⚡ CLASS 10 CBSE PHYSICS — LIGHT (CHANNEL READY MASTER NOTES)

Reflection • Refraction • Mirrors • Lenses • Full Theory + Tricks + PYQs

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🌟 1. LIGHT — BASIC CONCEPT

Light is a form of energy that enables vision. It travels in straight lines in a uniform medium.

📌 KEY PROPERTIES

✔ Rectilinear propagation (straight path)

✔ Reflection ✔ Refraction ✔ Formation of images using mirrors & lenses

🪞 2. REFLECTION OF LIGHT

Reflection = bouncing back of light from a smooth surface.

📌 TYPES

✔ Regular reflection → smooth surface (mirror)

✔ Diffused reflection → rough surface

📌 LAWS OF REFLECTION (VERY IMPORTANT)

1) i = r ✔ Incident ray,

2) reflected ray & normal lie in same plane

🧠 TRICK:

“Same Plane, Same Angle”

🪞 3. SPHERICAL MIRRORS

A mirror which is a part of a hollow sphere.

🌙 CONCAVE MIRROR (CONVERGING MIRROR)

✔ Reflecting surface inward

✔ Converges light rays to focus

✔ Can form real & virtual images

📍 IMAGE FORMATION (MUST LEARN)

✔ Infinity → at F (point image)

✔ Beyond C → between C & F (real, inverted)

✔ At C → at C (same size)

✔ Between C & F → beyond C (enlarged)

✔ Between F & P → virtual, erect, magnified

📌 USES

✔ Torch, headlights

✔ Shaving mirror

✔ Solar concentrator

☀️ CONVEX MIRROR (DIVERGING MIRROR)

✔ Reflecting surface outward

✔ Always forms virtual, erect, diminished image

✔ Wide field of view

📌 USES

✔ Rear view mirrors ✔ Security mirrors

📐 4. IMPORTANT TERMS (DEFINITIONS)

✔ Pole (P): centre of mirror

✔ Focus (F): point where rays meet/appear to meet

✔ Centre of curvature (C): centre of sphere

✔ Radius (R): distance PC

✔ Principal axis: line through P and C

📊 5. MIRROR FORMULA (HIGH SCORING)

*Mirror Formula & Magnification - Complete Notes*

*1. Mirror Formula*

1/f=1/v+1/u

*Where:*

- *f* = Focal length of mirror

- *v* = Image distance from pole

- *u* = Object distance from pole

*Main Formula:*

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

*Formula Variations:*

- *To Find Image Distance (v):* $$\frac{1}{v} = \frac{1}{f} - \frac{1}{u}$$ or $$v = \frac{uf}{u - f}$$

- *To Find Object Distance (u):* $$\frac{1}{u} = \frac{1}{f} - \frac{1}{v}$$ or $$u = \frac{vf}{v - f}$$

- *To Find Focal Length (f):* $$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$ or $$f = \frac{uv}{u + v}$$

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*2. Magnification Formula*

*Main Formula:*

m = -\frac{v}{u} = \frac{h_i}{h_o}

Where $h_i$ = Image height, $h_o$ = Object height

*Other Forms:*

- *Image Distance Using Magnification:* \$\$v = -mu\$\$

- *Object Distance Using Magnification:* $$u = -\frac{v}{m}$$

*Meaning of 'm':*

- *m = +ve* → Image is virtual & erect

- *m = -ve* → Image is real & inverted

- *|m| > 1* → Magnified image

- *|m| < 1* → Diminished image

- *|m| = 1* → Same size image

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*3. New Cartesian Sign Convention*

1. *Pole* is origin, *Principal axis* is x-axis

2. *Object* is always placed to *left* of mirror

3. *Distances against incident light* = *Negative*

4. *Distances along incident light* = *Positive*

5. *Object distance (u)* = Always *Negative*

6. *Real image (v)* = *Negative* - in front of mirror

7. *Virtual image (v)* = *Positive* - behind mirror

8. *Focal length*: Concave mirror = *Negative*, Convex mirror = *Positive*

9. *Height*: Upward = *Positive*, Downward = *Negative

🌊 6. REFRACTION OF LIGHT

Refraction = bending of light due to change in speed when medium changes.

