# Light - Grade 10 (SEE) Physics
## Complete Chapter Notes with All Exercises - NEB Curriculum 2080

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## 🎯 Chapter Overview
**Chapter 5: Light** is one of the most crucial chapters in Grade 10 Physics for SEE examination. This chapter covers fundamental concepts of reflection, refraction, mirrors, lenses, and their practical applications in daily life.

**Weightage:** 15-18 marks in SEE examination
**Time Required:** 30-35 periods

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## 📚 Learning Objectives
After studying this chapter, students will be able to:
- Understand properties and behavior of light
- Explain laws of reflection and refraction
- Analyze image formation in mirrors and lenses
- Apply mirror and lens formulas to solve problems
- Draw accurate ray diagrams for various optical situations

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## 💡 1. INTRODUCTION TO LIGHT

### 1.1 Definition
Light is a form of energy that enables us to see objects around us. It travels in straight lines and shows both wave and particle nature.

**Properties of Light:**
- Light travels in straight lines (rectilinear propagation)
- Speed of light in vacuum = 3 × 10⁸ m/s
- Light shows reflection, refraction, and dispersion
- Light carries energy and momentum

### 1.2 Sources of Light
**Natural Sources:** Sun, stars, fireflies, lightning
**Artificial Sources:** Electric bulb, candle, LED, laser

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## 🪞 2. REFLECTION OF LIGHT

### 2.1 Definition
The bouncing back of light when it strikes a surface is called reflection of light.

### 2.2 Laws of Reflection
1. **First Law:** The incident ray, reflected ray, and normal to the surface at the point of incidence all lie in the same plane.
2. **Second Law:** The angle of incidence is equal to the angle of reflection.
   **∠i = ∠r**

### 2.3 Types of Reflection
**Regular Reflection:** Occurs on smooth polished surfaces (plane mirror)
**Irregular Reflection:** Occurs on rough surfaces (diffused reflection)

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## 🔄 3. SPHERICAL MIRRORS

### 3.1 Definition
Spherical mirrors are mirrors whose reflecting surfaces are curved like a sphere.

### 3.2 Types of Spherical Mirrors

#### A. Concave Mirror (Converging Mirror)
- Reflecting surface curves inward
- Converges parallel rays to a point
- Real and virtual images both possible

#### B. Convex Mirror (Diverging Mirror)
- Reflecting surface curves outward  
- Diverges parallel rays
- Always forms virtual, erect, diminished images

### 3.3 Important Terms

**Centre of Curvature (C):** Centre of the sphere of which mirror is a part
**Radius of Curvature (R):** Radius of the sphere of which mirror is a part
**Pole (P):** Centre point of the mirror surface
**Principal Axis:** Imaginary line passing through pole and centre of curvature
**Principal Focus (F):** Point on principal axis where parallel rays converge or appear to diverge
**Focal Length (f):** Distance between pole and principal focus
**Aperture:** Effective diameter of the mirror

**Relationship:** f = R/2

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## 📐 4. RAY DIAGRAMS AND IMAGE FORMATION

### 4.1 Rules for Drawing Ray Diagrams

#### For Concave Mirror:
1. **Ray parallel to principal axis** → reflects through focus
2. **Ray through focus** → reflects parallel to principal axis
3. **Ray through centre of curvature** → reflects back along same path
4. **Ray hitting pole** → reflects symmetrically about principal axis

#### For Convex Mirror:
1. **Ray parallel to principal axis** → appears to come from focus after reflection
2. **Ray toward focus** → reflects parallel to principal axis
3. **Ray toward centre** → reflects back along same path

### 4.2 Image Formation in Concave Mirror

| Object Position | Image Position | Image Nature | Image Size |
|-----------------|----------------|--------------|------------|
| At infinity | At F | Real, inverted | Point sized |
| Beyond C | Between F and C | Real, inverted | Diminished |
| At C | At C | Real, inverted | Same size |
| Between C and F | Beyond C | Real, inverted | Enlarged |
| At F | At infinity | Real, inverted | Highly enlarged |
| Between F and P | Behind mirror | Virtual, erect | Enlarged |

### 4.3 Image Formation in Convex Mirror
- **Always:** Virtual, erect, diminished
- **Position:** Between pole and focus
- **Applications:** Rear-view mirrors, security mirrors

