Reflection of Light Class 10 Complete Chapter Notes

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Before reading about the reflection of light, we have to read about light. Let’s start with an explanation of the Definition of light.

Light: Light is the source of energy that gives us a sensation of vision. The object has broadly been classified as luminous (light-emitting) and non-luminous (non-light-emitting). The luminous objects further based on the source and mode of emission of light have further been classified.

Electric discharge such as electric sparks in internal combustion engine lightning, fluorescent lamps, mercury-vapor lamps, sodium-vapor lamps, etc.

Properties of light       

• Light is a form of energy. It brings the sensation of sight.
• Light travels in straight lines.
• It is a form of electromagnetic radiation.
• Light travels with a speed of 3 x 10⁸ m/s in the vacuum in a straight-line path.
• Light also provides us means of communication (fiber optics).

Lightwave

Lightwave travels with a speed of 3 x 10⁸ m/s in free space. Its speed depends on the medium. For example, in water or glass, its speed is considerably less than the speed in air or space. It is transverse in nature and does not require any medium to propagate.

Ray and Beam

Light travels in a straight line –Rectilinear propagation. The straight line indicating the path of the light (arrow direction) is known as a ray. A bundle of rays originating from the same source of light in a particular direction is known as a beam of light.

1. Parallel beam: When the rays which constitute the bean are parallel to one –another, then it is called a parallel beam of

1. Convergent beam: When the rays actually meet or appear to meet at a point, then the bean containing such rays is known as convergent

1. Divergent beam: When the rays are actually diverging or appear to diverge from a point, then beans containing such rays is called a divergent bean, and rays are called a divergent beam.

What is the Reflection of light?

When light falls on a surface and gets back into the same medium, it is called the reflection of light. A highly polished surface or mirror reflects the light. Reflection takes place when light bounces off by an opaque object. If the surface is shiny and smooth, like glass, water, or polished metal, the light will reflect at the same angle as it hits the surface. This is called specular reflection.

Reflection of light diagram:

1. Incident ray: The ray of light that falls on the reflecting surface is known as the incident ray.

2. Reflected Ray: The ray of light which gets reflected back in the same medium from the reflecting surface is known as a reflected ray.

3. Normal: A-line segment perpendicular to the plane of the reflecting or reflecting surface at the point of incidence is called normal.

4. The angle of Incidence: The angle between the incident ray and the normal at the point of incidence is called the angle of incidence.

5. The angle of reflection: The angle between the reflected ray and the normal at the point of reflection is called the angle of reflection.

Explain all Types of Reflection

The reflection of light based on the nature of the reflecting surface can be regular or diffused.

Regular reflection: It takes place when the surface is smooth or plane. It is difficult for the person to see in the regularly reflected light because of the glare produced or the dazzling effect.

Diffuse reflection: Diffuse reflection takes place when light hits an object and reflects it in lots of different directions. This happens when the surface is slightly rough. Most of the things we see are because light from a source has been reflected by it.

Specular reflection: The angle at which light strikes a reflecting surface is called the angle of incidence, and the angle at which a light ray bounces off a reflecting surface is called the angle of reflection. The angle of incidence and reflection are measured by reflecting the normal. Normal is the perpendicular drawn on the reflecting surface at the point of incidence.

Laws of Reflection

These laws proposed by Euclid, state that

• The reflected ray, incident ray, and the normal lie in the same plane.
• The angle of incidence is always equal to the angle of reflection. For example, the angle of incidence is 50 degrees then the angle of reflection is always 50 degrees.

Plane mirror

A plane mirror always forms an erect, virtual, size-to-size image, at the same distance as the object is, laterally inverted, but at the back of the mirror.

Properties of Image formation in the plane mirror

The different features of the image formed by a plane mirror include:

• The image is formed behind the plane mirror.
• The image is virtual, i.e., cannot be taken on-screen.
• The image of a plane mirror is of the same size as the object.
• The image is upright, with the same altitude as the object.
• The image undergoes a lateral inversion in relation to the object, i.e. right side appears left and vice versa.
• The image is situated at the same distance in the mirror as the object in front of it.
• When the plane mirror is turned by an angle, the reflected ray will turn by an angle.
• The radius of the curvature of a plane mirror is infinity. Its focal length is, therefore, infinity.
• Its magnification is +1.
• When a person approaches a mirror with a velocity of v m/s, his image approaches the person with a velocity of 2v m/s.
• When a person stands with his right hand holding a pen, the image appears to show the left hand holding the pen.
• If a person standing in front of a mirror wishes to see his full-size image, the size of the mirror has to be a minimum of half of his length in height.
• The number of images formed when two plane mirrors are placed at an angle is

      Number of images = (360 degree/angle between them) – 1

What do you mean by image?

The point of convergence or the point from where the light appears to diverge after reflection or refraction is known as an image.

Types of Image

There are two types of images:

• Real image
• Virtual image

Real Image

A real image is formed by the light rays after reflection or refraction when they actually meet or intersect with each other or actually converge at a point.

