CBSE Class 8 Science Light Chapter Notes

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LIGHT

LIGHT

Light is a form of energy which produces the sensation of sight. Light is an electromagnetic wave which does not require any material medium for its propagation. The speed of light waves depends on the nature of the medium through which they pass.

REFLECTION OF LIGHT

The process of bouncing back of light into the same medium after striking the surface of another medium is called reflection. A surface which reflects light is called a reflector. Silver metal is one of the best reflectors of light. A highly polished surface, such as a mirror, reflects most of the light falling on it.

Types of reflection. There are two types of reflection:

1. Regular reflection: when a parallel beam of light falls on a smooth and highly polished surface, the reflected beam is also parallel and is directed in a fixed direction. Example: light reflected from a searchlight or automobile headlights.
2. Diffused reflection: when a parallel beam of light falls on a rough surface, the reflected light is not parallel but spreads in all directions. This is called irregular or diffused reflection. Example: light reflected from a wooden surface.

Laws of reflection. The reflection of light from a plane surface or from a spherical surface takes place according to two laws:

1. The incident ray, the normal to the mirror at the point of incidence and the reflected ray all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection, i.e. \(i=r\).

These laws of reflection are applicable to all types of reflecting surfaces, such as plane surfaces, spherical surfaces or any irregular surface.

FORMATION OF IMAGE BY A PLANE MIRROR

Consider a point object O placed in front of a plane mirror MM′. The mirror forms an image I of the object O. Two reflected rays, when produced backwards, meet at a point behind the mirror, and hence form the virtual image I at that point.

Real image and virtual image

Properties of the image formed by a plane mirror

1. The image is virtual and erect.
2. The distance of the object from the mirror is equal to the distance of the image from the mirror.
3. The size of the image is equal to the size of the object.
4. The linear magnification produced by a plane mirror is unity, i.e. \(m=\dfrac{h'}{h}=1\).
5. The image is laterally inverted, i.e. the left side of the image appears on the right and vice versa.

Uses of a plane mirror

1. A plane mirror is used as a looking glass.
2. A plane mirror is used in a solar cooker to reflect sunlight.
3. Plane mirrors are used in periscopes, usually used in submarines.
4. Plane mirrors are used in barber’s shops to see the back portion of the head.

SPHERICAL MIRRORS

A spherical mirror is a mirror whose reflecting surface is a part of a hollow sphere of glass. Spherical mirrors are of two types: concave and convex. A spherical mirror whose reflecting surface is curved inwards, i.e. faces towards the centre of the sphere, is called a concave mirror. A spherical mirror whose reflecting surface is curved outwards is called a convex mirror.

Important terms used in spherical mirrors

1. Centre of curvature (C): the centre of the hollow sphere of which the mirror forms a part. The centre of curvature of a concave mirror lies in front of it, but that of a convex mirror lies behind the mirror.
2. Radius of curvature (R): the radius of the hollow sphere of which the mirror forms a part.
3. Pole (P): the centre of the reflecting surface of a spherical mirror.
4. Principal axis: the straight line passing through the centre of curvature and the pole of a spherical mirror.
5. Aperture: the part of the mirror from which reflection of light actually takes place; in other words, the diameter of the spherical mirror.
6. Principal focus (F): the point on the principal axis of a spherical mirror where parallel rays of light close to the principal axis meet, or appear to meet, after reflection from the mirror.
7. Focal length (f): the distance between the pole and the principal focus of a spherical mirror.

Note: for spherical mirrors of small aperture, the radius of curvature is equal to twice the focal length:

\[R=2f\quad\text{or}\quad f=\dfrac R2\]

Rules to draw a ray diagram. For a concave mirror: (i) a ray parallel to the principal axis passes through the focus F after reflection; (ii) a ray passing through the focus F becomes parallel to the principal axis after reflection; (iii) a ray passing through the centre of curvature C is reflected back along the same path; (iv) a ray incident at the pole P is reflected making an equal angle with the principal axis on the other side. For a convex mirror the same rules hold with the rays appearing to pass through F or C behind the mirror: a ray parallel to the principal axis appears to come from F after reflection; a ray directed towards F becomes parallel to the principal axis; a ray directed towards C is reflected back along the same path.

Representation of images formed by spherical mirrors using ray diagrams. To locate the image of an object, it is more convenient to consider only two rays, chosen so that their directions after reflection are easy to know. The intersection of at least two reflected rays gives the position of the image of the object.

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

When the object is beyond C

Uses of concave mirrors

1. Concave mirrors are commonly used in torches, searchlights and vehicle 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

When the object is at infinity

Uses of convex mirrors

1. Convex mirrors are commonly used as rear-view (wing) mirrors in automobiles to see the traffic at the back.
2. 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.

SIGN CONVENTION FOR REFLECTION BY SPHERICAL MIRRORS

1. The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
2. All distances parallel to the principal axis are measured from the pole of the mirror.
3. All distances measured to the right of the origin (along the +x axis) are taken as positive, while those measured to the left of the origin (along the –x axis) are taken as negative.
4. Distances measured perpendicular to and above the principal axis (along the +y axis) are taken as positive.
5. Distances measured perpendicular to and below the principal axis (along the –y axis) are taken as negative.

