Light - Reflection and Refraction (Part - I) – Class 10 Notes, Formulae, and Numerical (Class 10 Science Notes PDF, CBSE Board)

Light - Reflection and Refraction (Part-1) – Class 10 Notes, Formulae, and Numerical

Class 10 Science Notes PDF, CBSE Board 

Target: CBSE Board Exams | State Board Exams | NTSE | Olympiads | Foundation 

Introduction:

                            प्रकाश – परावर्तन और अपवर्तन (भाग–I) अध्याय में हम प्रकाश के मूल सिद्धांतों का अध्ययन करते हैं, जैसे परावर्तन (Reflection), अपवर्तन (Refraction) और उनसे जुड़े नियम। इस भाग में समतल दर्पण, गोलाकार दर्पण (अवतल एवं उत्तल), छवि निर्माण, फोकस दूरी, वक्रता त्रिज्या तथा महत्वपूर्ण सूत्र शामिल हैं। विशेष रूप से Mirror Formula और आवर्धन (Magnification) के सूत्रों का प्रयोग करके संख्यात्मक प्रश्नों को हल करना सिखाया जाता है। दिए गए नोट्स सरल भाषा में अवधारणाओं को स्पष्ट करते हैं, जबकि सॉल्व्ड न्यूमेरिकल्स छात्रों को प्रश्नों को चरणबद्ध तरीके से हल करने में सहायता करते हैं। यह सामग्री CBSE बोर्ड के अनुसार तैयार की गई है और PDF फॉर्मेट में आसानी से डाउनलोड व उपयोग की जा सकती है।

in short - प्रकाश एक प्रकार की ऊर्जा है जो हमें वस्तुओं को देखने में सहायता करती है। जब प्रकाश किसी सतह पर पड़ता है, तो वह या तो परावर्तित (वापस लौटता है [Reflection]) होता है या अपवर्तित (अपना मार्ग बदलता है [Refraction]) होता है। इस अध्याय में हम इन दोनों घटनाओं का अध्ययन करेंगे।

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 (falls) a shiny or polished surface and bounces back, this phenomenon is called reflection.

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.
• in short - जब प्रकाश किसी सतह से टकराकर वापस लौटता है, तो इस प्रक्रिया को परावर्तन (Reflection) कहते हैं।

Types of Reflection:

1. Regular Reflection: From a smooth surface, like a plane mirror.

2. Diffuse Reflection: From a rough surface, scattering light in all directions.

Laws of Reflection:

There are two fundamental laws:

1. The incident ray, the reflected ray, and the normal all lie in the same plane.

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

         उदाहरण: दर्पण में अपनी छवि देखना

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

Mirrors:

• A mirror is a smooth and shiny surface that reflects light back, allowing us to see the image of ourselves or other objects.

In very easy words:

• A surface in which you can clearly see your face is called a mirror.

Examples:

• Bathroom mirror
• Car side mirror

Types of mirrors.

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 / 𝜃 ) - 1

2) Spherical Mirrors: 

• A spherical mirror is a mirror whose reflecting surface is part of a sphere (like a piece cut from a ball).

In very easy words,

• A mirror that looks like a curved piece of a ball is called a spherical mirror.

Types of Spherical Mirrors:

1. Concave Mirror (Converging-curved inward)

2. Convex Mirror (Diverging-curved outward       

3. 

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:

Where:

• f = Focal length
• u = Object distance
• v = Image distance

Sign Convention (New Cartesian): 

- Sign Convention एक नियम प्रणाली है, जिसके द्वारा हम दूरियों (Distances) को + (Positive) और – (Negative) चिन्ह देते हैं, ताकि संख्यात्मक (Numerical) प्रश्नों को हल करना आसान हो जाए।

Distance Rule:

• सभी दूरियाँ (Distances) ध्रुव (Pole) से मापी जाती हैं।
• Mirror ke left side ki distances → Negative (–)
• Mirror ke right side ki distances → Positive (+)

Height Rule:

• Principal axis ke upar → Positive (+)
• Principal axis ke neeche → Negative (–)

Easy समझने का तरीका:

• “Left side –ve, Right side +ve”
• “Above axis +ve, Below axis –ve”

Magnification Formula

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

Where:

• h' = height of image

• h = height of object

Ray Diagrams for Concave Mirror (Important Cases)

1. Object at infinity → Image at focus (real, inverted, highly diminished)

1. Object beyond C → Image between F and C (real, inverted, diminished)

2. Object at C → Image at C (real, inverted, same size)

3. Object between C and F → Image beyond C (real, inverted, enlarged)

4. Object at F → Image at infinity

5. 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 $�=\frac{\text{Height of Image}}{\text{Height of Object}}$

$$�=\frac{10}{2}=5$$

                       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:

$$\frac{1}{f}=\frac{1}{v}+\frac{1}{\mathrm{u<span></span>}}$$

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.

Read: Light-Reflection and Refraction Part-2

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, $u=-20\text{cm }$

• 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

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:

                            अंत में, Light – Reflection and Refraction (Part–I) अध्याय का अध्ययन छात्रों के लिए अत्यंत महत्वपूर्ण है, क्योंकि यह प्रकाश के व्यवहार और दर्पणों के कार्य सिद्धांत को समझने की मजबूत नींव प्रदान करता है। सूत्रों और सॉल्व्ड न्यूमेरिकल्स का नियमित अभ्यास करने से छात्रों की गणना क्षमता, सटीकता और आत्मविश्वास में वृद्धि होती है। स्पष्ट अवधारणाओं और निरंतर अभ्यास के साथ छात्र इस अध्याय में अच्छी पकड़ बना सकते हैं और CBSE बोर्ड परीक्षा में बेहतर प्रदर्शन कर सकते हैं।

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

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

Also Read: How to study 11th & 12th Physics

Also Read: class 11th Physics chapter - 1 Physical World

Also Read: Online Education : The way of Future Learning

Also Read: Motion class 9 Notes, Formulae, Numericals

                                                    About us | Contact us | Privacy Policy | Disclaimer