Light is one of the most important chapters in Class 10 Physics because it combines theory, ray diagrams, formulas, sign conventions, and numericals in one place. It is also one of the most frequently asked chapters in CBSE board exams, where students often see 3-mark or 5-mark questions based on mirrors, lenses, refraction, and image formation.
The best way to revise this chapter is not by reading long explanations again and again, but by focusing on the most important concepts: what light is, why we see objects, laws of reflection and refraction, image formation by mirrors and lenses, sign conventions, formulas, refractive index, and power of lens. Once these are clear, the chapter becomes much easier to handle in the exam.
At Deeksha Vedantu, we always encourage students to revise Light in a compact and visual way. This chapter is easiest when students remember the ray-diagram patterns and apply formulas calmly.
Why Light Is Important for CBSE Board Exams
Light is a high-value chapter because it appears regularly in board exams and tests both concept clarity and numerical accuracy.
Why Students Should Revise This Chapter Well
- it is a regular CBSE board chapter
- it includes 3-mark and 5-mark questions frequently
- it contains formulas and diagram-based questions
- it includes reflection, refraction, mirrors, and lenses
- it is important for theory as well as numericals
Understanding Light and Vision
Light is a form of energy that helps us see the world around us. The chapter becomes easier when students first understand what light is, how it behaves, and why objects become visible to us.
Basic Ideas at a Glance
| Concept | Quick explanation |
| Light | A form of energy that enables vision |
| Dual nature of light | Light can behave as a particle and as a wave |
| Why we see objects | Light reflects from objects and enters our eyes |
| Main phenomenon involved | Reflection of light |
Dual Nature of Light
Light shows dual nature. It can behave as:
- a particle
- a wave
This behaviour depends on the situation.
Why Do We See Objects
We see objects because light falls on them, gets reflected, and then enters our eyes. This is why reflection of light is the first major idea in the chapter.
Reflection of Light and Plane Mirrors
Reflection is the bouncing back of light from a surface. When light falls on a reflecting surface and returns into the same medium, the phenomenon is called reflection.
Laws of Reflection
These are the most basic and important laws in the chapter.
| Law | Statement |
| First law | The angle of incidence is equal to the angle of reflection |
| Second law | The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane |
Plane Mirror Image Summary
A plane mirror forms a simple image with fixed properties.
| Property | Description |
| Nature | Virtual |
| Orientation | Erect |
| Size | Same as the object |
| Position | As far behind the mirror as the object is in front |
| Special effect | Lateral inversion |
Spherical Mirrors
The chapter mainly studies two types of spherical mirrors: concave mirror and convex mirror.
Types of Spherical Mirrors
| Mirror type | Description | Nature |
| Concave mirror | Reflecting surface is curved inward | Converging mirror |
| Convex mirror | Reflecting surface is curved outward | Diverging mirror |
Important Terms for Mirrors
Students should revise these mirror terms clearly because they are used in ray diagrams and formulas.
| Term | Meaning |
| Pole | The center point of the reflecting surface |
| Principal axis | The straight line passing through the pole and center of curvature |
| Center of curvature | The center of the sphere of which the mirror is a part |
| Radius of curvature | The distance between the pole and the center of curvature |
| Principal focus | The point where parallel rays meet or appear to meet after reflection |
| Focal length | Distance between pole and principal focus |
Relation Between Focal Length and Radius of Curvature
This is one of the most important formulas for mirrors.
Formula
f = R/2
Ray-Diagram Rules for Mirrors
These rules are used to draw all mirror image-formation diagrams.
| Rule number | Main idea |
| Rule 1 | A ray parallel to the principal axis passes through the focus after reflection in a concave mirror and appears to come from the focus in a convex mirror |
| Rule 2 | A ray passing through the focus in a concave mirror becomes parallel to the principal axis after reflection; in a convex mirror, a ray directed towards the focus reflects parallel to the principal axis |
| Rule 3 | A ray passing through the center of curvature retraces its path after reflection |
| Rule 4 | A ray striking the pole reflects with equal angles on both sides of the principal axis |
Image Formation by Concave Mirror
Students must revise all six standard cases.
| Object position | Image position | Nature | Size |
| At infinity | At focus | Real, inverted | Highly diminished or point-sized |
| Beyond C | Between C and F | 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 the mirror | Virtual, erect | Enlarged |
Image Formation by Convex Mirror
A convex mirror has only two standard image cases and always forms a virtual, erect, and diminished image.
