Chapter – 9

Light – Reflection and Refraction

In this post we have given the detailed notes of class 10 Science Chapter 9 (Light – Reflection and Refraction) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Chapter 9: Light – Reflection and Refraction

How Do We See an Object? 👁️

An object reflects the light falling on it, and when this reflected light enters our eyes, it makes the object visible to us.

• Ray of light: When light travels from its source, it travels in a straight line. This straight line along which light travels from its source is called a ray of light.
• Shadow: When light falls on an opaque object, the object blocks the light, and a shadow of the opaque object is formed.
• Diffraction of light: If the obstacle in the path of light is extremely small, light shows a tendency to bend around its edges instead of travelling in a perfectly straight line. This effect is called diffraction of light.
• Reflection of light: When a ray of light strikes a shiny or reflecting surface, it bounces back into the same medium from which it came. This phenomenon is called reflection of light.

Reflection of light always occurs from opaque objects, while refraction of light occurs through transparent objects.

Laws of Reflection of Light

1. The angle of incidence is always equal to the angle of reflection.

∠i = ∠r

2. The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.

Note: These laws of reflection are applicable to all reflecting surfaces, including spherical mirrors.

Some Common and Interesting Phenomena

Reflection of light is responsible for several common and fascinating phenomena, such as the formation of images by mirrors, the twinkling of stars, the beautiful colours of a rainbow, and the bending of light by different media.

Types of reflection:

(i) Regular reflection: This type of reflection occurs from a smooth surface, and parallel incident rays remain parallel even after reflection.

(ii) Irregular (diffuse) reflection: This type of reflection occurs from a rough surface, and parallel incident rays are no longer parallel after reflection.

• Regular reflection
• Diffuse reflection

Mirrors 🪞

A mirror is a highly polished, shiny reflecting surface that forms an image of an object placed in front of it. Mirrors are of two types.

A) Plane Mirror

Its reflecting surface is flat.

Definition: A mirror whose reflecting surface is flat is called a plane mirror.

Uses of a plane mirror:

• Used at home for looking at one’s face.
• Used in salons and beauty parlours.

Nature of image formed by a plane mirror: The image formed is virtual and erect, and it forms behind the mirror at the same distance as the object is placed in front of the mirror.

B) Spherical Mirror

Its reflecting surface is curved. The reflecting surface of a spherical mirror can be curved inward or outward.

Definition: A mirror whose reflecting surface is spherical (part of a sphere) is called a spherical mirror.

Based on the direction of curvature, spherical mirrors are of two types.

Types of spherical mirrors:

• Concave mirror: Its reflecting surface is curved inward, i.e. towards the centre of the sphere.
• Convex mirror: Its reflecting surface is curved outward.

Parts of a Spherical Mirror

• Pole: The centre of the reflecting surface of a spherical mirror is called its pole. It is denoted by P.
• Centre of curvature: The reflecting surface of a spherical mirror forms part of a sphere; the centre of this sphere is called the centre of curvature of the mirror, denoted by the letter C.
• Radius of curvature: The distance between the pole and the centre of curvature of a spherical mirror is called the radius of curvature.
• Principal axis: A straight line passing through the pole and the centre of curvature of a spherical mirror is called the principal axis.
• Principal focus: There is another point F between the pole and the centre of curvature, called the principal focus. Rays parallel to the principal axis, after reflection, converge at (or appear to diverge from) this principal focus — converging in the case of a concave mirror, and appearing to diverge in the case of a convex mirror.
• Focal length: The distance between the pole and the principal focus of a mirror is called the focal length, denoted by the small letter f. This distance is half the radius of curvature.
• Aperture: The reflecting surface of a spherical mirror is largely spherical, bounded by a circular outline; the diameter of this circular boundary of the reflecting surface is called the aperture of the mirror.

Position, Nature, and Size of the Image

• Position of the object: The place where the object is kept.
• Position of the image: The place where the image is formed by the mirror.
• Size of the image: This tells us whether the image of the object is smaller than, equal to, or larger than the object itself.
• Nature of the image: This tells us whether the image formed by the mirror is virtual or real, and erect or inverted.

Nature of the Image is of Two Types

1. Real and inverted: This image always forms in front of the mirror and is inverted.

2. Virtual and erect: This image always forms behind the mirror and is erect.

Image Formed by a Concave Mirror

The nature of the image depends on the position of the object. Only when the object is placed between the pole (P) and the principal focus (F) is the image virtual and erect; for the object placed at any other position, a concave mirror always forms a real and inverted image.

• Object at infinity — image forms at focus F, real and inverted, and highly diminished (point-sized).
• Object beyond the centre of curvature C — image forms between F and C, real and inverted, and diminished.
• Object at the centre of curvature C — image forms at C, real and inverted, and of the same size as the object.
• Object between the centre of curvature C and the principal focus F — image forms beyond C, real and inverted, and magnified (enlarged).
• Object at the principal focus F — image forms at infinity, real and inverted, and highly magnified.
• Object between the pole P and the principal focus F — image forms behind the mirror, virtual and erect, and magnified (larger than the object).

