Rajasthan Board RBSE Class 10 Science Notes Chapter 9 Light
Light:
Light is a kind of energy which helps us to see the wonderful world around us. We cannot see in the absence of light.
Reflection of Light:
When light rays fall on a surface then most of the rays travel in definite directions from the surface. This is called reflection of light. There are two types of reflection, viz. regular reflection and diffused reflection.
- Regular Reflection: When rays of light fall on a smooth surface then all the reflected rays are parallel to each other. This results in regular reflection.
- Irregular Reflection or Diffused Reflection: When rays of light fall on a rough .surface then reflected rays are not parallel to each other.This is called irregular or diffused reflection.
Laws of Reflection : Following are the laws of reflection:
- Incident ray, reflected ray and normal at the point of incidence, all lie in the same plane.
- Angle of reflection is equal to angle of incidence, ∠i = ∠r
Image Formation in Plane Mirror:
The image formed in plane mirror is virtual. Image distance is equal to object distance. Size of image is equal to size of object.
Lateral Inversion:
When you will see your image in a plane mirror, you will notice that your left hand appears to be the right hand of image. This phenomenon is called lateral inversion.
Spherical Mirror:
A mirror in which reflective surface is part of a sphere is called a spherical mirror. Such mirrors can be assumed as part of a hollow sphere. When the reflective surface is inside the sphere, it is called a concave mirror. When the reflective surface is outside the sphere, it is called a convex mirror.
- Focus: When a light beam consisting of parallel rays is reflected from a concave mirror, the reflected rays converge on a point in front of mirror. This point is called the focus of the concave mirror. In case of a convex mirror, such rays of light appear to diverge from a point (behind the mirror). This point is called the focus of the convex mirror. This is denoted by F.
- Pole of Mirror: The centre of reflective surface is called the pole of the mirror. This is denoted by P. ‘
- Centre of curvature: A spherical mirror is a part of a sphere. The centre of that sphere is called the centre of curvature of the mirror. This is denoted by C.
- Radius of Curvature: The distance between centre of curvature and any point on mirror is called the radius of curvature. This is denoted by R.
- Principal Axis: The straight line joining the pole and centre of curvature is called the principal axis of mirror.
- Focal Length: The distance between pole and focus is called focal length of mirror. This is denoted by f.
Cartesian Sign Conventions:
In this system, the pole of the mirrror is considered as the origin and principal axis is taken as X-axis.
Image formation by concave mirror | |||
Object position | Image position | Nature of image | Size of image |
At infinity | At F | Real and inverted | Highly diminished |
Between infinity and C | Between F and C | Real and inverted | Smaller |
At C | At C | Real and inverted | Same size as object |
Between C and F | Beyond C | Real and inverted | Enlarged |
At F | At infinity | Real and inverted | Highly enlarged |
Between F and pole of mirror | Behind the mirror | Virtual and erect | Enlarged |
Object position | Image position | Nature of image | Size of image |
At infinity | At focus behind the mirror | Real and erect | Highly diminished and point sized |
Between infinity and pole | Between F and P behind the mirror | Real and erect | Smaller |
Uses of Concave Mirror:
- The dish antenna which receives TV signals and focus them to the receiver.
- Concave mirrors are also used in high resolution telescopes to take images of celestial bodies.
- Concave mirror is used by barbers as shaving mirror.
- Concave mirror is used in reflector of headlight.
- Dentists and ENT specialists use concave mirror to direct a beam of light inside the mouth and ear to have clear visibility.
Uses of Convex Mirror:
- Convex mirror is used in rear view mirror and side mirror in vehicles.
- Convex mirrors are also installed near ATM machines, in shops and on hairpin bends so that a wider area can be viewed.
Mirror Formula:
In a spherical mirror, relationship between object distance, image distance and focal length is given by following mirror formula:
\(\cfrac { 1 }{ \upsilon } +\cfrac { 1 }{ \mu } =\cfrac { 1 }{ f } \quad \)
Magnification:
- The ratio of image height to object height is called magnification. It is usually denoted by m
- The capacity of a mirror to enlarge an image is called its magnification.
- If h is object height and h’ is image height then magnification by spherical mirror is given by following equation.
Refraction :
When a ray of light travels from one medium to another medium then it changes direction at the junction of two media. This phenomenon is called refraction of light. If incident ray is perpendicular to the surface (separating the two media) then there would be no refraction and the ray of light will continue to travel without a change in its direction.
First Law of Refraction:
Incident ray, refracted ray and normal at the point of incidence, all lie in the same plane.
Second Law of Refraction:
Ratio of sine of angle of incidence to sine of angle of refraction is constant. This is also called Snell’s Law.
\(\cfrac { sin\quad i }{ sin\quad r } =constant\)
This constant is called refractive index of medium 2 in relation to medium 1.
\(\quad { \mu }_{ 21 }=\cfrac { sin\quad i }{ sin\quad r } \)
Absolute refractive index:
If light from vacuum is entering a medium then its refractive index in relation to vacuum is called absolute refractive index. Similarly, refractive index of a medium in relation to air is shown by ratio of speed of light in air to speed of light in given medium.
\(\quad { \mu }_{ 21 }=\cfrac { speed\quad of\quad light\quad in\quad air }{ speed\quad of\quad light\quad in\quad given\quad medium } \cfrac { { \upsilon }_{ 1 } }{ { \upsilon }_{ 2 } } \)
Convex Lens:
A convex lens is thin at edges and thick at centre. They usually converge the refracted rays to a point.
