Exploring the Wonder of Light: A Comprehensive Guide for CBSE Class 10 Science Students at Future Classes.
Light-reflection and refraction are important topics covered in CBSE Class 10 Science notes. Reflection is the bouncing back of light rays when they hit a surface, while refraction is the bending of light when it passes through one material to another. Both processes are important for understanding the behaviour of light, and are extensively covered in CBSE Class 10 Science notes.
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LIGHT (REFLECTION AND REFRACTION)
Light is the form of energy that enables us to see.
Properties of Light
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Electromagnetic wave so does not require any medium to travel.
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Light tends to travel in straight line.
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Light has dual nature i.e., wave as well as particle
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Light casts shadow.
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Speed of light is maximum in vacuum. Its value is 3 × 10 m/s.
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When light falls on a surface, following may happen
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Reflection
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Refraction
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Absorption
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Reflection
Bouncing back of light when it strikes on a polished surface like mirror.
Laws of Reflection
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Angle of incidence is equal to the angle of reflection.
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The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.
Lateral inversion
Lateral inversion is the real or apparent reversal of left and right. For example, the letter b when laterally inverted becomes the letter d (more or less). It is well-known that a plane mirror causes the apparent lateral inversion of objects.
Spherical Mirrors
Mirrors whose reflecting surface is curved.
Convex mirror
A mirror whose reflecting surface is curved outwards is called convex mirror. It is also known as a diverging mirror.
Concave mirror
A mirror whose reflecting surface is inwards is called concave mirror. It is also known as a converging mirror.
Some definition related to spherical mirror
Principal axis
The line joining the pole and center of curvature.
Pole (P)
The center of the spherical mirror.
Aperture (MN)
It is the effective diameter of the spherical mirror.
Centre of Curvature (C)
The center of the hollow glass sphere of which the mirror was a part.
Radius of Curvature (R)
The distance between the pole and the center of curvature.
Focus (F)
The point on principal axis where all the parallel light rays actually meet or appear to meet after reflection.
Focal length (f)
The distance between the pole and the focus.
Relationship between focal length and radius of curvature
Real image
The image formed when rays of light meet at a point after reflection/refraction is called real image.
Virtual image
The image formed when rays of light appear to meet at a point after reflection/refraction is called virtual image.
Ray diagrams for images formed by concave mirror
When object is at infinity
When the object is at infinity then its image is formed at F and its nature is real and inverted and the size of the image is pointing size or highly diminished.
When object is beyond C
When the object is beyond C then its image is formed Between ‘F’ and ‘C’ and its nature is real and inverted and the size of the image is diminished.
When object is at ‘C’
When the object is at C then its image is formed at ‘C’ and its nature is real and inverted and the size of the image is Same size as that of object.
When object is placed between ‘F’ and ‘C’
When the object is between F and C then its image is formed beyond C and its nature is real and inverted and the size of the image is Enlarged.
When object is placed at ‘F’
When the object is at F then its image is formed at behind the mirror and its nature is virtual, erect and the size of the image is Enlarged.
When object is between P and F
When the object is at F then its image is formed at behind the mirror and its nature is virtual, erect and the size of the image is Enlarged.
Image formation by a concave mirror for different positions of the object
When object is between P and F
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Used in torches, search lights and vehicles headlights to get powerful parallel beam of light.
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Concave mirrors are used by dentists to see large image of teeth of patients. (Teeth have to be placed between pole and focus).
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Large concave mirrors are used to concentrate sunlight to produce heat in solar furnace.
Ray diagrams of images formed by convex mirror
When object is placed at infinity
When the object is at infinity then its image is formed at F and its nature is virtual, erect and the size of the image is point sized.
When object is placed between pole and infinity
When the object is between pole and infinity then its image is formed between P and F and its nature is virtual, erect and the size of the image is diminished.
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A full-length image of a tall building/tree can be seen in a small convex mirror.
Ray diagrams of images formed by convex mirror
Uses of Convex Mirror
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Convex mirrors are used as rear-view mirrors in vehicles because they always give an erect and diminished image.
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They have a wider field of view as they are curved outwards.
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Convex mirrors are used at blind turns and on points of merging traffic to facilitate vision of both side traffic.
Mirror Formula
Magnification of Spherical Mirrors
It is the ratio of the height of image to the height of object.
Magnification of Spherical Mirrors
Bending of light when it enters obliquely from one transparent medium to another.
