The Human Eye and the Colourful World — Practice

Question 1

What is the least distance of distinct vision for a normal eye?

Accepted Answer

Step 1 — The near point of a normal eye is 25 cm.

✦ Answer: 25 cm.

Question 2

Which lens corrects myopia?

Accepted Answer

Step 1 — Myopia is corrected with a concave (diverging) lens.

✦ Answer: concave lens.

Question 3

Why does the sky appear blue?

Accepted Answer

Step 1 — Air molecules scatter shorter (blue) wavelengths most.

✦ Answer: preferential scattering of blue light.

Question 4

What is accommodation of the eye? Which part achieves it?

Accepted Answer

Step 1 — Accommodation is the eye's ability to adjust its lens focal length to focus objects at different distances.

Step 2 — The ciliary muscles change the curvature of the eye lens.

✦ Answer: focal-length adjustment by the ciliary muscles.

Question 5

A person cannot read a book held closer than 50 cm. Name the defect and the corrective lens.

Accepted Answer

Step 1 — Difficulty with near objects is hypermetropia (long-sightedness).

Step 2 — Corrected with a convex (converging) lens.

✦ Answer: hypermetropia; convex lens.

Question 6

What is dispersion of light? Why does it occur in a prism?

Accepted Answer

Step 1 — Dispersion is the splitting of white light into its component colours.

Step 2 — Different colours have different speeds/refractive indices in glass, so they bend by different amounts.

✦ Answer: colour-splitting because each colour refracts differently in the prism.

Question 7

Explain why the sun appears reddish at sunrise and sunset.

Accepted Answer

Step 1 — Near the horizon, sunlight travels a long path through the atmosphere.

Step 2 — Most of the shorter-wavelength blue light is scattered away.

Step 3 — Mainly the longer-wavelength red light reaches the eye, so the sun looks red.

✦ Answer: long atmospheric path scatters away blue, leaving red.

Question 8

Why do stars twinkle but planets do not?

Accepted Answer

Step 1 — Starlight from a point source refracts through fluctuating atmospheric layers, so its apparent brightness and position vary — twinkling.

Step 2 — Planets are much closer and seen as a collection of points (a disc); the variations average out.

✦ Answer: atmospheric refraction of a point source twinkles; a planet's disc averages it out.

Question 9

Explain advanced sunrise and delayed sunset.

Accepted Answer

Step 1 — Atmospheric refraction bends light coming over the horizon towards the observer.

Step 2 — So the sun is seen about 2 minutes before it actually rises and after it actually sets.

✦ Answer: refraction lets us see the sun before rise and after set (~2 min each).

Question 10

What is the Tyndall effect? Give an everyday example.

Accepted Answer

Step 1 — The Tyndall effect is the scattering of light by fine particles that makes a beam's path visible.

Step 2 — Example: sunlight streaming through a canopy of trees or a smoke-filled room.

✦ Answer: scattering by fine particles making the light path visible (e.g. sunbeams through mist).

Practice — The Human Eye and the Colourful World

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