Light: Reflection, Refraction, Lenses, Mirrors, Human Eye and Eye Defects Notes for SSC General Science

Light is one of the most scoring chapters of physics in SSC general science. Questions come on mirrors, lenses, total internal reflection, the sky and rainbow, and defects of the eye. This guide on light for SSC covers the laws of reflection and refraction, the mirror and lens formulas, dispersion and scattering and the human eye, with solved examples and practice questions.

Basic Facts

• Light is an electromagnetic wave. Its speed in vacuum is about 3 × 10⁸ m/s.
• Light travels in a straight line (rectilinear propagation) and does not need a medium.
• The speed of light is highest in vacuum and lowest in dense media like glass.
• The sun’s light takes about 8 minutes 20 seconds to reach the earth, and moonlight takes about 1.3 seconds.

Reflection of Light

Laws of reflection: (1) The angle of incidence is equal to the angle of reflection. (2) The incident ray, the reflected ray and the normal lie in the same plane.

Mirror formula: 1/v + 1/u = 1/f. Magnification: m = −v/u = h′/h. The focal length f = R/2 (R is the radius of curvature).

Refraction of Light

The bending of light when it passes from one medium to another is called refraction. Snell’s law: sin i / sin r = constant = refractive index n. The refractive index n = speed of light in vacuum / speed of light in the medium. Light bends towards the normal when it enters a denser medium.

Total Internal Reflection (TIR)

When light travels from a denser to a rarer medium at an angle greater than the critical angle, it is completely reflected back. Applications and examples of TIR:

• Optical fibres used in communication and endoscopy.
• The sparkling of diamond (very small critical angle).
• Mirage in deserts and the shining of an air bubble in water.
• Totally reflecting prisms in binoculars and periscopes.

Lenses

Lens formula: 1/v − 1/u = 1/f. Power of a lens: P = 1/f (f in metres); the unit is dioptre (D). The power of a convex lens is positive and that of a concave lens is negative.

Dispersion and Scattering

• Dispersion: splitting of white light into its seven colours (VIBGYOR: violet, indigo, blue, green, yellow, orange, red) by a prism, first shown by Newton. Violet bends the most and red the least.
• Rainbow: formed by dispersion, refraction and internal reflection of sunlight in water droplets.
• Scattering: the sky is blue because the shorter wavelength (blue) is scattered more (Rayleigh scattering). At sunrise and sunset the sun looks red because blue light is scattered away.
• Danger signals are red because red is scattered the least and is visible from far.
• Twinkling of stars is due to atmospheric refraction.

The Human Eye

The near point of a normal eye is 25 cm and the far point is infinity.

Defects of the Eye

Solved Examples

Example 1. A concave mirror has a focal length of 20 cm. What is its radius of curvature?
R = 2f = 40 cm.

Example 2. A lens has a focal length of 50 cm. What is its power?
P = 1/0.5 = +2 D (convex lens).

Example 3. The refractive index of glass is 1.5. What is the speed of light in glass?
v = c/n = 3 × 10⁸ / 1.5 = 2 × 10⁸ m/s.

Quick Tips

• Remember the uses of the three types of mirrors.
• Learn the correction of each eye defect.
• Remember that violet bends the most and red the least.
• Memorise the refractive indices of water, glass and diamond.

Quick Revision Points

• Angle of incidence = angle of reflection.
• Mirror formula: 1/v + 1/u = 1/f; lens formula: 1/v − 1/u = 1/f.
• Myopia is corrected by a concave lens; hypermetropia by a convex lens.
• TIR needs light going from a denser to a rarer medium above the critical angle.
• The sky is blue due to scattering.

Practice Questions With Answers

1. Which mirror is used as a rear-view mirror?
   Answer: Convex mirror
2. Which lens corrects myopia?
   Answer: Concave lens
3. What is the unit of the power of a lens?
   Answer: Dioptre
4. Which colour of light bends the most in a prism?
   Answer: Violet
5. Which phenomenon causes the sparkling of diamond?
   Answer: Total internal reflection
6. Which part of the eye controls the amount of light?
   Answer: Iris (pupil size)
7. Why is the sky blue?
   Answer: Scattering of light by the atmosphere
8. What is the speed of light in vacuum?
   Answer: 3 × 10⁸ m/s
9. Which mirror is used in a solar cooker and headlights?
   Answer: Concave mirror
10. Which lens is used to correct hypermetropia?
    Answer: Convex lens

How to Practise This Chapter

Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the General Science notes page and the next topic Current Electricity and Magnetism.

Frequently Asked Questions

What is reflection of light?

The bouncing back of light from a surface.

What is refraction of light?

The bending of light as it passes from one medium to another.

What is the refractive index?

The ratio of the speed of light in vacuum to its speed in the medium.

What is total internal reflection?

The complete reflection of light at the boundary when light moves from a denser to a rarer medium at an angle above the critical angle.

Why is the sky blue?

Because the atmosphere scatters blue light more than red.

What is the power of a lens?

The reciprocal of its focal length in metres, measured in dioptre.

What is myopia?

Short-sightedness, in which distant objects look blurred.

What is hypermetropia?

Long-sightedness, in which near objects look blurred.

Which mirror gives an always virtual and diminished image?

The convex mirror.

What are rods and cones?

The light-sensitive cells of the retina, with rods for dim light and cones for colours.

What is dispersion?

The splitting of white light into its colours.

Is light asked in SSC?

Yes. Questions on mirrors, lenses and the eye are asked in SSC general science.