Some examples of refraction
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The bottom of swimming pool appears higher.
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A pencil partially immersed in water appears to be bent at the interface of water and air.
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Lemons placed in a glass tumbler appear bigger.
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Letters of a book appear to be raised when seen through a glass slab.
Magnification of Spherical Mirrors
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The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
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Snell’s law: The ratio of sine of angle of incidence to the sine of angle of refraction is a constant.
This constant is called the refractive index of the second medium with respect to the first medium and is represented by letter n or µ.
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When light travels from a rarer medium to a denser one, it bends towards the normal (i > r) and when travels from a denser medium to a rarer one, it bends away from the normal (i < r).
Cause of refraction
Speed of light is different in different media, i.e., more in rarer medium and comparatively less in denser medium. So, when light enters a denser medium, its speed reduces and it bends towards the normal and when it enters a rarer medium, its speed increases and it bends away from the normal.
Relative Refractive index (n)
The ratio of speed of light in a given pair of media
Absolute Refractive Index
Refractive index of a medium with respect to vacuum or air.
Refractive index of diamond is the highest till date. It is 2.42.
It means speed of light is times less in diamond than in vacuum.
Lens
It is a transparent medium bound by two surfaces, at least one of which is curved. Lens are of two types
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Convex or convergent lens
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Concave or diverging lens
Convex or convergent lens
A lens which is thicker at the center and thinner at its end is called convex lens. Convex lenses are of three types (as shown)
A convex lens is also known as converging lens because it converges a parallel beam of light rays passing through it.
Concave or diverging lens
A lens which is thinner at the center and thicker at its ends is called concave lens. Concave lenses are of three types (as shown)
A concave lens is also known as diverging lens because it diverges a parallel beam of light rays passing through it.
Some definitions related to lens
Optical center
The center point of a Jens is known as its optical center. It is represented by O.
Centre of curvature
The centers of the two imaginary spheres of which the lens is a part are called centers of curvature of the lens. It is represented by C.
Radii of curvature
The radii of the two imaginary spheres of which the lens is a part are called radii of the curvature of lens.
Principal focus
Lens has two principal foci
First principal focus It is a point on the principal axis of lens, from where the directed rays become parallel to principal axis after refraction.
Second principal focus It is the point on the principal axis at which the rays coming parallel to the principal axis, converge on the other side of lens (convex) or appear to meet on the same side of lens (concave), after refraction from the lens.
Focal length
The distance between focus and optical center of lens is called focal length of lens.
Aperture
The effective diameter of the circular outline of a spherical lens is called its aperture.
Some definitions related to lens
When object is at infinity
When the object is at infinity then its image is formed at F2 and its nature is real and inverted and the size of the image is pointing size.
When object is beyond 2F1
When the object is beyond 2F1 then its image is formed Between F2 and 2F2 and its nature is real and inverted and the size of the image is diminished.
When object is at 2F1
When the object is at 2F1 then its image is formed at 2F2 and its nature is real and inverted and the size of the image is same size.
When object is between F1 and 2F1
When the object is between F1 and 2F1 then its image is formed Beyond 2F2 and its nature is real and inverted and the size of the image is Enlarged.
When object is at F1
When the object is at F1 then its image is formed At Infinity and its nature is real and inverted and the size of the image is Highly enlarged.
When object is between F1 and optical center
When the object is between ‘F1’ and optical center then its image is formed on the same side of the lens as object and its nature is Virtual and erect and the size of the image is enlarged.
Nature, position and relative size of the image formed by a convex lens
Ray Diagrams of Imaged formed by Concave Lens
When object is between F1 and optical center
When the object is at infinity and optical center then its image is formed on the same side of the lens as object and its nature is Virtual and erect and the size of the image is Point sized or highly diminished.
When object is placed between infinity and optical center
When the object is infinity and optical center and optical center then its image is formed Between ‘F’ and ‘O’ and its nature is Virtual and erect and the size of the image is diminished.
Nature, position and relative size of the image formed by a concave lens for various positions of the object
Lens Formula
Magnification
​The ratio of height of image to height of object is called linear magnification.
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If m is negative, image is real.
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If m is positive, image is virtual.
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If m = 1 then image is equal to object.
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If m > 1 then image is enlarged.
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If m < 1 then image is diminished.
Power of a lens
The ability of a lens to converge or diverge light ray is called power(P) of the lens. It is defined as the reciprocal of focal length in meter. SI unit of power diopter (D).