Image Formed by a Convex Mirror

A convex mirror always forms a virtual, erect, and diminished image, located behind the mirror, regardless of the position of the object (except when the object is very close to the mirror, when the image size approaches the object’s size).

Uses of Concave Mirrors

• Concave mirrors are commonly used in torches, searchlights, and vehicle headlights to obtain a powerful parallel beam of light.
• They are often used as shaving mirrors to see a larger image of the face.
• Dentists use concave mirrors to see a magnified image of the teeth.
• Large concave mirrors are used to focus sunlight in solar furnaces.

Uses of Convex Mirrors

• Convex mirrors are commonly used as rear-view mirrors in vehicles.
• These mirrors are fitted on the sides of vehicles so that the driver can see vehicles behind them, enabling safe driving.
• They are also used in telescopes.
• Convex mirrors are also used as street light reflectors, as they can spread light over a large area.

Why Convex Mirrors are Preferred as Side Mirrors in Vehicles

Convex mirrors are preferred because they always form an erect image, even though it is smaller in size. They also have a much wider field of view, since they are curved outward. Hence, compared to a plane mirror, a convex mirror allows the driver to view a much larger area behind the vehicle.

Representation of Images Formed by Spherical Mirrors

• The position of the image of a point object can be found from the intersection of at least two reflected rays.
• Any two of a few standard rays can be considered for locating the image.

Sign convention for reflection by spherical mirrors (also called the New Cartesian Sign Convention):

According to this sign convention:

• The pole (P) of the mirror is taken as the origin, i.e. all distances are measured from the origin (P).
• The principal axis is taken as the x-axis (X-X′) of the coordinate system.
• The object is always placed to the left of the mirror, meaning light from the object falls on the mirror travelling from the left to the right.
• All distances measured to the right of the origin (along the +x-axis) are taken as positive, while all distances measured to the left of the origin (along the −x-axis) are taken as negative.

All distances in front of the mirror are taken as negative (−), and all distances behind the mirror are taken as positive (+).

For a Concave Mirror

All distances that lie in front of the mirror:

• Object distance (u) = −u [taken as negative]
• Focal length (f) = −f [taken as negative]
• Image distance (v) = −v [taken as negative, if the image is real and inverted]

For a Convex Mirror

Distances that lie both in front of and behind the mirror:

• Object distance (u) = −u [always negative, since the object is always kept in front of the mirror]
• Focal length (f) = +f [positive, because the centre of curvature of a convex mirror lies behind the mirror, so the focal length is also behind the mirror]
• Image distance (v) = +v [positive, since a convex mirror always forms a virtual, erect image behind the mirror; it would be negative only for a real, inverted image]

Object distance: The distance between the object placed in front of a spherical mirror and its pole is called the object distance (u).

Image distance: The distance between the pole of the mirror and the image formed is called the image distance (v).

Focal length (f): The distance between the pole of the mirror and the principal focus is called the focal length.

Mirror formula: The sum of the reciprocal of the image distance (v) and the reciprocal of the object distance (u) is equal to the reciprocal of the focal length (f):

1/v + 1/u = 1/f

Magnification: The extent to which the image of an object is enlarged or diminished compared to the object is called the magnification.

For magnification, the height of the object is always taken as positive, since the object is always placed above the principal axis and upright. The height of a virtual and erect image is taken as positive (+), while the height of a real and inverted image is taken as negative (−).

Value of magnification: A positive value of magnification indicates that the image is virtual and erect. A negative value of magnification indicates that the image is real and inverted.

Refraction of Light 💧

When a ray of light travels from one medium into another, it deviates (bends) from its original path. This bending of the ray of light from its path is called refraction of light. Refraction of light occurs only in transparent substances, such as glass, air, water, etc.

Cause of refraction of light: Refraction occurs due to a change in the speed of light as it passes from one transparent medium to another.

Laws of Refraction of Light

There are two laws of refraction of light:

• The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
• When a ray of light travels obliquely from one medium to another, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction is a constant.

Snell’s Law of Refraction: When a ray of light passes obliquely from one medium into another, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction is a constant. This law is also called Snell’s Law of Refraction.

Path of light during refraction: When a ray of light travels from a rarer medium to a denser medium, it bends towards the normal. When the same ray of light travels from a denser medium to a rarer medium, it bends away from the normal.

Denser medium: A medium with a higher refractive index is called a denser medium. The particles of this medium are more densely packed.

Rarer medium: A medium with a lower refractive index is called a rarer medium. The particles of this medium have a lower density.

• Whether a medium is denser or rarer is a comparative property between two media — it depends on which medium is denser or rarer relative to the other.

Phenomena Caused by the Refraction of Light

1. A pencil or spoon appearing bent in a glass of water: When we partially dip a pencil into a glass half-filled with water, it appears bent at the water surface. This happens because of refraction of light. The part of the pencil under water, which should appear straight, appears bent instead. Here, the same law of refraction applies — when a ray of light passes from a denser medium to a rarer medium, it bends away from the normal (in this everyday observation, it is the apparent bending of the object as viewed that results from this change in the light’s path).

