Light: Reflection, Refraction, Lenses and the Eye
Light: Reflection, Refraction, Lenses and the Eye
Light questions in NTPC ask the uses of mirrors and lenses, everyday effects of refraction and scattering, eye defects and their correcting lenses, and simple mirror or lens power calculations. Each effect has a standard real-life example, and the question usually gives the example and asks the effect.
1. Mirrors
| Mirror | Image and use |
|---|---|
| Plane mirror | Virtual, erect, same size, laterally inverted |
| Concave mirror | Converging; real or virtual image; used in torches, headlights, shaving and dentist's mirrors, solar cookers |
| Convex mirror | Diverging; always virtual, erect and smaller; wide field of view, so used as rear-view mirrors in vehicles and in shops |
Mirror formula: 1/f = 1/v + 1/u; focal length f = R/2
2. Refraction and Lenses
| Idea | Fact or example |
|---|---|
| Refraction | Bending of light when it passes from one medium to another; a pool looks shallower, a stick looks bent in water |
| Refractive index | n = speed of light in vacuum ÷ speed in the medium; glass about 1.5, water about 1.33, diamond about 2.42 |
| Total internal reflection | Light from a denser medium hits the boundary beyond the critical angle; sparkle of diamond, optical fibres, mirage |
| Convex lens | Converging; used in magnifying glasses, cameras and to correct long sight |
| Concave lens | Diverging; used to correct short sight |
| Power of a lens | P = 1/f (f in metres); unit dioptre; convex lens positive, concave negative |
3. Dispersion, Scattering and the Eye
| Effect | Example |
|---|---|
| Dispersion | A prism splits white light into seven colours; violet bends most, red least |
| Rainbow | Dispersion, refraction and internal reflection in raindrops |
| Scattering | Blue sky; red Sun at sunrise and sunset; danger signals are red because red scatters least |
| Tyndall effect | Scattering of light by colloid particles, such as a beam in a dusty room |
| Twinkling of stars | Atmospheric refraction |
| Eye defect | Problem and correction |
|---|---|
| Myopia (short sight) | Distant objects blurred; concave lens |
| Hypermetropia (long sight) | Near objects blurred; convex lens |
| Presbyopia | Age-related loss of focusing; bifocal lens |
| Astigmatism | Unequal curvature of cornea; cylindrical lens |
| Image on the retina | Real and inverted |
✗ A concave mirror is used as a rear-view mirror | ✓ A convex mirror is used, because it gives an erect image of a wide area
✗ The sky looks blue because of reflection by oceans | ✓ It looks blue because air molecules scatter blue light more than red
हिंदी नोट: वाहनों में पीछे देखने के लिए उत्तल दर्पण लगाया जाता है। आकाश का नीला रंग प्रकाश के प्रकीर्णन के कारण है। निकट दृष्टि दोष अवतल लेंस से और दूर दृष्टि दोष उत्तल लेंस से ठीक होता है। लेंस की क्षमता का मात्रक डायोप्टर है।
Exam Pointer: Verified NTPC patterns: the use of a convex mirror (April 2016 CBT-1) and an example of scattering of light (April 2021 CBT-1). Lens power and eye-defect questions are tagged syllabus-based.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Uses of mirrors
[PYQ: NTPC CBT-1 18-Apr-2016 Shift-2]
EXAM LEVEL
Q. Which mirror do dentists use to see a large image of a tooth?
A concave mirror; held close, it gives an enlarged, erect, virtual image.
Answer: Concave mirror
EXAMATLAS LEVEL
Q. A concave mirror has a radius of curvature of 30 cm. Find its focal length. Which type of mirror is used at sharp road bends and why?
f = R/2 = 15 cm. Convex mirrors are used at bends because they always form an erect image and show a wide area.
Answer: 15 cm; convex mirror
Pattern 2: Scattering and dispersion
[PYQ: NTPC CBT-1 7-Apr-2021 Shift-1]
EXAM LEVEL
Q. Why does the Sun look reddish at sunrise and sunset?
Sunlight travels a longer path through the air, so blue light is scattered away and mostly red reaches us.
Answer: Scattering of light
EXAMATLAS LEVEL
Q. Classify each as dispersion, scattering or total internal reflection: rainbow, blue sky, sparkle of a diamond, red danger signals visible from far.
A rainbow is mainly dispersion. Blue sky is scattering, and red is chosen for danger signals because it scatters least. A diamond sparkles by total internal reflection.
Answer: Dispersion; scattering; total internal reflection; scattering (least for red)
Pattern 3: Refraction, refractive index and optical fibres
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Light travels at 3 × 10⁸ m/s in vacuum and 2 × 10⁸ m/s in glass. Find the refractive index of glass.
n = 3 × 10⁸ ÷ 2 × 10⁸ = 1.5.
Answer: 1.5
EXAMATLAS LEVEL
Q. On what principle do optical fibres work, and why does a swimming pool look shallower than it is?
Optical fibres work by total internal reflection. A pool looks shallower because light from the bottom bends away from the normal as it leaves water, so the bottom appears raised.
Answer: Total internal reflection; refraction
Pattern 4: Lens power and eye defects
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Find the power of a convex lens of focal length 50 cm.
P = 1/f = 1/0.5 = +2 D.
Answer: +2 D
EXAMATLAS LEVEL
Q. A person cannot see distant objects clearly and is prescribed a lens of −2.5 D. Name the defect, the type of lens and its focal length.
Blurred distant vision is myopia. A negative power means a concave lens, with f = 1/(−2.5) = −0.4 m, that is −40 cm.
Answer: Myopia; concave lens; −40 cm
60-Second Revision
- Convex mirror: rear-view; concave mirror: headlights, dentists, solar cookers.
- Blue sky and red sunset: scattering; rainbow: dispersion; diamond and optical fibre: total internal reflection.
- n = c ÷ v; P = 1/f in dioptres.
- Myopia: concave lens; hypermetropia: convex lens; presbyopia: bifocal.