MIRROR FORMULA AND MAGNIFICATION

In a spherical mirror, the relationship between the image distance (v) from the pole, the object distance (u) from the pole and the focal length (f) is known as the mirror formula:

\[\dfrac1v+\dfrac1u=\dfrac1f\]

Magnification is defined as the ratio of the height of the image to the height of the object:

\[m=\dfrac{\text{height of the image }(h')}{\text{height of the object }(h)}\]

The magnification m is also related to the object distance (u) and image distance (v). It can be expressed as

\[m=\dfrac{h'}{h}=-\dfrac vu\]

A negative sign in the value of the magnification indicates that the image is real. A positive sign indicates that the image is virtual.

REFRACTION OF LIGHT

The change in direction of a light ray when it passes from one medium to another is called refraction of light.

1. When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal.
2. When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal.

REFRACTIVE INDEX

The ratio of the speed of light in vacuum to the speed of light in a medium is called the refractive index of that medium. The refractive index depends on the nature of the material medium and on the wavelength of light used. The refractive index of medium 2 with respect to medium 1 can be expressed as

\[n_{21}=\dfrac{\text{speed of light in medium 1}}{\text{speed of light in medium 2}}=\dfrac{v_1}{v_2}\qquad\text{or}\qquad n_{12}=\dfrac{v_2}{v_1}\]

\[n_m=\dfrac{\text{speed of light in air (vacuum)}}{\text{speed of light in the medium}}=\dfrac cv\]

Absolute refractive index of some material media

LAWS OF REFRACTION

1. The incident ray, the refracted ray and the normal to the interface of two media at the point of incidence all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction. If \(i\) is the angle of incidence and \(r\) is the angle of refraction, then \(\dfrac{\sin i}{\sin r}=\) constant. The value of this constant is called the refractive index of the second medium with respect to the first.

Note: refraction of light takes place because the speed of light is different in different media.

REFRACTION THROUGH A RECTANGULAR GLASS SLAB

Consider a ray of light AB going from air into a glass slab at face PQ, travelling along BC in the glass and coming out into air along CD from the opposite face SR. Refraction of light occurs at both surfaces of the slab. It is observed that the emergent ray CD is parallel to the incident ray AB.

When the ray is refracted from air to glass, the refractive index of glass with respect to air is

\[n_{ga}=\dfrac{\sin i_1}{\sin r_1}\quad\ldots(i)\]

When the ray is refracted from glass to air, the refractive index of air with respect to glass is

\[n_{ag}=\dfrac{\sin i_2}{\sin r_2}\quad\ldots(ii)\]

Multiplying (i) and (ii), and using \(n_{ga}\times n_{ag}=1\):

\[\dfrac{\sin i_1}{\sin r_1}\times\dfrac{\sin i_2}{\sin r_2}=1\]

Since \(i_2=r_1\), this gives \(\sin r_2=\sin i_1\), i.e. \(i_1=r_2\). Thus the ray of light leaves the glass slab at the same angle at which it entered it on the opposite face. However, it gets laterally displaced.

REFRACTION BY SPHERICAL LENSES

A transparent material bound by two surfaces, of which one or both are spherical, forms a lens. There are two types of lenses: convex and concave. A convex lens is thicker at the middle than at the edges. It is also known as a converging lens because it converges a parallel beam of light. A double concave lens is bounded by two spherical surfaces curved inwards. It is thicker at the edges than at the middle. Such lenses are called diverging lenses. A double concave lens is simply called a concave lens.

Rules to draw ray diagrams. For drawing ray diagrams in lenses we can consider any two of the following rays. For a convex lens: (i) a ray parallel to the principal axis passes through the second focus F₂ after refraction; (ii) a ray passing through the first focus F₁ emerges parallel to the principal axis; (iii) a ray passing through the optical centre O goes straight without deviation. For a concave lens: (i) a ray parallel to the principal axis appears to diverge from F₁ after refraction; (ii) a ray directed towards F₂ emerges parallel to the principal axis; (iii) a ray passing through the optical centre O goes straight without deviation.

Formation of images by a convex lens

When the object is beyond 2F₁

Formation of images by a concave lens

When the object is beyond 2F₁

LENS FORMULA AND MAGNIFICATION

The formula which gives the relationship between the object distance (u) from the optical centre, the image distance (v) from the optical centre and the focal length (f) of a lens is known as the lens formula:

\[\dfrac1v-\dfrac1u=\dfrac1f\]

Note: \(\dfrac1v-\dfrac1u=\dfrac1f\) or \(\dfrac vv-\dfrac vu=\dfrac vf\) or \(1-\dfrac vu=\dfrac vf\) \(\Rightarrow m=\dfrac{f-v}{f}\), where \(\dfrac vu=m\) (magnification).

