photo-electric effect'.

This was explained by Einstein. According to plank light consisted of packets or quanta's of energy called photons. The rest mass of photon is zero. Each quanta carries energy 

Some phenomenons like interference of light, diffraction of light are explained with the help of wave theory but wave theory was failed to explain the photo electric effect of light. It was explained with the help of quantum theory. So, light has dual nature.

(i) Wave nature  (ii)  Particle nature

 (B) SOURCES OF LIGHT.

♦ The objects which emit (give) light are called luminous objects. It may be natural or man-made. Sun is a natural source of light and electric lamp, and oil lamp, etc. are man-made source of light.

♦ The Non-luminous objects do not emit light. However, such objects become visible due to the reflection of the light falling on them. Moon does not emit light. It becomes visible due to the reflection of the sunlight falling on it.

(C)PROPAGATION OF LIGHT

Light travels along straight lines in a medium or in vacuum. The path of light changes only when there is an object in its path or where the medium changes. Apart from vacuum and gases, light can travel through some liquids and solids as well.

♦ Transparent medium : A medium in which light can travel freely over large distances is called a transparent medium.
Examples: Water, glycerin, glass and clear plastics are transparent.

♦ Opaque : A medium in which light cannot travel is called opaque.
Examples : Wood, metals, bricks, etc., are opaque.

♦ Translucent : A medium in which light can travel some distance, but its intensity reduces rapidly. Such materials are called translucent.
Examples : Oil

(D) THE CHARACTERISTICS OF LIGHT

♦ Light is an electromagnetic wave.

♦ Light travels in a straight line.

♦ Light is a transverse wave, and does not need any medium to travel. Light can travel through vaccum.
Its speed through vaccum is 3 × 108 m/s.

♦ The velocity of light changes when it travels from one medium to another.

♦ The wavelength (λ) of light changes when it goes from one medium to another.

♦ The frequency (f) of the light wave remains the same in all media.

♦ Light gets reflected back from polished surfaces, such as mirrors, polished metal surfaces, etc.

♦ Light undergoes refraction (bending) when it travels from one transparent medium to another.

♦ Light does not need a material medium to travel, that is, it can travel through a vacuum too. Scientists have assigned a value of 299, 792, 458 m/s to the speed of light in vacuum.

♦ According to current scientific theories, no material particle can travel at a speed greater than that of light in vacuum.

REFLECTION OF LIGHT

♦ Definition. When light rays are incident on an opaque polished surface (medium), these are returned back in the same medium.
This phenomenon of returning of ray of light in the same medium, is called reflection of light.

♦ Reflecting surface : The surface from which the light is reflected, is called the reflecting surface. In diagram, XY is the reflecting surface.

♦ Point of incidence : The point on the reflecting surface at which a ray of light strikes, is called the point of incidence. In diagram, O is the point of incidence.

♦ Normal : A perpendicular drawn on the reflecting surface at the point of incidence, is called the normal. In diagram, ON is the normal.

♦ Incident ray : The ray of light which strikes the reflecting surface at the point of incidence is called the incident ray. In diagram, PO is the incident ray.

♦ Reflected ray : The ray of light reflected from the reflecting surface from the point of incidence, is called the reflected ray. In diagram, OQ is the reflected ray.

♦ Angle of incidence : The angle that the incident ray makes with the normal, is called the angle of incidence. It is represented by the symbol i. In diagram, angle PON is the angle of incidence.

♦ Angle of reflection : The angle that the reflected ray makes with the normal, is called the angle of reflection. It is represented by the symbol r. In diagram, ∠QON is the angle of reflection.
♦ Plane of incidence : The plane in which the normal and the incident ray lie, is called the plane of incidence. In diagram, the plane of the bookpage, is the plane of incidence.

♦ Plane of reflection : The plane in which the normal and the reflected ray lie, is called the plane of reflection. In diagram, the plane of the book page, is the plane of reflection.

LAWS OF REFLECTION OF LIGHT

♦ 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 reflection ( ∠r) is always equal to the angle of incidence ( ∠i). i.e.,  ∠r =  ∠i

(For normal incidence, i = 0, r = 0. The ray is reflected back along normal).

(i) A ray of light striking the surface normally retraces its path.

When a ray of light strikes a surface normally, then angle of incidence is zero i.e.,  ∠i = 0. According to the law of reflection,
 ∠r =  ∠i,  ∠r = 0 i.e. the reflected ray is also perpendicular to the surface. Thus, an incident ray normal to the surface (i.e. perpendicular to the surface) retraces its path as shown in figure.

(ii) Laws of reflection are also obeyed when light is reflected from the spherical or curved surfaces as shown in figure (a) and (b)

(iii) Regular and Irregular Reflection :

 ♦ Regular Reflection – The phenomenon due to which a parallel beam of light travelling through a certain medium, on striking some smooth polished surface, bounces off from it, as parallel beam, in some other fixed direction is called Regular reflection

Regular reflection takes place from the objects like looking glass, still water, oil, highly polished metals, etc.

Regular reflection is useful in the formation of images, e.g., we can see our face in a mirror only on account of regular reflection. However, it causes a very strong glare in our eyes.

Irregular reflection or Diffused reflection

The phenomenon due to which a parallel beam of light, travelling through some medium, gets reflected in various possible directions, on striking some rough surface is called irregular reflection or diffused reflection.

The reflection which takes places from ground, walls, trees, suspended particles in air, and a variety of other objects, which are not very smooth, is irregular reflection.

Irregular reflection helps in spreading light energy over a vast region and also decreases its intensity. Thus, it helps in the general illumination of places and helps us to see things around us.

♦ Note : Laws of reflection are always valid no matter whether reflection is regular or irregular.

NATURE OF IMAGE

♦ Definition : Incident rays starting from a point object, and reflected from a mirror, either actually meet at or appear to come from a point. The other point is called the image of the point object.

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