Light - Reflection and Refraction – Class 10 Notes, Formulae, and Numericals

Light - Reflection and Refraction Part-1 – Class 10 Notes, Formulae, and Numericals

Chapter: Light – Reflection and Refraction (Part 1: Reflection of Light)

Class: 10 Science
Target: CBSE Board Exams | NTSE | Olympiads

Introduction to Reflection of Light

Reflection of light is a very important concept not only in Class 10 Board Exams but also in real life. Whenever light hits a shiny or polished surface and bounces back, this phenomenon is called reflection.

Important Keywords:

Reflection of Light
Laws of Reflection
Mirror Formula
Ray Diagrams
Class 10 Science Chapter 10
CBSE Class 10 Physics Numericals

Definition of Reflection of Light

When light rays strike a smooth surface like a mirror and bounce back in the same medium, the phenomenon is known as Reflection of Light.

Types of Reflection

Regular Reflection: From a smooth surface, like a plane mirror.
Diffuse Reflection: From a rough surface, scattering light in all directions.

Laws of Reflection

There are two fundamental laws:

The incident ray, the reflected ray, and the normal all lie in the same plane.
The angle of incidence (i) is always equal to the angle of reflection (r).
i = r

if we want to study reflection of light in detail we can use device called mirrors because it has highly reflecting surface.

there are two types of mirror,

1) Plane Mirror

2) Spherical Mirror

1) Plane Mirror:-

A plane mirror is a mirror which is flat, smooth, reflective surface, typically made of glass with a reflective coating, that forms images through reflection.

How image is formed by plane mirror:

When incident rays from an object strike a plane mirror, they reflect as per laws of reflection but our brain traces these reflected rays behind the mirror, and they will meet at a point where actually image is formed.

Plane Mirror Characteristics

Image is formed behind the mirror
Image is virtual, erect
same size.
Image distance from mirror = Object distance from the mirror.
Lateral inversion occurs

Multiple images formed by plane Mirrors:

When two plane mirrors are placed at an angle to each other, multiple images of an object placed between them can be formed due to multiple reflections. The number of images (n) depends on the angle (𝜃) between the mirrors.

The number of images formed can be calculated using the formula,

n = (360 o / θ) - 1

Spherical Mirrors: Concave and Convex

Concave Mirror (Converging)
Convex Mirror (Diverging)

Important Terms

Term

Definition

Pole (P)

Centre of mirror surface

Center of Curvature (C)

Centre of sphere from which mirror is cut

Principal Axis

Line joining pole and center of curvature

Focus (F)

Midway between P and C

Focal Length (f)

Distance between P and F

Mirror Formula

\frac{1}{f} = \frac{1}{v} + \frac{1}{u}

Where:

f = focal length
u = object distance
v = image distance

Sign Convention (New Cartesian):

All distances are measured from the pole (P).
Left side of mirror is taken as negative, right side as positive.

Magnification Formula

$m = \frac{h^{'}}{h} = \frac{- v}{u}$

Where:

h' = height of image
h = height of object

Ray Diagrams for Concave Mirror (Important Cases)

Object at infinity → Image at focus (real, inverted, highly diminished)
Object beyond C → Image between F and C (real, inverted, diminished)
Object at C → Image at C (real, inverted, same size)
Object between C and F → Image beyond C (real, inverted, enlarged)
Object at F → Image at infinity
Object between P and F → Image behind mirror (virtual, erect, enlarged)

Ray Diagrams for Convex Mirror

Always forms virtual, erect, and diminished image between F and P.

Q1. An object is placed 20 cm in front of a concave mirror. The image is formed 30 cm in front of the mirror. Find the focal length.

Solution:
Given:
u = –20 cm, v = –30 cm

\frac{1}{f} = \frac{1}{v} + \frac{1}{u} = \frac{1}{-30} + \frac{1}{-20} = \frac{-5 - 3}{60} = \frac{-8}{60} \Rightarrow f = -7.5\,cm

Q2. A convex mirror forms an image 10 cm behind the mirror when an object is placed 20 cm in front. Find focal length.

Solution:
u = –20 cm, v = +10 cm

$\frac{1}{f} = \frac{1}{v} + \frac{1}{u} = \frac{1}{10} + \frac{1}{-20} = \frac{2 - 1}{20} = \frac{1}{20} \Rightarrow f = 20\,cm$

Q3. A concave mirror produces a 10 cm long image of an object of height 2 cm. What is the magnification produced?

