LIGHT-REFLECTION AND REFRACTION Class 10 Notes

• Light is a form of energy, which enable us to see the object.
• In this chapter we will study the phenomena of reflection and refraction using the property of light i.e. straight-line propagation (Light wave travel from one point to another, along a straight line).

Reflection of Light

When the light is allowed to fall on highly polished surface, such mirror, most of the light gets reflected

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Laws of Reflection

1. The angle of incidence is always equal to angle of reflection. (i = r)

2. The incident ray reflected ray and the normal to the reflecting surface at the point of incidence lie in the same plane.

Image formed by Plane Mirror (Plane reflecting surface)

1. Virtual (imaginary) & Erect (Virtual: The image that do not form on screen.)
2. Laterally inverted (The left side of object appear on right side of image)
3. The size of image is equal to that of object
4. The image formed is as for behind the mirror as the object is in front of it.

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Reflection of light by spherical Mirrors

• Mirrors, whose reflecting surface are curved inward or outward spherically are called spherical mirror.
• For example – Spoon} The curved surface of shinning spoon can be considered as curved mirror.

If it is curved inward Act as a concave mirror

If it is curved outward Act as a convex mirror.

Few Basic terms related to Spherical Mirror

1. Principal axis: Line joining the pole and Centre of curvature of the spherical mirror.

2. Pole: The geometrical central point of the reflecting spherical surface. (aperture), denoted by (P).
3. Aperture: The width of reflecting spherical surface.
4. Centre of curvature: The reflecting surface of a spherical mirror form a part of sphere. It has a Centre, which is known as Centre of curvature, denoted by (C)
5. Radius of curvature: The separation between the pole and the Centre of curvature. i.e.. 
6. Focus (Focal point): The point on the principal axis, where all parallel rays meet after reflection, denoted by (F)
7. Focal length: The length between the pole and focus point i.e.
8. Relationship between focal length and Radius of curvature

F = R/2

Image formation by spherical Mirror

Before we learn the formation of image or ray diagram, let us go through few tips.

1. Remember, A say of light which is parallel to principle axis always pass through focus (meet at focus) or vice-versa

2.A ray of light which passes through centre of curvature (it is also known as normal at the point of incidence on spherical mirror) will retrace their path after reflection

3.A ray of light falling on pole get reflected at the same angle on the other side of principal axis

NOTE: A ray of light passes through centre of curvature reflecting spherical surface is always act as normal at the point of incidence. If we know the normal, we can draw angle of incidence and angle of reflection.

NOTE: The image will only form when two or more rays meets at a point.

Uses of Concave Mirror

1. Used in torches, search light and headlight of vehicle. 
2. Used to see large image of face as shaving mirror
3. Used by dentist to see large images of the teeth 
4. Large concave mirror used to focus sunlight (heat) in solar furnaces.

Uses of Convex Mirror

1. Used as rear-view mirror in vehicles because it gives erect image. It also helps the driver to view large area. 

Sign Convention for Reflection by Spherical Mirror

1. The object is always placed to the left side of mirror.
2. All distance should be measured from pole (P); parallel to principal axis.
3. Take ‘P’ as origin. Distances measured

Right of the origin (+x-Axis) are taken positive 

Left of the origin (-x-Axis) are taken negative 

Perpendicular to and above principal axis (+y-Axis) are taken positive 

Perpendicular to and below principal axis (-y-Axis) are taken negative

REFRACTION OF LIGHT

Refraction of Light: Happens in Transparent medium when a light travels from one medium to another, refraction takes place.

• A ray of light bends as it moves from one medium to another
• Refraction is due to change in the speed of light as it enters from one transparent medium to another.

Speed of light decreases as the beam of light travel from rarer medium to the denser medium

Some Commonly observed phenomenon due to Refraction

1. The stone at the bottom of water tub appear to be raised.
2. A fish kept in aquarium appear to be bigger than its actual size.
3. A pencil partially immersed in water appears to be displaced at the interface of air and water.

Refraction through a Rectangular Glass Slab

When a incident ray of light AO passes from a rarer medium (air) to a denser medium (glass) at point. O on interface PQ, it will bend towards the normal. At point B, on interface SR the light ray entered from denser medium (glass) to rarer medium (air) here the light ray will bend away from normal OB is a refracted ray emergent ray. If the incident ray is extended to D, we will observe BC is an that emergent ray parallel to incident ray. The ray will slightly be displaced laterally after refraction.

Note: When a ray of light is incident normally to the interface of two media it will go straight, without any deviation.

Laws of refraction of light-

1. 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.
2. The ratio of sine of angle of incidence to the sine of angle of refraction is a constant i.e..