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## 📝 5. MIRROR FORMULA AND MAGNIFICATION

### 5.1 Mirror Formula
```
1/f = 1/v + 1/u

Where:
f = focal length
v = image distance
u = object distance
```

### 5.2 Sign Convention (New Cartesian)
- All distances measured from pole of mirror
- Distances measured in direction of incident light are positive
- Distances measured opposite to incident light are negative
- Heights measured upward are positive, downward are negative

**For Concave Mirror:** f = negative
**For Convex Mirror:** f = positive

### 5.3 Magnification
```
m = -v/u = h'/h

Where:
m = magnification
v = image distance
u = object distance  
h' = image height
h = object height
```

**Magnification Types:**
- **m > 1:** Enlarged image
- **m < 1:** Diminished image
- **m = 1:** Same size image
- **m = positive:** Virtual, erect image
- **m = negative:** Real, inverted image

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## 🌈 6. REFRACTION OF LIGHT

### 6.1 Definition
The bending of light when it passes from one medium to another is called refraction.

### 6.2 Laws of Refraction (Snell's Laws)
1. **First Law:** Incident ray, refracted ray, and normal lie in same plane
2. **Second Law:** For any two given media, the ratio of sine of angle of incidence to sine of angle of refraction is constant
   ```
   sin i/sin r = μ = constant
   ```

### 6.3 Refractive Index
```
μ = c/v = sin i/sin r

Where:
μ = refractive index
c = speed of light in vacuum
v = speed of light in medium
i = angle of incidence
r = angle of refraction
```

**Absolute Refractive Index:** Refractive index with respect to vacuum
**Relative Refractive Index:** ₁μ₂ = μ₂/μ₁

### 6.4 Refraction through Glass Slab
- Light bends toward normal when entering denser medium
- Light bends away from normal when entering rarer medium
- Emergent ray is parallel to incident ray but laterally displaced

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## 🔍 7. LENSES

### 7.1 Definition
A transparent medium bounded by two spherical surfaces is called a lens.

### 7.2 Types of Lenses

#### A. Convex Lens (Converging Lens)
- Thicker at middle, thinner at edges
- Converges parallel rays
- Can form real and virtual images

#### B. Concave Lens (Diverging Lens)
- Thinner at middle, thicker at edges
- Diverges parallel rays
- Always forms virtual images

### 7.3 Image Formation by Convex Lens

| Object Position | Image Position | Image Nature | Image Size |
|-----------------|----------------|--------------|------------|
| At infinity | At F | Real, inverted | Point sized |
| Beyond 2F | Between F and 2F | Real, inverted | Diminished |
| At 2F | At 2F | Real, inverted | Same size |
| Between 2F and F | Beyond 2F | Real, inverted | Enlarged |
| At F | At infinity | Real, inverted | Highly enlarged |
| Between F and O | Same side | Virtual, erect | Enlarged |

### 7.4 Image Formation by Concave Lens
- **Always:** Virtual, erect, diminished
- **Position:** Between object and lens

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## 📊 8. LENS FORMULA AND POWER

### 8.1 Lens Formula
```
1/f = 1/v - 1/u

Where:
f = focal length
v = image distance
u = object distance
```

### 8.2 Sign Convention for Lenses
- Object distance (u) is always negative
- For real images: v is positive
- For virtual images: v is negative
- For convex lens: f is positive
- For concave lens: f is negative

### 8.3 Power of Lens
```
P = 1/f (in meters)

SI Unit: Dioptre (D)
1 D = 1 m⁻¹
```

**Power Applications:**
- Convex lens has positive power
- Concave lens has negative power
- Higher power = shorter focal length

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## 📋 9. COMPLETE EXERCISE SOLUTIONS

### **2. Answer the following questions:**

**a. What is light? Write any two properties of light.**

**Answer:** Light is a form of energy that enables us to see objects around us.

**Properties:**
1. Light travels in straight lines
2. Light shows reflection and refraction

**b. State the laws of reflection.**

**Answer:** 
1. The incident ray, reflected ray, and normal all lie in the same plane
2. The angle of incidence is equal to the angle of reflection (∠i = ∠r)

**c. What is the difference between regular and irregular reflection?**

**Answer:**
- **Regular Reflection:** Occurs on smooth surfaces, all reflected rays are parallel
- **Irregular Reflection:** Occurs on rough surfaces, reflected rays scatter in different directions