• It can be obtained on the screen.
• It is always inverted i.e. upside down with respect to the object.
• The size of the real image depends on the position of the object, so, it can be diminished, or the same size as that of the object or enlarged.
• It is formed by both the convex lens and the concave mirror.

Virtual Image

A virtual image is formed by the light rays after reflection or refraction when they appear to diverge from a point or appear to meet when they are produced in the backward direction or appear intersection of the rays observed.

• A virtual image cannot be trapped on the screen.
• It is always erect i.e., upside up with respect to the object.
• The size of the virtual image depends on the nature of the mirror or lens.
• It is formed by both concave and a convex lens. It is also formed by concave, convex, and plane mirrors.

Spherical Mirror

A spherical mirror is part of a hollow sphere with one side having silver/mercury coated, further coated with paint to protect it from damage. According to the position of the silvered surface, the spherical mirror is of two types:

• Concave mirror
• Convex mirror

Concave mirror

The inside curve of a spoon is an example of a concave mirror. These mirrors are used in certain types of astronomical telescopes called reflecting telescopes. The mirrors condense lots of light from faint sources in space onto a much smaller viewing area and allow the viewer to see far-off objects and events in space that would be invisible to the naked eye.

Light rays travel towards the mirror in a straight line and are reflected inwards to meet at a point called the focal length.

Uses of Concave mirror

• Dentists use concave mirrors for the magnified view of a tooth.
• The concave shape is also useful for car headlights and satellite dishes.
• Concave mirrors are useful as make-up mirrors because they can make things seem larger.
• A concave mirror can be used as a shaving mirror.
• A concave mirror can be used in the searchlight and headlight of the automobile.
• The concave mirror can be used in a reflecting types astronomical telescope.

Convex mirror

Convex mirrors curve outwards, like the outside of a spoon or a balloon. Parallel rays of light hit the mirror and are reflected outwards. If imaginary lines are traced back, they appear to come from a focal point behind the mirror. Convex mirrors irrespective of the size or position of an object from the mirror always produced a diminished and virtual image between the pole of the mirror and its focus.

Uses of Convex Mirror

(i) Convex mirrors are useful for shop security as an anti-theft mirror and rear-view mirror on vehicles because they give a wider field of vision.

(ii) The convex mirror can be used as a device to check theft in shops.

(iii)  The convex mirror can be used to light a large area.

(iv) The convex mirror can be used as a rear-view mirror in automobiles because it gives a wider field of view as the mirror is curved outwards and produces an erect and diminished image of the traffic behind the driver of the vehicle.

The important term associated with mirror

Aperture: The width of the reflecting surface from which reflection takes place is called the aperture.

Pole: The central point of the reflecting surface spherical surface is called the pole (P). It lies on the surface of the mirror.

Center of curvature: The center of the hollow sphere of which, the spherical mirror is a part, is called the center of curvature(C).

The radius of Curvature: The separation between the pole and the center of curvature or the radius of the hollow sphere, of which the mirror is a part, is called the radius of curvature(R).

Principal axis: The straight line joining the pole and the center of curvature is called the principal axis.

Focus: The point F on the principal axis, where a beam of light parallel to the principal axis actually meets after reflection.

Focal Length: The length of separation between the pole and the focus is known as the focal length.

The relation between the Radius of curvature and focal length.

The radius of curvature is twice the focal length, R=2f.

Rules for obtaining Images formed by Concave mirror

• A ray of light that is parallel to the principal axis of a concave mirror passes through its focus after reflection from the mirror.
• A ray of light passing through the center of curvature of a concave mirror is reflected back along the same path.
• When a ray of light passes through the focus of a concave mirror becomes parallel to the principal axis after reflection.
• A ray of light which is incident at the pole of the concave mirror is reflected back making the same angle with the principal axis.

New Cartesian Sign Conventions

The set of rules, to use ‘+’ or ‘−‘ signs with the values while doing any problem in optics, is called sign convention. They are –

• The object is always placed to the left of the mirror so that the incident light moves left to
• All distances are to be measured from the pole of the mirror e., from the origin of the coordinate axis.
• The distances measured in the direction of the incident light will be taken as positive (along the positive X-axis) while those measured to the left of the origin (along the negative X-axis) will be taken as
• All measurements of heights above the principal axis (along the Y-axis) are to be taken as positive and below it (along the negative Y-axis) are taken as

Mirror Formula

A formula that gives the relationship between the focal length(f), object distance(u), and image distance(v) of a spherical mirror is known as the mirror formula. This formula is valid for concave and convex mirrors in all situations for various positions of this object.

       1/f = 1/v + 1/u

Linear magnification

It is defined as the ratio of the size of the image (h_I) to the size of the object (h_o). Magnification is denoted by m. Linear magnification formula:

     m = h_I/h_o = – v/u

Magnification (m) is negative for real and inverted images and positive for virtual and erect images. So, magnification is always positive for the convex mirror, while it depends on the position of the object in a concave mirror. If,

• m<1, the image is diminished.
• m>1, the image is enlarged.
• m=1, the image is of the same size as that of the object.

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