📌 WHY IT HAPPENS?

✔ Speed changes in different media ✔ Light bends towards/away from normal

📌 LAWS

✔ i, r and normal lie in same plane

✔ Snell’s law:


\frac{sin i}{sin r} = constant

🧠 TRICK:

Denser → bends towards normal Rarer → bends away

🔍 7. LENSES

Transparent material with at least one curved surface.

🔥 CONVEX LENS (CONVERGING)

✔ Thick in centre

✔ Forms real & virtual images

📍 IMAGE FORMATION

✔ Beyond 2F → between F & 2F (real, inverted)

✔ At 2F → same size

✔ Between F & 2F → beyond 2F (enlarged)

✔ At F → infinity

✔ Between F & O → virtual, erect, magnified

📌 USES

✔ Camera

✔ Microscope

✔ Magnifier

❄️ CONCAVE LENS (DIVERGING)

✔ Thin in centre

✔ Always virtual, erect, diminished image

📌 USES

✔ Door peephole

✔ Optical devices

📊 8. LENS FORMULA

1/f=1/v-1/u

Where:

� = focal length of lens

� = image distance

� = object distance

📌 MAGNIFICATION


m =-v/u

⚡ 9. POWER OF LENS


P = 1/f

✔ Unit = Dioptre (D)

✔ Convex → +

✔ Concave → –

📌 10. SIGN CONVENTION (SUPER IMPORTANT)

🌟 COMMON RULE

✔ Object always left side → u negative

✔ Above axis → positive

✔ Below axis → negative

🪞 MIRROR

✔ Concave mirror: f (–), R (–)

✔ Convex mirror: f (+), R (+)

✔ Real image → v (–)

✔ Virtual image → v (+)

🔍 LENS

✔ Convex lens: f (+)

✔ Concave lens: f (–)

✔ Real image → v (+)

✔ Virtual image → v (–)

🎨 11. RAY DIAGRAM TRICK

✔ Ray parallel → passes through F

✔ Ray through F → becomes parallel

✔ Ray through C → reflects back

🧠 TRICK: “FPC RULE”

🔥 12. MOST IMPORTANT QUESTIONS

✔ Mirror formula numericals

✔ Lens formula numericals

✔ Power of lens

✔ Ray diagrams

✔ Refraction reasoning

⚡ 13. QUICK REVISION BOX (ULTIMATE)

✔ i = r

✔ 1/f = 1/v + 1/u

✔ 1/f = 1/v - 1/u

✔ P = 1/f

✔ Convex mirror → always virtual

✔ Concave lens → always virtual

✔ Denser → bends towards normal

🚀 14. EXAM STRATEGY (TOP MARKS)

✔ Write formula first

✔ Apply sign convention

✔ Draw neat diagram

✔ Mention unit

✔ Box final answer

🌟 FINAL LINE

✨ Concepts + diagrams + practice = Full marks guaranteed ✨

🧠⚡ CLASS 10 CBSE — LIGHT CHAPTER MOCK TEST (70 MARKS)

🪞 FULL QUESTION + STEPWISE SOLUTION PAPER

📘 SECTION A — MCQs (1 × 20 = 20)

📌 Q: According to the law of reflection, the angle of incidence is equal to the angle of reflection. What is the correct relation?

A) i > r

B) i < r

C) i = r

D) i + r = 90°

👉 Ans: C) i = r

📌 Q: A convex mirror always forms which type of image?

👉 Ans: Virtual, erect and diminished image

📌 Q: Unit of power of a lens is:

👉 Ans: Dioptre (D)

📌 Q: Light bends when it goes from one medium to another due to:

👉 Ans: Refraction

📌 Q: A concave lens always forms:

👉 Ans: Virtual, erect, diminished image

📌 Q: The mirror formula is:

👉 Ans: 1/f = 1/v + 1/u

📌 Q: A convex lens has focal length:

👉 Ans: Positive

📌 Q: Speed of light is maximum in:

👉 Ans: Vacuum/air

📌 Q: Image formed by plane mirror is:

👉 Ans: Virtual and laterally inverted

10.

