CBSE Class 10 Science – Chapter 10: Light Reflection And Refraction – Detailed Notes & Explanation

Light: Light is a type of electromagnetic radiation that allows the human eye to see or makes objects visible. It is also defined as visible radiation to the human eye. Photons, which are tiny packets of energy, are found in light.

Reflection of light: The bouncing back of light in the same medium is called reflection of light.

Terms related to Reflection of Light

• Incident Ray: The light ray striking a reflecting surface is called the incident ray.
• Point of Incidence: The point at which the incidence ray strikes the reflecting surface is called the Point of Incidence.
• Reflected Ray: The light ray obtained after reflection from the surface in the same medium in which the incident ray is traveling is called Reflected ray.
• Normal: The perpendicular drawn to the surface at the point of incidence is called the Normal.
• Angle of Incidence: The angle which the incident ray makes with the normal at the point of incidence is called the angle of incidence.
• Angle of Reflection: The angle which the reflected ray makes with the normal at the point of incidence is called the angle of reflection .

Laws of Reflection

The laws of reflection states that,

(a) The incident ray, the reflected ray and the normal all lie in the same plane.

(b) The angle of incidence ∠i =∠r  Angle of reflection.

Types of Reflection:

There are two types of reflection

    (a) Regular Reflection

    (b) Irregular Reflection

(a) Regular Reflection: When all reflected rays are parallel to each other, the reflection is called regular reflection. Clear image is formed in case of regular reflection.

(b) Irregular Reflection: When reflected rays are not parallel to each other, the reflection is called irregular reflection.We call it as diffused reflection.

Reflection of Light by Plane Mirror

Following are the characteristics of an image formed by the plane mirror:

• Image is always virtual and erect.
• Size of the image is equal to the size of the object.
• Image is as far behind the mirror as the object is in front of it.
• Image is laterally inverted.

Lateral Inversion: When an object is placed in front of a plane mirror, then the right side of the object appears to become the left side of the image; and the left side of the object appears to become the right side of the image.This change of sides of an object and its mirror image is called as Lateral Inversion.

Spherical Mirror

There are two types of spherical mirrors,

1. Concave Mirror
2. Convex Mirror

(a) Concave Mirror: A concave mirror has a reflective surface that is curved inward.

(b) Convex Mirror: A convex mirror is a diverging mirror in which the reflective surface bulges towards the light source.

Reflection of Light by Concave Mirror

Some Important terms

1. Pole: The center of the reflecting surface of a spherical mirror is called pole.
2. Centre of curvature: The center of the sphere formed by the reflecting part of a spherical mirror is called the center of curvature. It is generally denoted by C.
3. Principal axis: This is normal to the mirror at its pole.The straight line joining the Pole of the mirror and the Centre of curvature of a spherical mirror is called the Principal Axis.
4. Radius of curvature: It is the radius of the sphere formed by the reflecting part of the sphere.It is represented as R.
5. Focus: The Focus of a Concave Mirror is a point on the Principal Axis at which the light rays incident parallel to the principal axis meet after reflection from the mirror.It is represented as F .
6. Focal length: The distance of focus from the Pole of the mirror is called the Focal Length of the mirror.It is represented by f.
7. Aperture: The part of the mirror which can be exposed to the incident light is called the aperture.

Relation between focal length and Radius of curvature R = 2f

Rules for Obtaining Images of Concave Mirror

1. A line joining the center of curvature to any point on the surface of the mirror is always normal to it. 

2. A Ray of light incident parallel to the Principal Axis , after Reflection  will either pass through focus.

3. A Ray of light passing through the Focus , gets reflected parallel to the principal axis of the concave mirror.

Formation of Images of Concave Mirrors

Position of the Object	Position of the Image	Size of the Image	Nature of the Image
At infinity	At focus, F	Highly diminished and pointed in size	Inverted and real
Beyond C	Between F and C	Diminished	Inverted and real
At C	At C	Same size	Inverted and real
Between C and F	Beyond C	Enlarged	Inverted and real
At F	At infinity	Highly enlarged	Inverted and real
Between F and P	Behind the mirror	Enlarged	Erect and virtual