**d. Define the following terms related to spherical mirrors:**
**i. Centre of curvature:** Centre of the sphere of which mirror is a part
**ii. Radius of curvature:** Radius of the sphere of which mirror is a part
**iii. Principal focus:** Point where parallel rays converge or appear to diverge
**iv. Focal length:** Distance between pole and principal focus

**e. What is the relationship between focal length and radius of curvature?**

**Answer:** f = R/2 (Focal length is half of radius of curvature)

**f. Where should an object be placed in front of a concave mirror to get a real image of the same size?**

**Answer:** At the centre of curvature (C)

**g. Where should an object be placed to get a virtual and enlarged image in a concave mirror?**

**Answer:** Between focus (F) and pole (P)

**h. Write two uses each of concave and convex mirrors.**

**Answer:**
**Concave mirrors:**
1. Used in headlights of vehicles
2. Used by doctors in examination

**Convex mirrors:**
1. Used as rear-view mirrors in vehicles
2. Used as security mirrors in shops

**i. State the laws of refraction.**

**Answer:**
1. Incident ray, refracted ray, and normal lie in same plane
2. sin i/sin r = constant (Snell's law)

**j. Define refractive index. Write its formula.**

**Answer:** Refractive index is the ratio of speed of light in vacuum to speed of light in the medium.
**Formula:** μ = c/v = sin i/sin r

**k. Light travels faster in which medium - air or water? Why?**

**Answer:** Light travels faster in air because air is rarer medium (lower refractive index) than water.

**l. What happens to the path of light when it travels from:**
**i. Rarer to denser medium:** Bends toward normal
**ii. Denser to rarer medium:** Bends away from normal

**m. A ray of light is incident normally on a glass surface. What happens to it?**

**Answer:** It passes through without deviation because angle of incidence is 0°.

**n. What is a lens? Mention the types of lenses.**

**Answer:** A lens is a transparent medium bounded by two spherical surfaces.
**Types:**
1. Convex lens (converging)
2. Concave lens (diverging)

**o. Where should an object be placed to get a real, inverted and same size image from a convex lens?**

**Answer:** At 2F (twice the focal length)

**p. Define power of a lens. Write its SI unit.**

**Answer:** Power of a lens is the reciprocal of its focal length in meters.
**SI Unit:** Dioptre (D)
**Formula:** P = 1/f

**q. A convex lens has positive power while concave lens has negative power. Why?**

**Answer:** Because convex lens has positive focal length (converging) while concave lens has negative focal length (diverging).

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### **3. Distinguish between:**

**a. Real and Virtual Image**

| Real Image | Virtual Image |
|------------|---------------|
| Can be obtained on screen | Cannot be obtained on screen |
| Formed by actual intersection of rays | Formed by apparent intersection of rays |
| Always inverted | Can be erect or inverted |
| Example: Image on cinema screen | Example: Image in plane mirror |

**b. Concave and Convex Mirror**

| Concave Mirror | Convex Mirror |
|----------------|---------------|
| Reflecting surface curves inward | Reflecting surface curves outward |
| Converges parallel rays | Diverges parallel rays |
| Forms real and virtual images | Forms only virtual images |
| Focal length is negative | Focal length is positive |

**c. Concave and Convex Lens**

| Concave Lens | Convex Lens |
|--------------|-------------|
| Thinner at center, thicker at edges | Thicker at center, thinner at edges |
| Diverges parallel rays | Converges parallel rays |
| Always forms virtual images | Forms real and virtual images |
| Has negative power | Has positive power |

**d. Real and Apparent Depth**

| Real Depth | Apparent Depth |
|------------|----------------|
| Actual depth of object in medium | Depth as seen by observer |
| Does not change | Changes due to refraction |
| Example: Actual depth of pool | Example: Pool appears shallow |

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### **4. Give reasons:**

**a. A concave mirror is used in headlights of cars.**

**Answer:** Because concave mirror produces powerful parallel beam of light when bulb is placed at focus.

**b. Convex mirrors are used as rear-view mirrors in vehicles.**

**Answer:** Because convex mirrors give wider field of view and always form erect, virtual images.

**c. A ray passing through center of curvature retraces its path.**

**Answer:** Because it hits the mirror normally (perpendicular), so angle of incidence is 0°, hence angle of reflection is also 0°.

