Is it correct to say: Prescribing a convex lens for myopia and concave for hypermetropia?

Myopia (short-sighted, image forms IN FRONT of retina) → CONCAVE lens (diverges rays). Hypermetropia → CONVEX.

Is it correct to say: Forgetting the sign of focal length while computing power?

Concave lens f < 0 ⇒ P < 0. Always keep sign in P = 1/f.

Is it correct to say: Assuming presbyopia is corrected with a single lens?

Presbyopia is age-related loss of accommodation — needs BIFOCAL lens (concave upper for distance, convex lower for reading).

Is it correct to say: Reversing the VIBGYOR sequence?

Violet bends MOST, red bends LEAST. On the screen: Red on top, Violet at bottom (for a normal prism setup).

Is it correct to say: Saying sky is blue because water absorbs red light?

It's atmospheric SCATTERING, not absorption. Rayleigh's 1/λ⁴ rule.

Is it correct to say: Confusing accommodation with adaptation?

Accommodation = change of focal length by ciliary muscles. Adaptation = adjusting to change of brightness by pupil size.

Formula sheet (PDF) →

Physics · Chapter 10

Human Eye and Colourful World

Q: Is it correct to say: Forgetting the sign of focal length while computing power?

Concave lens f < 0 ⇒ P < 0. Always keep sign in P = 1/f.

Q: Is it correct to say: Assuming presbyopia is corrected with a single lens?

Presbyopia is age-related loss of accommodation — needs BIFOCAL lens (concave upper for distance, convex lower for reading).

Q: Is it correct to say: Reversing the VIBGYOR sequence?

Violet bends MOST, red bends LEAST. On the screen: Red on top, Violet at bottom (for a normal prism setup).

Q: Is it correct to say: Saying sky is blue because water absorbs red light?

It's atmospheric SCATTERING, not absorption. Rayleigh's 1/λ⁴ rule.

Q: Is it correct to say: Confusing accommodation with adaptation?

Accommodation = change of focal length by ciliary muscles. Adaptation = adjusting to change of brightness by pupil size.

Human eye structure, defects of vision and their correction, dispersion, scattering — NCERT Class 10 Science Ch 10

8 formulas · 3 solved examples · 6 practice questions · Board weight: 5 marks · CBSE Class 10 · 2025–26 syllabus

Board Exam Tips

→Defects of vision — myopia, hypermetropia, presbyopia — a 3-mark question is almost guaranteed.
→Draw ray diagrams for defect and its correction using lens: mark the defective far/near point clearly.
→Power of correcting lens: use P = 1/f with sign. Concave lens for myopia (negative P), convex for hypermetropia (positive P).
→Rayleigh's scattering ∝ 1/λ⁴ — explain blue sky and red sunrise/sunset in your own words.
→Prism dispersion: red bends least, violet bends most. Do NOT reverse the order.

Diagram

Ray diagram of dispersion of white light by a glass prism into VIBGYOR

Formulas(8)

Power of Accommodation

P = \frac{1}{f}

Least Distance of Distinct Vision

D = 25 cm

Focal Length for Myopia CorrectionBoard favourite

f = - x (far point at distance x)

Focal Length for Hypermetropia CorrectionBoard favourite

f = \frac{D \cdot y}{y - D}

Power of Lens (Dioptres)

P = \frac{1}{f (in metres)}

Angle of Deviation in a Prism

δ = (i_{1} + i_{2}) - A

Rayleigh Scattering Intensity

I \propto \frac{1}{λ ^{4}}

Snell's Law at Prism Faces

n = \frac{s i n i}{s i n r}

Solved Examples

1Solved Exampleeasy3 steps

A person's far point is 80 cm. What lens should be prescribed?

This is myopia — the far point is at a finite distance, not infinity.

Far point x = 80 cm = 0.80 m

2Solved Exampleboard4 steps

A hypermetropic person cannot see objects closer than 100 cm. Calculate the focal length and power of the correcting lens (D = 25 cm).

Defective near point y = 100 cm; normal D = 25 cm

y = 100 cm, D = 25 cm

3Solved ExampleHOTS4 steps

Why does the Sun appear reddish at sunrise and sunset while the sky at midday appears blue? Answer using Rayleigh's law.

Rayleigh's law: intensity of scattered light is inversely proportional to the fourth power of the wavelength

I \propto \frac{1}{λ ^{4}}

Traps & Common Mistakes

Common Mistakes6

1
Prescribing a convex lens for myopia and concave for hypermetropia
✓Myopia (short-sighted, image forms IN FRONT of retina) → CONCAVE lens (diverges rays). Hypermetropia → CONVEX.
2
Forgetting the sign of focal length while computing power
✓Concave lens f < 0 ⇒ P < 0. Always keep sign in P = 1/f.
3
Assuming presbyopia is corrected with a single lens
✓Presbyopia is age-related loss of accommodation — needs BIFOCAL lens (concave upper for distance, convex lower for reading).
4
Reversing the VIBGYOR sequence
✓Violet bends MOST, red bends LEAST. On the screen: Red on top, Violet at bottom (for a normal prism setup).
5
Saying sky is blue because water absorbs red light
✓It's atmospheric SCATTERING, not absorption. Rayleigh's 1/λ⁴ rule.
6
Confusing accommodation with adaptation
✓Accommodation = change of focal length by ciliary muscles. Adaptation = adjusting to change of brightness by pupil size.

Practice Yourself

Practice Yourself6 questions

Q1
A myopic person cannot see beyond 50 cm. Find the focal length and power of the corrective lens.
Q2
The near point of a hypermetropic eye is 75 cm. Find the power of the correcting lens (D = 25 cm).
Q3
Why do stars twinkle but planets do not?
Q4
Why is the danger signal red?
Q5
State two functions of the ciliary muscles.
Q6
A prism produces a spectrum from white light. Name the seven colours and state which bends most and least.

Notes

Human Eye and Colourful World

Our eye is a remarkable optical instrument — a self-adjusting camera with a variable-focus lens.

Structure of the eye (must-know parts)

Cornea: transparent front bulge; does most of the refraction.
Iris & pupil: iris controls pupil size, regulating light entering.
Eye lens: convex, flexible; ciliary muscles change its focal length (accommodation).
Retina: screen at the back with rods (dim light) and cones (colour).
Optic nerve: carries signals to the brain.

Three common defects

Defect	Cause	Correction
Myopia (short sight)	Eyeball too long / lens too curved	Concave lens
Hypermetropia (long sight)	Eyeball too short / lens too flat	Convex lens
Presbyopia	Ciliary muscles weaken with age	Bifocal lens

Dispersion and prism

White light is a mixture of seven colours. When it enters a glass prism, each wavelength refracts by a slightly different amount because n depends on λ. Result: VIBGYOR spectrum on a screen. Recombining with an inverted prism gives back white light — proving colours are constituents of white light (Newton's experiment).

Atmospheric optics — Rayleigh scattering

I ∝ 1/λ⁴ explains:

Blue sky: blue scatters ~10× more than red at midday.
Red Sun at horizon: long slant path scatters blue away, only red reaches us.
White clouds: water droplets are large enough that all colours scatter equally.
Red danger signal: longest wavelength, least scattered — visible farthest.