Light – Reflection and Refraction notes

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CLASS 10 • SCIENCE • CHAPTER 10

Light – Reflection and Refraction

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📖 Chapter Roadmap

1. Light and Reflection 2. Basic Terms of Reflection 3. Laws of Reflection 4. Plane Mirror 5. Spherical Mirrors 6. Concave Mirror 7. Convex Mirror 8. Mirror Formula and Magnification 9. Ray Diagrams and Image Formation 10. Uses of Spherical Mirrors 11. Refraction of Light 12. Laws of Refraction 13. Refractive Index 14. Refraction Through a Glass Slab 15. Refraction by Spherical Lenses 16. Convex and Concave Lenses 17. Lens Formula and Magnification 18. Power of a Lens 19. Important Board Questions 20. Final 96%-Target Revision 21. Practice Corner

High-score rule: Learn the laws, sign convention, ray diagrams, mirror formula, magnification, lens formula, refractive index and power of lens. In numericals, always write formula → substitution → calculation → unit.

1. Light and Reflection

Light is a form of energy that enables us to see objects.

Reflection of Light

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

Examples

• We see our face in a mirror because light is reflected from the mirror.
• Objects around us are visible because light reflected from them reaches our eyes.
• A polished metal surface gives regular reflection.

Memory: “Reflection = light comes back after hitting a surface.”

2. Basic Terms of Reflection

Term	Simple Meaning
Incident ray	The ray of light falling on the reflecting surface.
Reflected ray	The ray that bounces back from the surface.
Point of incidence	The point where the incident ray strikes the surface.
Normal	A line drawn perpendicular to the reflecting surface at the point of incidence.
Angle of incidence	Angle between incident ray and normal.
Angle of reflection	Angle between reflected ray and normal.

\(\angle i = \text{angle of incidence}\)

\(\angle r = \text{angle of reflection}\)

Board point: Angles of incidence and reflection are measured from the normal, not from the mirror surface.

3. Laws of Reflection

First Law

The incident ray, reflected ray and normal at the point of incidence all lie in the same plane.

Second Law

The angle of incidence is equal to the angle of reflection.

\[\boxed{i=r}\]

Regular and Diffuse Reflection

Regular Reflection	Diffuse Reflection
Occurs on a smooth surface.	Occurs on an irregular or rough surface.
Reflected rays remain orderly.	Reflected rays go in different directions.
Clear image can be formed.	Clear image is generally not formed.

Memory: “Smooth surface → regular reflection; rough surface → diffuse reflection.”

4. Plane Mirror

Plane mirror is a flat reflecting surface.

Characteristics of Image in a Plane Mirror

• Image is virtual and erect.
• Image is of the same size as the object.
• Image is formed at the same distance behind the mirror as the object is in front.
• Image is laterally inverted.

\[\text{Object distance}=\text{Image distance}\]

Lateral Inversion

In lateral inversion, the left side of an object appears as the right side in the mirror and the right side appears as the left side.

Example: The word “AMBULANCE” is written laterally reversed on the front of ambulances so that it can be read correctly in a rear-view mirror.

5. Spherical Mirrors

Spherical mirror is a part of a hollow sphere whose one surface is polished or silvered.

Two Types

Mirror	Nature
Concave mirror	Reflecting surface curves inward.
Convex mirror	Reflecting surface bulges outward.

Important Terms

• Pole (P): Centre of the reflecting surface.
• Centre of curvature (C): Centre of the sphere of which the mirror is a part.
• Radius of curvature (R): Distance between P and C.
• Principal axis: Straight line passing through P and C.
• Principal focus (F): Point where parallel rays meet or appear to meet after reflection.
• Focal length (f): Distance between P and F.

\[\boxed{R=2f}\]

Memory: “C is at \(2f\), F is at \(f\).”

6. Concave Mirror

Concave mirror has a reflecting surface curved towards the inside.

Ray Rules for a Concave Mirror

1. A ray parallel to the principal axis is reflected through the principal focus \(F\).
2. A ray passing through \(F\) is reflected parallel to the principal axis.
3. A ray passing through the centre of curvature \(C\) is reflected back along the same path.
4. A ray striking the pole follows the laws of reflection.