| Object position | Image position | Nature | Size |
| At infinity | At focus behind the mirror | Virtual, erect | Highly diminished |
| At any finite distance | Between P and F behind the mirror | Virtual, erect | Diminished |
Quick Summary of Mirror Images
| Mirror type | Main image pattern |
| Concave mirror | Can form real and inverted images, and in one special case a virtual and erect image |
| Convex mirror | Always forms a virtual, erect, and diminished image |
Sign Convention for Mirrors
This is extremely important for numericals.
| Quantity or direction | Sign rule |
| Distances measured in the direction of incident light | Positive |
| Distances measured opposite to the direction of incident light | Negative |
| Heights above the principal axis | Positive |
| Heights below the principal axis | Negative |
| Object distance u | Always negative |
| Focal length of concave mirror | Negative |
| Focal length of convex mirror | Positive |
| Real image distance v | Negative |
| Virtual image distance v | Positive |
Mirror Formula and Magnification
| Concept | Formula |
| Mirror formula | 1/f = 1/v + 1/u |
| Magnification for mirrors | m = -v/u = hᵢ/hₒ |
Where:
- m = magnification
- hᵢ = height of image
- hₒ = height of object
Refraction of Light and Refractive Index
Refraction is the bending of light when it passes from one medium to another. Refraction happens because the speed of light changes from one medium to another.
Refraction Rules at a Glance
| Transition | Behaviour of light |
| Rarer to denser medium | Light bends towards the normal |
| Denser to rarer medium | Light bends away from the normal |
Laws of Refraction
These are also called Snell’s laws.
| Law | Statement |
| First law | The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane |
| Second law | For a given pair of media, sin i / sin r = constant |
Refractive Index
Refractive index measures how much light bends or how much its speed changes in a medium.
Formula for Refractive Index
If light goes from medium 1 to medium 2, then:
n₂₁ = v₁/v₂
Where:
- n₂₁ = refractive index of medium 2 with respect to medium 1
- v₁ = speed of light in medium 1
- v₂ = speed of light in medium 2
Refraction Through a Glass Slab
When light enters a rectangular glass slab, it follows a standard pattern.
| Observation | Result |
| On entering the slab | Light bends towards the normal |
| On leaving the slab | Light bends away from the normal |
| Emergent ray | Becomes parallel to the incident ray |
| Overall effect | A sideways shift is produced |
Lateral Displacement
The sideways shift between the original path and the emergent path is called lateral displacement.
Lenses
The chapter studies two types of lenses: convex lens and concave lens.
Types of Lenses
| Lens type | Description | Nature |
| Convex lens | Thicker at the center and thinner at the edges | Converging lens |
| Concave lens | Thinner at the center and thicker at the edges | Diverging lens |
Important Terms for Lenses
| Term | Meaning |
| Optical center | The center point of a lens through which a ray passes without deviation |
| Principal focus | The point where rays converge or appear to diverge from after refraction |
| F₁ and F₂ | A lens has two principal foci, one on each side |
| Focal length | Distance between optical center and focus |
Ray-Diagram Rules for Lenses
| Rule number | Main idea |
| Rule 1 | A ray parallel to the principal axis passes through the focus after refraction in a convex lens and appears to come from the focus in a concave lens |
| Rule 2 | A ray passing through the focus of a convex lens emerges parallel to the principal axis; in a concave lens, a ray directed towards the focus emerges parallel |
| Rule 3 | A ray passing through the optical center goes straight without deviation |
Image Formation by Convex Lens
Students must revise all six standard cases here as well.
| Object position | Image position | Nature | Size |
| At infinity | At focus on the other side | Real, inverted | Highly diminished |
| Beyond 2F₁ | Between F₂ and 2F₂ | Real, inverted | Diminished |
| At 2F₁ | At 2F₂ | Real, inverted | Same size |
| Between F₁ and 2F₁ | Beyond 2F₂ | Real, inverted | Enlarged |
| At F₁ | At infinity | Real, inverted | Highly enlarged |
| Between F₁ and O | On the same side of lens | Virtual, erect | Enlarged |
Image Formation by Concave Lens
A concave lens has only two standard image cases and always forms a virtual, erect, and diminished image.
| Object position | Image position | Nature | Size |
| At infinity | At focus on the same side | Virtual, erect | Highly diminished |
| At any finite distance | Between optical center and focus | Virtual, erect | Diminished |
Quick Summary of Lens Images
| Lens type | Main image pattern |
| Convex lens | Can form real and inverted images, and in one special case a virtual and erect image |
| Concave lens | Always forms a virtual, erect, and diminished image |
Sign Convention for Lenses
This is different from mirror sign convention in image distance.