(SIN i/SIN r)=CONSTANT

   for given color and pair of media, this law is also known as Snell’s Law

Constant n is the refractive index for a given pair of medium. It is the refractive index of the second  medium with respect to first medium

Where 2 is for second medium and 1 is for first medium

Refractive Index

The refractive index of glass with respect is air is given by ratio of speed of light in air to the speed of light in glass.

C Speed of light in vacuum = 3• 10⁸  m/s

Speed of light in air is marginally less, compared to that in vacuum.

Refractive index of air with respect to glass is given by 

The absolute refractive index of a medium is simply called refractive index

Refractive index of water (n_w) = 1.33

Refractive index of glass (n_g) = 1.52

SPHERICAL LENS

A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens.

CONVEX LENS

A lens may have two spherical surfaces, bulging outwards, is called double convex lens (or simply convex lens.

It is also known as converging lens because it converges the light.

CONCAVE LENS

A lens bounded by two spherical surfaces, curved inwards is known as double concave lens (or simply concave lens)

It is also known as diverging lens because it diverges the light.

Few Basic Terms related to spherical lens.

1. Centre of curvature – A lens, either a convex lens or a concave lens has two spherical surfaces. Each of these surfaces form a part of sphere. The centre of these two spheres are called centre of curvature represented by C₁ and C₂

2. Principal axis – Imaginary straight line passing through the two centres of curvature

3. Optical Centre – The central point of lens is its optical centre (0). A ray of light, when passes through ‘O’ it remains undeviated i.e. it goes straight.

Aperture – The effective diameter of the circular outline of a spherical lens.

1. Focus of lens – Beam of light parallel is principal axis, after refraction from

1) Convex lens, converge to the point on principal axis, denoted by F, known as Principal focus

2) Concave lens, appear to diverge from a point on the principal axis, known as principal focus.

                                        The distance OF₂ and  OF₂ is called as focal length

Tips for drawing Ray diagram

1. After refraction, a ray parallel to principal axis will pass through F.

Image formation by a convex lens for various position of object
Position of the Object : At InfinityPosition of the Image : At Focus F2Nature of the Image : Real & InvertedSize of the Image : Highly Diminished	Position of the Object : Beyond 2F1Position of the Image : Between F2 and 2F2Nature of the Image : Real & InvertedSize of the Image : Diminished
Position of the Object : At 2F1Position of the Image : At 2F2Nature of the Image : Real & InvertedSize of the Image : Same Size of Object	Position of the Object : Between F1 and 2F1Position of the Image : Beyond 2F2Nature of the Image : Real & InvertedSize of the Image : Enlarged
Position of the Object : At F1Position of the Image : At InfinityNature of the Image : Real & InvertedSize of the Image : Highly Enlarged	Position of the Object : Between F1 & OPosition of the Image : Beyond 2F1Nature of the Image : Virtual & ErectSize of the Image : Enlarged

Image formation by a Concave Lens for different position of the object.
Position of the Object : At InfinityPosition of the Image : At FocusNature of the Image : Virtual & ErectSize of the Image : Highly Diminished	Position of the Object : Between infinity & OPosition of the Image : Between F1 & ONature of the Image : Virtual & ErectSize of the Image : Diminished

Sign Convention for Reflection by Spherical Lens

1. The object is always placed to the left side of mirror.
2. All distance should be measured from Optical Center (O); parallel to principal axis.
3. Take ‘O’ as origin. Distances measured

Right of the origin (+x-Axis) are taken positive 

Left of the origin (-x-Axis) are taken negative 

Perpendicular to and above principal axis (+y-Axis) are taken positive 

Perpendicular to and below principal axis (-y-Axis) are taken negative

POWER OF LENS

The degree of convergence or divergence of light Ray achieved by a lens is known as power of a lens.It is define as the reciprocal of its focal length represented by P.

If any optical instrument has many lens, then net power will be

Light:

A form of electromagnetic radiation emitted by hot objects like lasers, bulbs, as well as the sun is called light.

It is a form of kinetic energy that allows the human eye to see or make the situation apparent or viewable.

Properties of light:

1. Reflection of light: Each light obeys the law of reflection, which either stipulates that the angle of incidence equivalent to the angle of reflection.
2. Refraction of light: It occurs whenever a light ray’s velocity fluctuates even though it travels from one medium to another one. The index of refraction is a measurement of how much light refracts.
3. Dispersion: Whenever white light has been sent through a prism, it divides into different shades dependent on its own wavelength.
4. Diffraction: Diffraction seems to be the phenomenon of bending light across tiny obstacle angles and therefore its penetration further into geometric extent refers.
5. Polarization of light: Ordinary light resonates perpendicular to the transmission of light throughout all orientations. When light is restricted to vibrating in just one plane, it is referred to be polarised light.
6. Interference of light: This would be the phenomenon of light intensity alteration caused by the rearrangement of light sources of energy of a combination of two or more visible light.