📌 Q: A concave mirror focus is:

👉 Ans: Real focus

11–20 (Answers only)

Lens formula → 1/f = 1/v – 1/u

Convex mirror focus → virtual

Refraction occurs due to → change in speed

Magnification (mirror) → m = –v/u

Convex lens is → converging lens

Concave lens is → diverging lens

Object at infinity → image at → focus

Denser medium → light bends → towards normal

Rare medium → light bends → away from normal

Power formula → P = 1/f

📘 SECTION B — VERY SHORT ANSWERS (2 × 7 = 14)

21.

📌 Q: Define reflection of light.

👉 Ans: Reflection is bouncing back of light into the same medium after striking a surface.

22.

📌 Q: State laws of reflection.

✏️ Solution:

✔ Angle of incidence = angle of reflection

✔ Incident ray, reflected ray and normal lie in same plane

👉 Ans: i = r and all rays lie in same plane

23.

📌 Q: Why convex mirror is used in vehicles?

👉 Ans: It gives wide field of view and always forms small upright image.

24.

📌 Q: Define refraction of light.

👉 Ans: Bending of light when it passes from one medium to another due to change in speed.

25.

📌 Q: Write mirror formula.

👉 Ans: 1/f = 1/v + 1/u

26.

📌 Q: Difference between real and virtual image.

👉 Ans:

Real → formed on screen, inverted

Virtual → cannot be formed on screen, erect

27.

📌 Q: Define power of lens.

👉 Ans: Power is reciprocal of focal length in meters.

📘 SECTION C — SHORT NUMERICALS (3 × 7 = 21)

28.

📌 Q: A concave mirror has focal length 10 cm. An object is placed at 20 cm. Find image position.

✏️ Solution:

Given: u = –20 cm, f = –10 cm

👉 Ans: v = –20 cm

29.

📌 Q: A convex lens has focal length 15 cm and object distance 30 cm. Find image distance.

✏️ Solution:

u = –30 cm, f = +15 cm

LCM = 30:

👉 Ans: v = +30 cm

30.

📌 Q: Find power of lens having focal length 50 cm.

✏️ Solution:

f = 0.5 m

P = 1/f = 1/0.5 = 2

👉 Ans: P = +2 D

31.

📌 Q: Why does light bend when it enters glass from air?

👉 Ans: Because speed of light decreases in glass.

32.

📌 Q: Object at infinity in concave mirror—image position?

👉 Ans: At focus (F)

33.

📌 Q: Write magnification formula for mirror.

👉 Ans: m = –v/u

34.

📌 Q: Convex lens image when object is between F and 2F.

👉 Ans: Real, inverted, enlarged image beyond 2F

📘 SECTION D — LONG ANSWERS (5 × 3 = 15)

35.

📌 Q: Explain image formation by concave mirror.

✏️ Solution:

✔ At infinity → image at F

✔ Beyond C → between C and F

✔ At C → same size

✔ Between F and C → enlarged image

✔ Between F and P → virtual image

👉 Ans: Case-wise image formation explained above

36.

📌 Q: Explain refraction of light.

✏️ Solution:

✔ Light changes speed when medium changes

✔ Hence direction changes

✔ Bends towards normal (denser) or away (rarer)

👉 Ans: Refraction due to change in speed of light

37.

📌 Q: Explain image formation by convex lens.

✏️ Solution:

✔ Beyond 2F → between F and 2F

✔ At 2F → same size

✔ Between F and 2F → enlarged

✔ At F → infinity

✔ Between O and F → virtual image

👉 Ans: Case-wise explained above

🎯 FINAL EXAM STRATEGY

✔ Always write “Given”

✔ Write formula first

✔ Apply sign convention carefully

✔ Show steps clearly

✔ Box final answer