Concave Lens:
A concave lens is thin at centre and thick at edges. They diverge the refracted rays.
The refracted rays appear to meet at a point when they are extended backwards.
- Centre of Curvature: The curved surface of lens is the part of a sphere. Centre of this sphere is called the centre of curvature. If a lens has two curved surfaces then it has two centres of curvature.
- Radius of curvature: The radii of curved surfaces are called radii of curvature. First surface is the surface on which a ray of light is incident. Light is refracted through the second surface of lens.
- Principal axis: The straight line joining the centres of curvature is called principal axis.
- Optical Centre: There is a point on principal axis through which a ray of light passes without deviation. This point is called optical centre of lens. If both the radii of curvature are equal then optical centre is at the middle of the lens.
- Principal Focus: When rays of light are parallel to the principal axis, they converge at a point or appear to diverge from a point after refraction. This point is called principal focus. One principal focus is present on each side of the lens.
- Focal Length: The distance between optical centre and principal focus is called focal length.
- Focal plane: A plane which is perpendicular to principal axis and passes through focal point is called focal plane.
Laws of Refraction through spherical lens
The incident ray which is parallel to principal axis passes through principal focus (convex lens) after refraction. In case of concave lens, the refracted ray appears to come from principal focus after refraction.
- The incident ray which passes through principal focus goes parallel to principal axis after refraction. In case of concave lens, if incident ray of light appears to be moving towards principal focus then it goes parallel to principal axis after refraction.
- The incident ray of light which passes through optical centre experiences no change in direction even after refraction.
Lens Formula:
For a spherical lens, the relationship between object distance, image distance and focal length is given by following lens formula.
\(\cfrac { 1 }{ f } =\cfrac { 1 }{ { \upsilon } } -\cfrac { 1 }{ u } \quad \)
Magnification:
Power of a lens to magnify the image is called magnification. This is given by ratio of image height to object height.
\(m=\cfrac { height\quad of\quad image }{ height\quad of\quad object } =\cfrac { h\prime }{ h } =\cfrac { \upsilon }{ \mu } \quad \)
Power of Lens :
Power of lens is reciprocal to its focal length.
\(p=\cfrac { 1 }{ f } \)
- Unit of power is Dioptre and written as D. Power of convex lens is taken as positive while that of concave lens is taken as negative. In common language, this is called as number of spectacles. If two or more lenses are combined then their combine power can be given as follows:
P = P1 + P2 + P3+ … - Structure of Eye : The eye is spherical in shape with about 2.5 cm diameter. Following are the main parts of eye.
- Sclera: This is a white and opaque protective layer around the eye.
- Cornea: It is a raised and transparent portion of sclera in front of eye. Rays of light enter the eye through cornea.
- Iris: This is situated behind cornea. Iris is made of muscle fibres and there is a hole at the centre of iris. Most portion of iris is black in colour.
- Pupil: The hole in iris is called pupil. Contraction or relaxation in muscles of iris regulates the size of pupil. Pupil becomes smaller in bright light and becomes larger in dim light.
Eye Lens: There is a flexible and transparent lens behind iris. Lens is held in its position by muscles. - Aqueous Humour: There is a transparent liquid filled between lens and cornea. This is called aqueous humour.
- Choroid: There is a membranous structure below sclera called choroid. It provides oxygen and nutrition to retina.
- Retina: It is a transparent membrane below choroid. Rays of light pass through cornea and lens and then are focused on retina. Image is formed on retina. There are numerous photosensitive cells on retina. These cells become active when light falls on them, and they emit signals. These signals are sent to brain through optical nerve. The brain interprets these signals and thus we are able to see something.
- Vitreous Humour: A transparent liquid is filled between lens and retina. This is called vitreous humour.
Power of Accommodation:
We can clearly see a distant object as well as a nearby object. This capacity of eyes is called the power of accommodation 3 For a normal person, minimum distance for clear vision is 25 cm. The maximum distance for clear vision is infinity.
Myopia or Short Sightedness:
A person who is suffering from myopia can clearly see nearby objects but is unable to see distant objects. Increased curvature of lens is the main reason for this defect. In a myopic eye, image of distant object is formed before the retina. Thus, the far point in myopic eye is near the eye rather than at infinity. This defect is rectified by using concave lens of suitable focal length.
Hypermetropia or Long Sightedness:
A person who is suffering from hypermetropia can easily see distant objects but finds it difficult to see nearbv objects. In this case, the near point of eye becomes more than 25 cm. Image of nearby object is formed behind the retina. This happens because curvature of lens becomes smaller. Hypermetropia can be corrected bv using a convex lens of suitable focal length.
Presbyopia:
Flexibility of lens and muscles reduces with advancing age. This reduces the capacity of adjustment of lenses. Such a person suffers from long sightedness and is unable to clearly see nearby objects. Sometimes, the person is also unable to clearly see distant objects. This condition is called presbyopia. Such a person is given bifocal lens. The upper portion of such a spectacles is made of concave lens and lower portion has convex lens.
Astigmatism:
This condition develops because of irregular curvature of cornea. A person suffering from astigmatism is unable to differentiate between horizontal and vertical lines at same distance. This defect is corrected by using cylindrical lens.
Cataract:
Transparency and flexibility of lens reduces with advancing age. A stage comes when the lens starts reflecting the light and the person loses clear vision. This condition is called cataract. This is corrected by removing the affected lens by surgery.
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