Nature of Image

The image formed by a concave mirror depends on the position of the object.

Board focus: Practise all six standard object positions: beyond C, at C, between C and F, at F, between F and P, and at infinity.

7. Convex Mirror

Convex mirror has a reflecting surface curved outward.

Ray Rules

1. A ray parallel to the principal axis appears to come from the principal focus behind the mirror.
2. A ray directed towards the principal focus is reflected parallel to the principal axis.
3. A ray directed towards the centre of curvature is reflected back along the same path.

Image Formed by a Convex Mirror

• Always virtual.
• Always erect.
• Always diminished.
• Formed behind the mirror between P and F.

Memory: “Convex mirror = virtual + erect + diminished, always.”

8. Mirror Formula and Magnification

Mirror Formula

\[\boxed{\frac{1}{f}=\frac{1}{v}+\frac{1}{u}}\]

Here \(f\) = focal length, \(v\) = image distance and \(u\) = object distance.

Magnification

\[\boxed{m=\frac{h_i}{h_o}=-\frac{v}{u}}\]

Here \(h_i\) is image height and \(h_o\) is object height.

Magnification	Meaning
\(m>1\)	Image is enlarged.
\(m=1\)	Image is same size.
\(0	Image is diminished and erect.
\(m<0\)	Image is inverted.

9. Ray Diagrams and Image Formation

Concave Mirror: Important Cases

Object Position	Image Position	Nature
At infinity	At F	Real, inverted, highly diminished
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 mirror	Virtual, erect, enlarged

Convex Mirror

For all object positions, the image is virtual, erect and diminished.

Exam warning: In ray-diagram questions, label the principal axis, P, F and C clearly. Use at least two correct rays whenever an image construction is required.

10. Uses of Spherical Mirrors

Mirror	Important Uses
Concave mirror	Shaving/makeup mirror, dentist's mirror, headlights, torches, searchlights, solar furnace.
Convex mirror	Rear-view mirrors in vehicles because they provide a wider field of view.

Why Convex Mirror in Vehicles?

A convex mirror forms an erect and diminished image and gives a wider field of view. Therefore, the driver can see a larger area behind the vehicle.

Board line: “Convex mirrors are used as rear-view mirrors because they provide a wider field of view and form erect, diminished images.”

11. Refraction of Light

Refraction is the change in direction of light when it passes obliquely from one transparent medium to another.

Why Does Refraction Occur?

Light travels with different speeds in different transparent media. When light enters a different medium obliquely, its speed changes and its direction can also change.

Important Examples

• A pencil partly immersed in water appears bent.
• The bottom of a pond appears raised.
• Objects under water may appear closer than they actually are.

Memory: “Different medium → speed changes → direction may change.”

12. Laws of Refraction

First Law

The incident ray, refracted ray and normal at the point of incidence all lie in the same plane.

Second Law: Snell's Law

For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.

\[\boxed{\frac{\sin i}{\sin r}=\text{constant}}\]

\[\boxed{n=\frac{\sin i}{\sin r}}\]

Here \(n\) represents the refractive index of one medium with respect to another under the chosen convention.

Board point: The angles \(i\) and \(r\) are measured from the normal.

13. Refractive Index

Refractive index tells us how much a medium can change the speed of light compared with vacuum.

Absolute Refractive Index

\[\boxed{n=\frac{c}{v}}\]

Here \(c\) is the speed of light in vacuum and \(v\) is the speed of light in the medium.

Important Facts

• Refractive index has no unit.
• Higher refractive index means light travels more slowly in that medium.
• For a medium, \(n\) is generally greater than or equal to 1 when measured relative to vacuum.

Numerical idea: If \(c=3\times10^8\,m/s\) and \(v=2\times10^8\,m/s\), then \(\displaystyle n=\frac{3\times10^8}{2\times10^8}=1.5\).

14. Refraction Through a Glass Slab

What Happens?

1. Light enters the glass from air and bends towards the normal.
2. It travels through the glass.
3. When it comes out from glass into air, it bends away from the normal.
4. The emergent ray is parallel to the incident ray for a rectangular glass slab.

Lateral Displacement

The emergent ray is shifted sideways from the original path. This sideways distance is called lateral displacement.