2. A coin appearing raised in a glass of water:

Similarly, when we place a coin in a glass filled with water, the coin appears to be raised. This too happens because of refraction of light. Hence, it becomes clear that due to refraction of light, the coin appears to be raised slightly above its actual position. Another example is a lemon kept in a glass vessel of water appearing larger than its actual size.

• The apparent bending of an object (like a pencil) or of light differs for different liquids.
• When light travels obliquely from one medium into another, its direction of travel changes in the second medium.

Refractive index: When a ray of light travels from one medium to another, it deviates from its path. This deviation depends on the medium and the speed of light in that medium. Hence, the refractive index is the ratio of the speeds of light in the two media. “When a ray of light passes obliquely from one medium into another, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction is a constant.” This constant value is called the refractive index of the second medium with respect to the first medium.

Speed of light and refractive index: The speed of light in any medium depends on the refractive index of that medium.

Refraction by Spherical Lenses 🔍

Lens: A transparent medium bound by two surfaces, of which one or both surfaces are spherical, is called a lens.

Convex lens: A lens in which both outer spherical surfaces bulge outward is called a convex lens. This lens is also called a converging lens, because it converges the light rays passing through it.

Concave lens: A lens in which both outer spherical surfaces are curved inward is called a concave lens. This lens is also called a diverging lens, because it diverges the light rays passing through it.

Centre of curvature: Each surface of a spherical lens is part of a sphere; the centre of this sphere is called the centre of curvature of the lens, denoted by C₁ and C₂.

Principal axis: The imaginary straight line passing through both centres of curvature of a lens is called its principal axis.

Optical centre: The central point of a lens is called its optical centre, usually denoted by the letter O. A ray of light passing through the optical centre of a lens emerges without any deviation.

Aperture: The effective diameter of the circular outline of a spherical lens is called its aperture.

Thin lens: A lens whose aperture is much smaller than its radius of curvature is called a thin lens with a small aperture.

Principal focus of a convex lens: Many rays of light parallel to the principal axis fall on a convex lens. After refraction by the lens, these rays converge to a point on the principal axis. This point on the principal axis is called the principal focus of the lens.

Principal focus of a concave lens: Many rays of light parallel to the principal axis fall on a concave lens. After refraction by the lens, these rays appear to diverge from a point on the principal axis. This point on the principal axis is called the principal focus of the concave lens.

Focal length of a lens: The distance of the principal focus of a lens from its optical centre is called the focal length.

Sign Convention for Spherical Lenses

• All distances for spherical lenses are measured from the optical centre.
• The focal length of a convex lens is positive (+).
• The focal length of a concave lens is negative (−).
• The side from which light enters the lens is taken as negative, whether the lens is convex or concave — that is, the side on which the object is placed is negative.
• All real and inverted images formed by a lens are taken as positive, and all virtual and erect images are taken as negative.
• Real and inverted images form on the positive side of the lens, while virtual and erect images form on the negative side of the lens.
• The height of the object (h) is taken as positive (+), since it is erect. If the image height (h′) is erect, the image is virtual and erect (positive); if the image height (h′) is inverted, the image is real and inverted (negative).

Power of a Lens

The degree to which a lens can converge or diverge rays of light is called the power of the lens. It is equal to the reciprocal of the focal length of the lens. It is represented by P, and its SI unit is the dioptre (D).

1 dioptre (D) is equal to a focal length of 1 m or 100 cm.

If the focal length (f) is expressed in metres, then the power is expressed in dioptres.

The power of a convex lens is positive (+).

The power of a concave lens is negative (−).

Example: Suppose a lens has a power of +2 D. This means it is a convex lens, and its focal length (f) is +0.50 m, i.e. +50 cm.

And if another lens has a power of −2 D, it is a concave lens, and its focal length (f) is −0.50 m, i.e. −50 cm.

Key Points to Remember

• We see an object when light reflected by it enters our eyes; the laws of reflection state ∠i = ∠r, with the incident ray, normal, and reflected ray in the same plane. 👁️
• Mirrors are of two types — plane mirrors (flat surface, virtual/erect/same-size image) and spherical mirrors (concave and convex). 🪞
• A concave mirror can form real or virtual images depending on the object’s position, while a convex mirror always forms a virtual, erect, and diminished image.
• Convex mirrors are used as vehicle side mirrors because they give a wider field of view, even though the image is smaller.
• The mirror formula is 1/v + 1/u = 1/f, and magnification m = h′/h = −v/u.
• Refraction is the bending of light as it passes from one transparent medium to another, caused by a change in the speed of light. 💧
• Light bends towards the normal when going from a rarer to a denser medium, and away from the normal when going from denser to rarer.
• Refractive index is the ratio of the sine of the angle of incidence to the sine of the angle of refraction (Snell’s Law).
• A convex (converging) lens has a positive focal length and power; a concave (diverging) lens has a negative focal length and power.
• Power of a lens P = 1/f (in metres), measured in dioptres (D); it indicates the degree of convergence or divergence produced by the lens.

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