The magnification produced by a lens is defined as the ratio of the height of the image to the height of the object. It is also equal to the ratio of the image distance (v) to the object distance (u):

\[m=\dfrac{h'}{h}=\dfrac vu\]

POWER OF A LENS

The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power. The power of a lens is defined as the reciprocal of its focal length. The power P of a lens of focal length f is

\[P=\dfrac1f\]

The SI unit of power of a lens is the dioptre, denoted by D. If f is expressed in metres, the power is expressed in dioptres. Thus 1 dioptre is the power of a lens whose focal length is 1 metre. The power of a convex lens is positive and that of a concave lens is negative.

Power of a combination of lenses. If a number of lenses are placed in close contact, the power of the combination is equal to the algebraic sum of the powers of the individual lenses. If two lenses of powers \(P_1\) and \(P_2\) are placed in contact, their resultant power is

\[P=P_1+P_2\]

THE HUMAN EYE

The human eye is one of the most valuable and sensitive sense organs. As far as physics is concerned, the human eye consists of a convex lens (called the eye lens) and a screen (called the retina). The eye lens forms an inverted and real image of the object on the retina, and we are able to see the object. The main parts of the eye and their functions are:

1. Cornea: the front part of the eye. It is a thin membrane made of transparent substance and bulges outwards. Light coming from objects enters the eye through the cornea.
2. Iris: a muscular diaphragm having a hole in its centre, called the pupil. The function of the iris is to control the size of the pupil, which regulates the amount of light entering the eye. The pupil becomes small when bright light falls on the eye and becomes wide in dim light.
3. Eye lens: a crystalline biconvex lens made of a transparent and flexible material like a jelly made of proteins. It focuses the images of objects on the retina.
4. Ciliary muscles: the eye lens is held in position by the ciliary muscles, which control the focal length of the eye lens. When the muscles relax, the eye lens becomes thinner and its focal length increases. When the ciliary muscles contract, the eye lens becomes thicker and its focal length decreases.
5. Retina: it is behind the eye lens and acts as a screen on which the image of the object is obtained. It is a delicate membrane having a large number of light-sensitive cells called rods and cones. Rods are responsible for dim-light vision, while cones are responsible for bright-light vision.
6. Optic nerves: the light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals and processes the information so that we perceive objects as they are.

POWER OF ACCOMMODATION

A normal eye can see near and far-off objects clearly if sharp images of these objects are formed on the retina. Since the distance between the eye lens and the retina is fixed, the focal length of the lens has to be changed to see objects at different positions and form sharp images on the retina. The ability of the eye lens to adjust its focal length is called accommodation. The focal length of the eye lens is adjusted automatically by the action of the ciliary muscles, so that the eye lens can focus distant as well as nearby objects on the retina.

RANGE OF VISION

The range of vision of a normal human eye is from infinity to about 25 cm; that is, a normal eye can see clearly any object lying between infinity and about 25 cm.

Near point and far point. The minimum distance at which objects can be seen most distinctly without strain is called the least distance of distinct vision, or the near point of the eye. For a normal human eye the near point is about 25 cm. The farthest point up to which the eye can see objects clearly is called the far point of the eye. For a normal human eye the far point is infinity.

Persistence of vision. The ability of the eye to continue to see the image of an object for a very short duration after the removal of the object is called persistence of vision. Normally the image of an object seen by our eyes persists on the retina for about \(\dfrac1{16}\) of a second after we have stopped seeing the object.

Cataract. A defect of the eye which usually comes in old age. Cataract develops when the eye lens of a person becomes hazy or cloudy due to the formation of a membrane over it. This causes partial or even complete loss of vision. It is possible to restore vision through cataract surgery.

Colour blindness. A defect of the eye due to which a person is unable to distinguish between certain colours. Colour-blind persons do not possess some of the cone cells that respond to certain colours. Colour blindness is a genetic disorder and cannot be cured.

DEFECTS OF VISION AND THEIR CORRECTION

There are three common refractive defects of vision: (i) myopia or near-sightedness, (ii) hypermetropia or far-sightedness, (iii) presbyopia.

Myopia (short-sightedness). A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. It is corrected by using a concave lens of suitable power, which brings the image back on to the retina.

Hypermetropia (long-sightedness). A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This defect may arise because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. It is corrected by using a convex lens of suitable power, which provides the additional focusing power required to form the image on the retina.

Presbyopia. The power of accommodation of the eye usually decreases with ageing due to the gradual weakening of the ciliary muscles and the diminishing flexibility of the crystalline lens. This defect is called presbyopia. Because of it most people cannot read comfortably and distinctly and need convex lenses of appropriate power.

Sometimes a person may suffer from both myopia and hypermetropia. Such persons often require bi-focal lenses. The upper portion consists of a concave lens, which facilitates distant vision. The lower part is a convex lens, which facilitates near vision. These days it is also possible to correct refractive defects with contact lenses or through surgical interventions.

In the following videos we have explained you about the concept of Light and its properties. Watch this video to understand the concepts and also post your questions and comments in the comments section

Part 1

Part 2

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