Notes Class 10 Science Chapter 9 Light Reflection and Refraction

Notes For All Chapters Science Class 10 CBSE

1. Introduction to Light

Light makes objects visible; in darkness, objects are invisible.

Light enables us to see by reflection (object reflects light to eyes) and transmission (through transparent media).

Common phenomena related to light:

• Image formation in mirrors
• Twinkling of stars
• Rainbow colours
• Bending of light

Light generally travels in straight lines → explained as rays.

Diffraction of light:

• When object is very small, light bends around it.
• Needs wave theory for explanation.

Modern theory: Quantum theory of light → reconciles wave and particle nature.

2. Reflection of Light

Laws of Reflection

• Angle of incidence = Angle of reflection.
• Incident ray, normal, and reflected ray lie in the same plane.
• Valid for all surfaces (plane & spherical).

Image by Plane Mirror

• Always virtual, erect, and laterally inverted.
• Size = same as object.
• Distance of image behind mirror = distance of object in front.

3. Spherical Mirrors

• Concave mirror: Reflecting surface curved inwards.
• Convex mirror: Reflecting surface curved outwards.

Terms Related to Spherical Mirrors

Pole (P): Centre of mirror surface.

Centre of Curvature (C): Centre of the sphere of which mirror is part.

Radius of Curvature (R): Distance PC.

Principal Axis: Line through P and C.

Principal Focus (F):

• Concave → Rays parallel to axis meet at F.
• Convex → Rays appear to diverge from F.

Focal Length (f): Distance PF; relation → R = 2f.

Aperture: Diameter of mirror’s reflecting surface.

4. Image Formation by Concave Mirrors

Depends on object’s position

At infinity

Beyond C

At C

Between C & F

At F

Between P & F

Uses of Concave Mirrors

• Torches, headlights, searchlights.
• Shaving mirrors.
• Dentist mirrors.
• Solar furnaces.

5. Image Formation by Convex Mirrors

At infinity

Between infinity & P

Uses of Convex Mirrors

Used as rear-view mirrors in vehicles:

• Always erect.
• Wider field of view.

6. Sign Convention (New Cartesian)

Object always to left of mirror.

Pole is origin; principal axis = X-axis.

Distances towards right (+x) → positive; left (–x) → negative.

Distances above axis (+y) → positive; below (–y) → negative.

7. Mirror Formula & Magnification

Mirror Formula:

Magnification (m):

• Negative m → real image.
• Positive m → virtual image.

8. Refraction of Light

• Change in direction of light when passing obliquely from one medium to another.
• Examples: Raised coin in water, bent pencil, raised letters under glass slab.

Laws of Refraction

Incident ray, refracted ray, and normal lie in the same plane.

Snell’s Law:

Refractive Index

Absolute refractive index:

Optical density: Larger refractive index → optically denser medium.

9. Lenses

• Transparent medium bounded by spherical surfaces.
• Convex lens: Thicker in middle, converges rays.
• Concave lens: Thicker at edges, diverges rays.

Important Terms

• Optical centre (O): Central point, no deviation.
• Principal axis: Line joining centres of curvature.
• Principal focus (F1, F2): Rays parallel to axis meet (convex) or appear to diverge (concave).
• Focal length (f): Distance between O and F.

10. Image Formation by Lenses

Convex Lens

• Forms real, inverted images except when object is between F and O (virtual, erect, enlarged).

Concave Lens

• Always forms virtual, erect, and diminished images.

11. Lens Formula & Magnification

Lens Formula:

Magnification:

12. Power of a Lens

Unit: Dioptre (D)

• 1D = focal length 1 m.

Convex lens → Positive power.
Concave lens → Negative power.

Combination of lenses: P = P1​+P2​+P3​+…