Light - Reflection and Refraction — Class 10 Science
What CBSE examines here (2026-27). Reflection, spherical mirrors, refraction (including Snell's law and absolute refractive index, both named explicitly in the book), and spherical lenses are all here, across four in-text question sets (14 questions, not 18 as some older manifests claim) and one end-of-chapter Exercise (17 questions, correctly counted). One topic is gone: total internal reflection — along with critical angle, optical fibres and the mirage/diamond-sparkle examples — doesn't appear anywhere in the current chapter. It's kept in the appendix, since it's genuinely useful and returns in Class 12.
"Light: the messenger that brings the universe to our eyes."
1. About the Chapter
This chapter explores how light BEHAVES:
- Reflection (bouncing back)
- Refraction (bending in different media)
- Mirrors (spherical: concave, convex)
- Lenses (concave, convex)
- Formulas for mirror/lens
Why Important
- All optics: spectacles, microscopes, cameras
- Visual perception
- Indian Raman Effect heritage
2. Reflection of Light
Definition
When light strikes a surface and BOUNCES BACK.
Laws of Reflection
- Angle of incidence = Angle of reflection (i = r)
- Incident ray, normal, reflected ray are all in the same plane.
Types of Reflection
- Regular (specular): smooth surfaces; clear images. Examples: mirrors.
- Diffuse: rough surfaces; scattered. Examples: paper, walls.
3. Spherical Mirrors
Two Types
1. Concave Mirror (curves inward — like inside of bowl)
- Converges parallel rays to a focal point
- Magnifies image when object close
2. Convex Mirror (curves outward — like back of spoon)
- Diverges parallel rays
- Always forms small, virtual image
Important Terms
- Centre of Curvature (C): centre of the sphere
- Radius of Curvature (R): distance from C to surface
- Pole (P): midpoint of mirror
- Focus (F): where parallel rays converge (or appear to)
- Focal Length (f): distance from pole to focus
- f = R/2 (for spherical mirrors)
Sign Conventions
- All distances measured from POLE
- Distances in DIRECTION OF INCIDENT LIGHT: positive
- Distances AGAINST direction of incident light: negative
- Heights ABOVE principal axis: positive
- Heights BELOW principal axis: negative
Mirror Formula
1/v + 1/u = 1/f
where:
- u = object distance
- v = image distance
- f = focal length
Magnification
m = h'/h = −v/u
where:
- h' = image height
- h = object height
- m positive: erect; m negative: inverted
4. Image Formation by Concave Mirror
Depends on OBJECT POSITION:
| Object Position | Image Position | Image Type | Size |
|---|---|---|---|
| At infinity | At F | Real, inverted | Highly diminished |
| 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 | Magnified |
| At F | At infinity | — | Highly magnified |
| Between F and P | Behind mirror | Virtual, erect | Magnified |
Uses
- Shaving mirror (close object: erect, magnified)
- Dentist's mirror
- Headlights (object at focus: parallel beam)
- Solar concentrator
- Astronomical telescope
5. Image Formation by Convex Mirror
ALWAYS:
- Virtual (behind mirror)
- Erect
- Diminished
- Smaller than object
Uses
- Rear-view mirror (wide field of view)
- Security mirrors in shops
- Vehicle side mirrors
6. Refraction of Light
Definition
Bending of light when passing from one medium to another.
Why Does It Happen?
Light travels at DIFFERENT SPEEDS in different media.
- Vacuum/air: ~3 × 10⁸ m/s
- Water: ~2.25 × 10⁸ m/s (slower)
- Glass: ~2 × 10⁸ m/s (even slower)
Laws of Refraction
- Incident ray, refracted ray, and normal — all in SAME PLANE
- sin(angle of incidence) / sin(angle of refraction) = constant
The constant = REFRACTIVE INDEX (n) of the second medium with respect to the first.
Snell's Law
n₁ sin θ₁ = n₂ sin θ₂
For light entering DENSER medium: bends TOWARDS normal. For light entering RARER medium: bends AWAY from normal.
Refractive Index
n = c / v
where:
- c = speed of light in vacuum
- v = speed in medium
Optical Density
The medium with the larger refractive index is optically denser; the other is optically rarer. This is not the same as mass density — kerosene (n = 1.44) is optically denser than water (n = 1.33) despite being less mass-dense.
Refractive Index Table (Table 9.3 — the chapter says you don't need to memorise this)
| Medium | n | Medium | n |
|---|---|---|---|
| Air | 1.0003 | Canada Balsam | 1.53 |
| Ice | 1.31 | Rock salt | 1.54 |
| Water | 1.33 | Carbon disulphide | 1.63 |
| Alcohol | 1.36 | Dense flint glass | 1.65 |
| Kerosene | 1.44 | Ruby | 1.71 |
| Fused quartz | 1.46 | Sapphire | 1.77 |
| Turpentine oil | 1.47 | Diamond | 2.42 |
| Benzene | 1.50 | ||
| Crown glass | 1.52 |
7. Refraction by Spherical Lenses
Types
1. Convex Lens (thicker in middle): CONVERGING
- Focuses parallel rays to focal point
2. Concave Lens (thinner in middle): DIVERGING
- Spreads parallel rays apart
Important Terms
Same as mirrors: optical centre, focal length, etc.
Lens Formula
1/v − 1/u = 1/f
(Note: different sign from mirror formula.)
Magnification (for lens)
m = h'/h = v/u
(Positive sign, unlike mirror.)
Power of a Lens
P = 1/f (in diopters)
where f is in METRES.
- Convex lens: positive power
- Concave lens: negative power
Combining Lenses
P = P₁ + P₂ + ...