**d. We can see our image in water.**

**Answer:** Because water surface acts like a plane mirror due to total internal reflection.

**e. A coin at the bottom of a glass appears to be raised when water is filled.**

**Answer:** Due to refraction of light from water to air, the light rays bend away from normal making coin appear closer.

**f. A thick glass slab appears thin when viewed from top.**

**Answer:** Due to refraction, light bends when coming from glass to air, making the slab appear thinner.

**g. Stars appear to twinkle.**

**Answer:** Due to atmospheric refraction, starlight continuously changes its path, making stars appear to twinkle.

**h. The sun is visible before actual sunrise and after actual sunset.**

**Answer:** Due to atmospheric refraction, sunlight bends and reaches us even when sun is below horizon.

**i. A lemon kept in water appears bigger than its actual size.**

**Answer:** Water acts as a magnifying glass due to refraction, making objects appear larger.

**j. Diamond sparkles brilliantly.**

**Answer:** Due to high refractive index and total internal reflection, light undergoes multiple reflections inside diamond.

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### **5. Numerical Problems:**

#### **Problem Type 1: Mirror Formula**

**Q1:** An object is placed 20 cm in front of a concave mirror of focal length 15 cm. Find the position and nature of image.

**Given:**
- u = -20 cm (object distance)
- f = -15 cm (concave mirror)
- v = ?

**Solution:**
Using mirror formula: 1/f = 1/v + 1/u
1/(-15) = 1/v + 1/(-20)
1/v = -1/15 + 1/20 = (-4 + 3)/60 = -1/60
v = -60 cm

**Magnification:** m = -v/u = -(-60)/(-20) = -3

**Answer:** Image is at 60 cm in front of mirror, real, inverted, and 3 times enlarged.

**Q2:** A convex mirror has focal length 20 cm. Where should an object be placed to get an image at 12 cm behind the mirror?

**Given:**
- f = +20 cm (convex mirror)
- v = +12 cm (behind mirror)
- u = ?

**Solution:**
1/f = 1/v + 1/u
1/20 = 1/12 + 1/u
1/u = 1/20 - 1/12 = (3-5)/60 = -2/60 = -1/30
u = -30 cm

**Answer:** Object should be placed 30 cm in front of the mirror.

#### **Problem Type 2: Lens Formula**

**Q3:** A convex lens of focal length 25 cm forms an image at 50 cm. Find object distance and magnification.

**Given:**
- f = +25 cm (convex lens)
- v = +50 cm (real image)
- u = ?

**Solution:**
1/f = 1/v - 1/u
1/25 = 1/50 - 1/u
1/u = 1/50 - 1/25 = (1-2)/50 = -1/50
u = -50 cm

**Magnification:** m = v/u = 50/(-50) = -1

**Answer:** Object distance = 50 cm, magnification = -1 (same size, inverted)

**Q4:** A concave lens of focal length 20 cm forms an image 12 cm from lens. Find object distance.

**Given:**
- f = -20 cm (concave lens)
- v = -12 cm (virtual image)
- u = ?

**Solution:**
1/f = 1/v - 1/u
1/(-20) = 1/(-12) - 1/u
1/u = -1/12 - (-1/20) = -1/12 + 1/20 = (-5+3)/60 = -2/60 = -1/30
u = -30 cm

**Answer:** Object distance = 30 cm

#### **Problem Type 3: Power of Lens**

**Q5:** Find the power of a convex lens of focal length 50 cm.

**Given:**
- f = 50 cm = 0.5 m

**Solution:**
P = 1/f = 1/0.5 = 2 D

**Answer:** Power = +2 D

**Q6:** A concave lens has power -4 D. Find its focal length.

**Given:**
- P = -4 D

**Solution:**
P = 1/f
f = 1/P = 1/(-4) = -0.25 m = -25 cm

**Answer:** Focal length = 25 cm

#### **Problem Type 4: Refractive Index**

**Q7:** Light enters from air into glass. If angle of incidence is 45° and angle of refraction is 30°, find refractive index of glass.