Formation of Image of Convex Mirror

Position of the object	Position of the image	Size of the image	Nature of the image
At infinity	At focus, F, behind the mirror	Highly diminished and pointed in size	Virtual and erect
Between infinity and pole of the mirror	Between P and F, behind the mirror	Diminished	Virtual and erect

Uses of concave mirror: Shaving mirror,dentist mirror,torch mirror

Uses of convex mirror: Rear view mirror,shop security mirror

Mirror Formula

A formula which gives the Relationship between Image distance ( v ) , object distance ( u ) and focal Length ( f ) of a mirror , is known as the mirror formula.

Where,

v = distance of image from mirror

u = distance of object from mirror

f = focal length of mirror

Magnification produced by a mirror is defined as the ratio of the size of the image to the size of the object.

Magnification: Height of image / Height of object

where h’ is the height of image

h is the height of the object.

v = image distance

u = object distance

If magnification is POSITIVE, then  the image is UPRIGHT.

If magnification is NEGATIVE then  the image is INVERTED.

Refraction: Change in the direction of propagation of a ray of light, when it travels obliquely from one transparent medium to another, is called refraction of light.

Laws of Refraction

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 the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

Refractive Index

The refractive index of medium 2 w.r.t medium 1 is given by the ratio of the speed  of light in medium 1 and the speed of light in medium 2.

n₂₁ = speed of light in medium ₁ / speed of light in medium ₂

Refractive index of a medium depends upon

1. Nature of the material of the medium ,
2. Density of the medium,
3. Colour or wavelength of light,

Spherical Lens

A lens is an optical device bounded by one or two spherical surfaces that is used to bend light in a specific way,

There are two type of lens

1. Concave lens
2. Convex lens

Converging lens: A converging lens bends light so that the light rays come together at a point

Diverging lens: A diverging lens bends light so it spreads light apart instead of coming together.

Terms Related to Spherical Lens:

Pole

The centre of the spherical refracting surface of the lens is called the pole. It is the point where the principal axis meets the surface of the lens.

Optical Centre

The point on the principal axis at the centre of the lens is called the optical centre.

Centre of Curvature

A lens has two spherical surfaces; these two spherical surfaces form a part of a sphere. The centre of these spheres is known as the centre of curvature.

Principal Axis

The principal axis is an imaginary line passing through the centres of curvature and the pole.

Aperture

The area of the lens suitable for refraction is called aperture. The aperture of the lens is the effective diameter of its light-transmitting area.

Focus

Focus is the point onto which collimated light parallel to the axis is focused. 

Image formation by Convex Lens
Object location	Image location	Image nature	Image size
Infinity	At F2	Real and Inverted	Diminished, point sized
Beyond 2F1	Between 2F2 and F2	Real and Inverted	Diminished
Between 2F1 and F1	Beyond 2F2	Real and Inverted	Enlarged
At F1	At infinity	Real and Inverted	Infinitely large or highly enlarged
At 2F1	At 2F2	Real and Inverted	Same size
Between F1 and Pole	On the same side as the object	Virtual and Erect	Enlarged

Image formation by Concave Lens
Object Location	Image Location	Image Nature	Image Size
Infinity	At F1	Virtual and Erect	Highly diminished, point-sized
Beyond Infinity and Optical Centre	Between Focus (F1) and Optical center (O)	Virtual and Erect	Diminished

Uses of Spherical Lens

Applications such as visual aids: spectacles, binoculars, magnifying lens, telescopes.

Lens Formula

The Relationship between  object distance ( u ) , Image Distance ( v ) and the Focal Length ( f ) of a lens is called Lens Formula .

Magnification Formula

The magnification of a lens is defined as the ratio of the height of an image to the height of an object. It is also given in terms of image distance and object distance. It is equal to the ratio of image distance to that of object distance.

Where m= magnification

h’= height of the image

h = height of an object

v is the image distance

u is the object distance

Power of a Lens

Power of a lens is the reciprocal of its focal length i.e

(in metre). The SI unit of power of a lens is dioptre (D).