Board point: For a rectangular glass slab, the incident and emergent rays are parallel, but the emergent ray is laterally displaced.

15. Refraction by Spherical Lenses

Lens is a transparent material bounded by two surfaces, at least one of which is curved.

Types

Lens	Shape	Action on Parallel Rays
Convex lens	Thicker at centre, thinner at edges	Converges rays
Concave lens	Thinner at centre, thicker at edges	Diverges rays

Important Terms

• Optical centre (O): Central point of a thin lens.
• Principal axis: Line passing through the optical centre and principal foci.
• Principal focus: Point where parallel rays meet or appear to diverge from after refraction.
• Focal length: Distance between optical centre and principal focus.

16. Convex and Concave Lenses

Convex Lens

A convex lens converges parallel rays of light. Therefore, it is also called a converging lens.

Concave Lens

A concave lens diverges parallel rays of light. Therefore, it is also called a diverging lens.

Standard Ray Rules for a Convex Lens

1. A ray parallel to the principal axis passes through the principal focus after refraction.
2. A ray passing through the optical centre travels approximately undeviated.
3. A ray passing through the principal focus emerges parallel to the principal axis.

Standard Ray Rules for a Concave Lens

1. A ray parallel to the principal axis appears to come from the principal focus.
2. A ray through the optical centre passes approximately undeviated.

17. Lens Formula and Magnification

Lens Formula

\[\boxed{\frac{1}{f}=\frac{1}{v}-\frac{1}{u}}\]

Here \(f\) = focal length, \(v\) = image distance and \(u\) = object distance.

Magnification

\[\boxed{m=\frac{h_i}{h_o}=\frac{v}{u}}\]

Here \(h_i\) is image height and \(h_o\) is object height.

Value of \(m\)	Meaning
\(m>1\)	Image enlarged.
\(m=1\)	Image same size.
\(0	Image diminished and erect.
\(m<0\)	Image inverted.

Exam warning: Do not use the mirror formula for a lens. For a lens use \(\displaystyle \frac1f=\frac1v-\frac1u\).

18. Power of a Lens

Power of a lens measures the ability of a lens to converge or diverge light.

Formula

\[\boxed{P=\frac{1}{f}}\]

Here \(f\) must be measured in metres.

Unit

The SI unit of power is dioptre (D).

\[\boxed{1\,D=1\,m^{-1}}\]

Sign of Power

• Convex lens has positive focal length, so its power is positive.
• Concave lens has negative focal length, so its power is negative.

Example: If \(f=0.5\,m\), then \(\displaystyle P=\frac{1}{0.5}=+2D\) for a convex lens.

19. ⭐ Important Board Exam Questions

1-Mark Questions

Q1. What is reflection of light?
Bouncing back of light into the same medium after striking a surface.

Q2. State the second law of reflection.
The angle of incidence is equal to the angle of reflection.

Q3. What is the relation between \(R\) and \(f\) for a spherical mirror?
\(R=2f\).

Q4. Which mirror is used as a rear-view mirror?
Convex mirror.

Q5. What is refraction?
Change in direction of light when it passes obliquely from one transparent medium to another.

Q6. What is the SI unit of power of a lens?
Dioptre (D).

3-Mark Questions

Q. Write the laws of reflection.
Answer: (1) Incident ray, reflected ray and normal lie in the same plane. (2) Angle of incidence equals angle of reflection.

Q. Why is a convex mirror used as a rear-view mirror?
Answer: It forms an erect and diminished image and provides a wider field of view.

Q. State the laws of refraction.
Answer: The incident ray, refracted ray and normal lie in one plane, and for a fixed pair of media \(\sin i/\sin r\) is constant.

5-Mark Questions

Q. Derive/use the mirror formula in a numerical.
Answer plan: Write sign convention → write \(\displaystyle 1/f=1/v+1/u\) → substitute signed values → calculate \(v\) → state nature of image if asked.

Q. Explain image formation by a convex lens for different object positions.
Answer plan: Draw principal axis → mark F and 2F → draw two standard rays → locate image → write position, size and nature.

Q. Explain refraction through a rectangular glass slab.
Answer plan: Incident ray → bending towards normal → travel through glass → bending away from normal → emergent ray parallel to incident ray → lateral displacement.