**Given:**
- i = 45°, r = 30°
- sin 45° = 0.707, sin 30° = 0.5

**Solution:**
μ = sin i/sin r = sin 45°/sin 30° = 0.707/0.5 = 1.414

**Answer:** Refractive index of glass = 1.414

**Q8:** Speed of light in water is 2.25 × 10⁸ m/s. Find refractive index of water.

**Given:**
- c = 3 × 10⁸ m/s
- v = 2.25 × 10⁸ m/s

**Solution:**
μ = c/v = (3 × 10⁸)/(2.25 × 10⁸) = 1.33

**Answer:** Refractive index of water = 1.33

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## 📐 6. DIAGRAMMATIC QUESTIONS

### **Question 1: Ray Diagram for Concave Mirror**
**Draw ray diagram when object is placed between F and C of concave mirror.**

**Answer:** 
- Object placed between F and C
- Image formed beyond C
- Image is real, inverted, and enlarged
- Draw two rays: one parallel to axis, another through focus

### **Question 2: Ray Diagram for Convex Lens**
**Draw ray diagram when object is placed at 2F of convex lens.**

**Answer:**
- Object at 2F
- Image formed at 2F on other side
- Image is real, inverted, same size
- Draw rays through optical center and parallel to axis

### **Question 3: Refraction through Glass Slab**
**Show the path of light ray through rectangular glass slab.**

**Answer:**
- Ray bends toward normal when entering glass
- Ray travels straight inside glass
- Ray bends away from normal when leaving glass
- Emergent ray is parallel to incident ray but displaced

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## 📈 7. IMPORTANT FORMULAS SUMMARY

### **Mirror Formulas:**
- Mirror formula: 1/f = 1/v + 1/u
- Magnification: m = -v/u = h'/h
- Relation: f = R/2

### **Lens Formulas:**
- Lens formula: 1/f = 1/v - 1/u
- Power: P = 1/f (in meters)
- Magnification: m = v/u = h'/h

### **Refraction Formulas:**
- Snell's law: sin i/sin r = μ
- Refractive index: μ = c/v
- Relative refractive index: ₁μ₂ = μ₂/μ₁

### **Sign Conventions:**
**Mirrors:** All distances from pole, real images have positive v
**Lenses:** Object distance always negative, real images have positive v

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## 🎯 8. EXAM PREPARATION TIPS

### **Theory Questions (2-3 marks)**
- Learn definitions with proper units
- Understand laws of reflection and refraction
- Practice distinguishing between concepts
- Remember applications of mirrors and lenses

### **Numerical Problems (3-4 marks)**
- Memorize all formulas with correct signs
- Apply sign conventions carefully
- Show all calculation steps clearly
- Check units in final answer

### **Ray Diagrams (4-5 marks)**
- Use ruler and pencil for neat diagrams
- Show at least 2 rays for image formation
- Mark all points and distances clearly
- Write image characteristics

### **Give Reason Questions (2-3 marks)**
- Connect to daily life examples
- Use scientific principles to explain
- Mention relevant optical phenomena

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## 🏆 9. PREVIOUS YEAR SEE QUESTIONS PATTERN

### **Theory Questions:**
- Laws of reflection and refraction
- Types and uses of mirrors/lenses
- Properties of images
- Definitions of optical terms

### **Numerical Questions:**
- Mirror and lens formula problems
- Power of lens calculations
- Refractive index problems
- Magnification calculations

### **Diagram Questions:**
- Ray diagrams for mirrors and lenses
- Refraction through glass slab
- Path of light in different media

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## ✅ 10. CHAPTER CHECKLIST

**Before SEE Exam, ensure you can:**
- [ ] State laws of reflection and refraction
- [ ] Draw ray diagrams for all cases
- [ ] Apply mirror and lens formulas correctly
- [ ] Calculate power and refractive index
- [ ] Distinguish between optical concepts
- [ ] Explain daily life optical phenomena
- [ ] Solve all types of numerical problems
- [ ] Remember uses of mirrors and lenses

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## 🎊 CONCLUSION

Light is one of the highest-scoring chapters if concepts and formulas are mastered properly. Focus on understanding ray diagrams, applying correct sign conventions, and connecting theory to practical applications. Practice numerical problems regularly and draw neat diagrams for maximum marks.

**Success Formula:** 
Clear Concepts + Formula Practice + Diagram Skills + Application Knowledge = Perfect SEE Score!

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*This complete note covers all exercises from the Light chapter of NEB Science textbook for SEE examination preparation. Study systematically and practice regularly for excellent results.*

**📞 Need Help?** Practice more ray diagrams and numerical problems for complete mastery of this important chapter.

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