CBSE Class 10 Physics — Chapter 9: Light — Reflection and Refraction — Revision Notes

Overview: This revision module covers the core NCERT topics in Light — Reflection and Refraction. It explains physical concepts, gives exact formulas (with correct subscripts and superscripts), provides step-by-step problem strategies and includes exam-oriented tips. Use this for focused board preparation and rapid revision.

1. Nature of Light — Ray Model

In geometric optics, light is treated as straight rays that travel in straight lines in a homogeneous medium. A light ray represents the direction of propagation. A beam may be parallel, convergent or divergent. This chapter uses the ray model to explain reflection and refraction which is sufficient for typical CBSE questions.

2. Reflection of Light

Reflection is the bouncing back of light from a surface. Two fundamental laws of reflection:

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

Plane mirrors: They produce virtual, erect images of the same size as the object. The image distance (v) equals the object distance (u) measured from the mirror, and the image is laterally inverted.

Image formation by plane mirror

• Image is virtual (cannot be obtained on a screen), upright and of same size.
• Image distance = object distance: v = u.

Spherical mirrors

Concave and convex mirrors are parts of a spherical surface. Key points and sign conventions (use centre of curvature C, pole P, focus F):

• Focal length: f = R/2 where R is radius of curvature.
• Mirror formula (applies to spherical mirrors): 1/v + 1/u = 1/f. Convention: object distance u is negative if object is in front of mirror (CBSE convention often uses signs; follow NCERT examples).
• Magnification: m = h_i/h_o = v/u. If m is negative, image is inverted; positive → image erect.

1/v + 1/u = 1/f → 1/v + 1/(−30) = 1/15 → 1/v = 1/15 + 1/30 = (2 + 1)/30 = 3/30 = 1/10 → v = 10 cm

3. Refraction of Light

Refraction is the bending of light when it passes from one transparent medium to another (e.g., air → glass). The bending occurs because the speed of light changes between media.

Snell's Law and Refractive Index

Snell's law: n₁ · sin i = n₂ · sin r, where n is refractive index, i is angle of incidence and r angle of refraction. Common expression:

n = sin i / sin r

Refractive index of a medium relative to vacuum is n = c / v where c is speed of light in vacuum and v is speed in the medium.

Refraction at a plane surface — apparent depth

Objects immersed in water appear raised. For small angles, apparent depth relates to real depth by refractive index: n = real depth / apparent depth (for air–water interface).

Total Internal Reflection (TIR)

TIR occurs when light travels from a denser medium to a rarer medium (e.g., glass → air) and the angle of incidence exceeds the critical angle (θ_c) given by:

sin θ_c = n₂ / n₁ (for n₁ > n₂)

Applications of TIR include optical fibres, binoculars and certain prisms used in periscopes and instruments.

4. Lenses — Refraction at Curved Surfaces

Lenses converge or diverge light by refraction. Convex (converging) lenses form real or virtual images; concave (diverging) lenses form only virtual, erect, diminished images for real objects.

Lens formula and magnification

Lens formula (thin lens approximation):

1/v − 1/u = 1/f

Power of a lens: P = 1/f (with f in metres), measured in dioptres (D). Example: a 0.5 m focal length lens has P = 2 D.

1/v − 1/u = 1/f → 1/v − 1/(−60) = 1/20 → 1/v = 1/20 − 1/60 = (3 − 1)/60 = 2/60 = 1/30 → v = 30 cm

5. Ray Diagrams — How to Draw

For mirrors and lenses, practice three principal rays for image construction:

• For mirrors: ray parallel to principal axis → reflects through focus; ray through focus → reflects parallel; ray through centre → retraces path.
• For lenses: ray parallel to axis → refracts through focus; ray through centre → passes undeviated; ray through focus on entering side → emerges parallel.

Tip: Neat ray diagrams, correctly labelled (O for object, I for image, F for focus, C for centre) score marks quickly in board answers.

6. Total Internal Reflection & Optical Fibres

For TIR to occur: light must move from denser to rarer medium and angle of incidence > critical angle (θ_c). Optical fibres exploit TIR to transmit light signals with minimal loss. Advantages: low signal loss, flexible routing and use in telecommunications and medical endoscopy.

7. Important Formulas & Sign Conventions

Keep these formulas memorised; use correct _{and ^{where necessary in answers:}}

• Mirror formula: 1/v + 1/u = 1/f
• Lens formula: 1/v − 1/u = 1/f
• Magnification: m = h_i/h_o = v/u
• Refractive index: n = sin i / sin r = c / v
• Power of lens: P (in D) = 1/f (in m)

8. Numerical Problem Strategy

1. Write down given quantities and units; convert to SI (metres) where needed (for power).
2. Choose correct formula (mirror or lens) and apply sign convention consistently (NCERT convention: object in front of mirror/lens → u < 0 for mirrors; check your teacher’s preferred convention).
3. Solve algebraically; interpret sign of v and m to determine real/virtual and erect/inverted.
4. Sketch a simple ray diagram to confirm the nature and location of image.

9. Common Numerical Examples (Practice)

1/v + 1/u = 1/f → 1/v + 1/(−18) = 1/(−12) → 1/v = (1/−12) + (1/18) = (−3 + 2)/36 = −1/36 → v = −36 cm

10. Practicals & Experiments

Typical NCERT practicals include:

• Verification of reflection laws using a plane mirror and pins.
• Finding focal length of a convex lens by focusing distant objects or using object-image distance pairs and applying lens formula.
• Observing refraction through transparent slab and measuring lateral displacement.

Record readings carefully, draw well-labelled diagrams and calculate mean values where applicable.

11. Common Board Questions & Answer Tips

• Always state laws (e.g., laws of reflection) explicitly before solving.
• Label diagrams (P, C, F, O, I) and show ray paths with arrows.
• For numerical answers show substitution steps clearly and include units.
• When asked about nature of image, give three properties: real/virtual, erect/inverted, magnified/diminished.

12. Quick Revision Checklist (One-page)

• Understand ray model and definitions: incident, reflected, refracted, normal.
• Memorise laws of reflection and Snell’s law.
• Practice mirror & lens formulas and sign conventions.
• Know focal length relations: f = R/2 for spherical mirrors.
• Be able to explain TIR and optical fibre working.
• Practice at least 10 numerical problems and 6 ray diagrams before exam.