20. ⭐ Final 96%-Target Board Revision

Must Learn Definitions

• Reflection
• Refraction
• Incident ray
• Normal
• Angle of incidence
• Angle of reflection
• Concave mirror
• Convex mirror
• Principal focus
• Focal length
• Refractive index
• Convex lens
• Concave lens
• Magnification
• Power of lens

Must Learn Formulas

Reflection

\(i=r\)

Mirror relation

\(R=2f\)

Mirror formula

\(\displaystyle \frac1f=\frac1v+\frac1u\)

Mirror magnification

\(\displaystyle m=-\frac vu=\frac{h_i}{h_o}\)

Refraction

\(\displaystyle n=\frac{\sin i}{\sin r}\)

Absolute refractive index

\(\displaystyle n=\frac cv\)

Lens formula

\(\displaystyle \frac1f=\frac1v-\frac1u\)

Lens magnification

\(\displaystyle m=\frac vu=\frac{h_i}{h_o}\)

Power

\(\displaystyle P=\frac1f\), \(f\) in metre

Must Learn Memory Lines

\[\text{Convex mirror} \rightarrow \text{virtual + erect + diminished}\]

\[\text{Concave mirror} \rightarrow \text{image depends on object position}\]

\[\text{Convex lens} \rightarrow \text{converging}\]

\[\text{Concave lens} \rightarrow \text{diverging}\]

\[\text{Higher }n \rightarrow \text{lower speed of light}\]

\[P=\frac1f\quad(f\text{ in metre})\]

⚠ Common Board Mistakes

• Measure angles from the normal, not from the mirror surface.
• Use the correct sign convention before substituting values.
• Do not confuse mirror formula with lens formula.
• Convert focal length into metres before calculating lens power.
• Remember \(R=2f\) for a spherical mirror.
• For a convex mirror, image is always virtual, erect and diminished.
• For a concave lens, the image is always virtual, erect and diminished.
• Label F, C, P and the principal axis clearly in mirror diagrams.
• Label F, 2F and O correctly in lens diagrams.
• Always write the unit in numerical answers.

📝 Last-Minute Checklist

• ☐ Laws of reflection
• ☐ Plane mirror
• ☐ Spherical mirrors
• ☐ Concave mirror
• ☐ Convex mirror
• ☐ Ray diagrams
• ☐ \(R=2f\)
• ☐ Mirror formula
• ☐ Mirror magnification
• ☐ Refraction
• ☐ Snell's law
• ☐ Refractive index
• ☐ Glass slab
• ☐ Convex lens
• ☐ Concave lens
• ☐ Lens formula
• ☐ Lens magnification
• ☐ Power of lens
• ☐ Sign convention
• ☐ Board numericals

🎯 96%-type board method: Learn definitions exactly, draw neat labelled ray diagrams, write formulas before numerical calculations, use the correct sign convention, show every step, and underline key terms.

21. ✍ Practice Corner

Write these answers without looking at the notes:

1. Define reflection and refraction.
2. State both laws of reflection.
3. Define pole, principal axis, focus and centre of curvature.
4. Write the relation between radius of curvature and focal length.
5. Write the image characteristics of a convex mirror.
6. Explain all important cases of image formation by a concave mirror.
7. Write the mirror formula and magnification formula.
8. Why is a convex mirror used as a rear-view mirror?
9. State Snell's law of refraction.
10. Define refractive index and write \(n=c/v\).
11. Explain refraction through a rectangular glass slab.
12. Differentiate between convex and concave lenses.
13. Write the lens formula and magnification formula.
14. Define power of a lens and its SI unit.
15. Solve numerical questions based on mirror formula.
16. Solve numerical questions based on lens formula.
17. Solve numerical questions based on refractive index.
18. Solve numerical questions based on power of lens.

Final answer tip: For 5 marks, use definition + law/formula + labelled diagram + steps + result. For numericals, never skip the formula, sign convention, substitution and unit.

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✍ Class 10 Science • Light – Reflection and Refraction
Handwritten-style educational notes • Very Simple English • Mobile friendly

✍ Class 10 Science • Light – Reflection and Refraction
Handwritten-style educational notes • Very Simple English • Mobile friendly