Answer:
Magnification $� = \frac{Height of Image}{Height of Object}$

� = \frac{10}{2} = 5

m = 10 / 2 = +5

(Positive sign indicates the image is virtual and erect

Q4. If the magnification of a body of size 1 m is 2, what is the size of the image?

Answer:
Magnification (m) = Size of Image / Size of Object

2 = Size of Image / 1

Size of Image = 2 x 1 = 2 m

Tips to Remember for Board Exam

Learn all ray diagrams by drawing repeatedly.
Always follow sign conventions carefully.
Practice Mirror Formula numericals daily.
Use magnification formula for height-based questions.

Class 10 Board Previous Year Questions (Reflection)

2024 (Expected / Sample Paper Based)

Q. What kind of mirror is used in shaving mirrors and why?
Answer: Concave mirror because it forms enlarged virtual image when object is close.

2023

Q.1 State mirror formula and define each term. (2 marks)

Answer:

Mirror Formula

Using Mirror Formula:

\frac{1}{f} = \frac{1}{v} + \frac{1}{u​}

Where:

f = focal length
u = object distance
v = image distance

Q.2 A concave mirror produces real image twice the size of object. If object is placed 15 cm from mirror, find image distance and focal length. (Numerical – 3 marks)

Answer:

Given:

Object distance, $u = -20 \, \text{cm}$
Image distance, $v = -30 \, \text{cm}$
(Note: both are in front of the mirror ⇒ negative as per sign convention)

Using Mirror Formula:

\frac{1}{f} = \frac{1}{v} + \frac{1}{u}

\frac{1}{f} = \frac{1}{-30} + \frac{1}{-20} = \frac{-1}{30} + \frac{-1}{20}

\frac{1}{f} = \frac{-2 - 3}{60} = \frac{-5}{60} \Rightarrow f = -12 \, \text{cm}

Final Answer:

Focal Length (f) = –12 cm

Negative sign indicates: Concave mirror

Read: Light-Reflection and Refraction Part-2

2022

Q. Draw ray diagram when object is placed between pole and focus of a concave mirror. (2 marks)
Answer: Virtual, erect and enlarged image behind mirror.

2021 (COVID year – internal assessment)

Q. A student places a candle 20 cm from a concave mirror. The image is formed 30 cm. Find focal length. (Numerical – 2 marks)

Answer:

Given:

Object distance, $u = - 20 cm$
Image distance, $v = - 30 cm$
(Note: both are in front of the mirror ⇒ negative as per sign convention)

Using Mirror Formula:

\frac{1}{f} = \frac{1}{v} + \frac{1}{u}

\frac{1}{f} = \frac{1}{-30} + \frac{1}{-20} = \frac{-1}{30} + \frac{-1}{20}

\frac{1}{f} = \frac{-2 - 3}{60} = \frac{-5}{60} \Rightarrow f = -12 \, \text{cm}

Final Answer:

Focal Length (f) = –12 cm

Negative sign indicates: Concave mirror

2020

Q.1 Define focal length and principal axis. (1 mark)

Answer:

The focal length of a spherical mirror is the distance between the pole (P) and the focus (F) of the mirror.
It is denoted by 'f'.

The principal axis is the straight line passing through the pole (P) and the center of curvature (C) of a spherical mirror.

It is the reference axis for drawing ray diagrams.

Q.2 Draw ray diagram for image formation when object is at centre of curvature of concave mirror. (2 marks)

FAQs on Reflection of Light – Class 10

Q.1: What is the difference between concave and convex mirror?
A: Concave converges light; convex diverges light. Concave forms real or virtual images; convex always virtual.

Q.2: How to identify mirror type from the image?
A: If image is virtual, erect, diminished → convex; if real and inverted → concave.

Q.3: What is lateral inversion?
A: The left and right sides appear reversed in a plane mirror.

Conclusion

Reflection of Light is one of the most scoring topics in Class 10 Physics. With basic understanding of formulas, ray diagrams, and laws, you can easily score full marks.

Next Part: Read: Light-Reflection and Refraction Part-2

Also Download Worksheet : Download: Light-Reflection and Refraction Part-1 Numericals PDF

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