| Quantity or direction | Sign rule |
| Distances measured in the direction of incident light | Positive |
| Distances measured opposite to the direction of incident light | Negative |
| Heights above the principal axis | Positive |
| Heights below the principal axis | Negative |
| Object distance u | Always negative |
| Focal length of convex lens | Positive |
| Focal length of concave lens | Negative |
| Real image distance v | Positive |
| Virtual image distance v | Negative |
Lens Formula, Magnification, and Power
| Concept | Formula |
| Lens formula | 1/f = 1/v – 1/u |
| Magnification for lenses | m = v/u = hᵢ/hₒ |
| Power of a lens | P = 1/f |
SI Unit and Sign of Power
The SI unit of power is dioptre.
| Lens type | Sign of power |
| Convex lens | Positive |
| Concave lens | Negative |
Most Important Last-Minute Revision Points
This section is useful for final board revision.
Quick Revision Table
| Topic | What to remember |
| Reflection | Angle of incidence = angle of reflection; incident ray, reflected ray, and normal lie in the same plane |
| Mirrors | Concave mirror is converging, convex mirror is diverging, f = R/2, mirror formula: 1/f = 1/v + 1/u, magnification: m = -v/u |
| Refraction | Rarer to denser bends towards normal, denser to rarer bends away from normal, Snell’s law: sin i / sin r = constant |
| Lenses | Convex lens is converging, concave lens is diverging, lens formula: 1/f = 1/v – 1/u, magnification: m = v/u, power: P = 1/f |
Common Mistakes Students Make in Light
Common Mistakes Table
| Mistake | Why it causes problems |
| Mixing up mirror and lens formulas | Mirror formula has + 1/u, while lens formula has – 1/u |
| Forgetting sign conventions | This causes direct errors in numericals |
| Mixing up concave and convex cases | It changes image nature and diagram logic |
| Confusing real and virtual image signs | Mirror and lens image signs are not the same |
| Forgetting the nature of images | Students miss whether the image is real or virtual, erect or inverted, enlarged or diminished |
Best Strategy to Revise Light Before Boards
A quick but structured revision works best for this chapter.
Step-by-Step Revision Plan
| Step | What to do |
| Step 1 | Revise all ray-diagram cases in one go |
| Step 2 | Learn sign convention separately as an independent table |
| Step 3 | Revise the four main formulas: mirror formula, lens formula, magnification formula, and power formula |
| Step 4 | Solve a few numericals right after revision |
| Step 5 | Revise key differences such as concave mirror vs convex mirror, convex lens vs concave lens, and reflection vs refraction |
Quick Practice Questions
Important Practice Questions
- What is light?
- State the two laws of reflection.
- What is the difference between concave mirror and convex mirror?
- Write the mirror formula.
- Write the lens formula.
- What is refractive index?
- State Snell’s law.
- What is lateral displacement?
- What kind of image is always formed by a convex mirror?
- What kind of image is always formed by a concave lens?
FAQs
Q1. What is light in Class 10 Physics?
Light is a form of energy that enables us to see objects around us.
Q2. Why do we see objects?
We see objects because light gets reflected from them and enters our eyes.
Q3. What are the two laws of reflection?
The angle of incidence is equal to the angle of reflection, and the incident ray, reflected ray, and normal lie in the same plane.
Q4. What is the mirror formula?
The mirror formula is 1/f = 1/v + 1/u.
Q5. What is the lens formula?
The lens formula is 1/f = 1/v – 1/u.
Q6. What is magnification for mirrors?
Magnification for mirrors is m = -v/u.
Q7. What is magnification for lenses?
Magnification for lenses is m = v/u.
Q8. What is the formula for power of a lens?
The formula is P = 1/f.
Conclusion
Light is one of the most important chapters in Class 10 Physics because it combines reflection, refraction, mirrors, lenses, formulas, diagrams, and numericals in one complete chapter. Once the core ideas are revised in a compact way, the whole chapter becomes much easier to handle during board preparation.
The best way to revise this chapter before CBSE boards is to keep the formulas, ray-diagram cases, image summaries, and sign conventions together in one place. At Deeksha Vedantu, we always remind students that Light is a scoring chapter when revision is visual, structured, and repeated calmly.
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