8. Image Formation by Lenses
Convex Lens (similar pattern to concave mirror):
| Object Position | Image | Type |
|---|---|---|
| At infinity | At F | Real, inverted, diminished |
| Beyond 2F | Between F and 2F | Real, inverted, diminished |
| At 2F | At 2F | Real, inverted, same size |
| Between F and 2F | Beyond 2F | Real, inverted, magnified |
| At F | At infinity | — |
| Between F and centre | Same side as object | Virtual, erect, magnified |
Try it: find the focal length of a convex lens
The table above, on a real bench. Place the candle, slide the screen until the flame is sharp, record u and v, and work out f.
Focal length of a convex lens
Procedure · 5 steps
- Place the candle well beyond the lens — try somewhere near the 10 cm mark.
- Slide the screen slowly until the image of the flame is sharp. Record u and v.
- Move the candle closer to the lens and focus the image again. Record at least three positions.
- Notice how the image changes as the candle comes closer — its size, its distance, its orientation.
- Use 1/f = 1/v − 1/u for each reading and write down the focal length.
Concave Lens
ALWAYS forms virtual, erect, diminished image.
Uses
Convex lens:
- Magnifying glass
- Camera
- Microscope (objective)
- Spectacles for hyperopia (far-sighted)
Concave lens:
- Spectacles for myopia (near-sighted)
- Peephole
9. Human Eye (Brief; covered in next chapter)
- Cornea: outer transparent layer
- Iris: coloured (controls pupil size)
- Pupil: opening
- Lens: convex; focuses light
- Retina: back of eye; light-sensitive cells
- Optic nerve: sends signals to brain
(Detailed in Chapter 10.)
10. Worked Examples
Example 1: Concave Mirror
An object is placed 30 cm from a concave mirror of focal length 10 cm. Find image distance and magnification.
Using mirror formula: 1/v + 1/(-30) = 1/(-10)
- 1/v = -1/10 + 1/30 = -3/30 + 1/30 = -2/30
- v = -15 cm
Magnification: m = -v/u = -(-15)/(-30) = -0.5
Image is REAL (v negative for concave mirror in this convention), INVERTED, HALF size.
Example 2: Refractive Index
Light enters water (n=1.33) from air. Speed in water?
- n = c/v
- 1.33 = (3×10⁸) / v
- v = 3×10⁸ / 1.33 ≈ 2.25 × 10⁸ m/s
Example 3: Lens Formula
A convex lens of f = 20 cm produces image at v = 30 cm. Find object distance.
1/v - 1/u = 1/f 1/30 - 1/u = 1/20 1/u = 1/30 - 1/20 = -1/60 u = -60 cm
Object is 60 cm from lens (on negative side).
Example 4: Power of Lens
A lens has focal length 25 cm. Find its power.
- P = 1/f = 1/0.25 = +4 diopters (convex)
11. Common Mistakes
-
Concave vs convex confusion
- CONCAVE mirror: curves INWARD (like bowl interior). CONVERGES.
- CONVEX mirror: curves OUTWARD (like back of spoon). DIVERGES.
- For LENSES, it's reversed: convex lens converges; concave lens diverges.
-
Sign convention errors
- Strict rules. Memorise. Distances behind mirror/in front of lens — be careful with signs.
-
Mirror vs lens formula
- Mirror: 1/v + 1/u = 1/f
- Lens: 1/v − 1/u = 1/f
-
Refractive index value
- Always ≥ 1 for media (vacuum is 1; all media > 1).
-
Magnification sign
- Negative m = inverted image. Positive m = erect.
12. Indian Heritage
C.V. Raman (Indian Physicist)
- Won NOBEL PRIZE 1930 for the RAMAN EFFECT
- Discovered that light changes wavelength when scattered by molecules
- 28 February celebrated as National Science Day in India
Aryabhata
Discussed reflection and refraction in ancient texts.
Modern Indian Optics
- ISRO uses optical instruments for satellite imaging
- Indian Astronomical Observatory (Hanle, Ladakh)
- World-class telescopes built by Indian scientists
13. Conclusion
Light's behaviour follows precise rules:
- Reflection: angle in = angle out
- Refraction: bends in new medium (Snell's law)
- Mirrors and lenses: form images per formulas
- Sign conventions: critical for problems
Master:
- Two laws (reflection, refraction)
- Mirror formula (1/v + 1/u = 1/f) and lens formula (1/v - 1/u = 1/f)
- Magnification
- Image formation tables
- Power of lens
Practice 15+ problems. This is HIGH-MARK chapter for board exam.
Optics: how the world reveals itself to your eyes.
Appendix — beyond the current chapter
Not examinable in CBSE 2026-27. Total internal reflection — along with critical angle, optical fibres, and the mirage/diamond-sparkle examples — has been removed from this chapter. It's kept here because it's genuinely useful physics that returns in Class 12.
Total Internal Reflection
When light travels from a denser medium to a rarer medium and the angle of incidence exceeds a threshold called the critical angle, the light doesn't refract out at all — it reflects entirely back into the denser medium. This is total internal reflection (TIR).
Applications:
- Optical fibres: light bounces along the inside of a thin glass/plastic fibre via repeated TIR, used in telecommunications and medical endoscopes
- Sparkle of diamonds: diamond's very high refractive index (2.42) gives it a small critical angle, so light entering a cut diamond undergoes TIR multiple times before emerging, producing its brilliance
- Mirage: layers of air at different temperatures (and hence different refractive indices) near a hot road bend light until it undergoes TIR, creating the illusion of a water pool
