NCERT Science: Light, Reflection, Refraction and the Human Eye

The one-line takeaway

Light travels in straight lines at the highest known speed. It reflects from mirrors and refracts (bends) when it passes between media, which is how mirrors and lenses form images and how the human eye works. Splitting white light into its colours (dispersion) explains the rainbow.

Reflection

• Laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal lie in the same plane.
• A plane mirror forms an image that is virtual, erect, the same size, and laterally inverted (left and right swapped), at the same distance behind the mirror as the object is in front.
• Spherical mirrors:
  • A concave mirror is converging; it can form real or virtual images. Uses: shaving and make-up mirrors (magnified erect image when close), torch and headlight reflectors, dentists' mirrors, and solar concentrators.
  • A convex mirror is diverging; it always forms a small, erect, virtual image and gives a wide field of view. Used as the rear-view (wing) mirror in vehicles because it shows more of the road behind.

Refraction

• Refraction is the bending of light when it passes from one transparent medium to another because its speed changes. Light bends toward the normal entering a denser medium and away from the normal leaving it.
• Effects: a pencil looks bent in water, a coin in a beaker seems to rise, stars twinkle (atmospheric refraction), and the Sun is seen a little before it rises and after it sets.
• The refractive index measures how much a medium slows light; the more it slows, the more it bends.

Lenses

• A convex (converging) lens is thicker in the middle; it converges light and can form real or virtual images. Uses: a magnifying glass, the human eye lens, cameras, and the correction of long-sight.
• A concave (diverging) lens is thinner in the middle; it always gives a diminished, erect, virtual image. Used to correct short-sight.
• The power of a lens is measured in dioptre (the reciprocal of focal length in metres); a converging lens has positive power, a diverging lens negative.

The human eye

• The eye works like a camera. Light enters through the cornea, passes the pupil (whose size the iris controls), is focused by the lens onto the retina, where light-sensitive cells (rods for dim light, cones for colour) send signals along the optic nerve to the brain.
• Accommodation is the eye's ability to change the focal length of its lens to focus near and far objects.

Defects of vision and their correction

• Myopia (short-sight): distant objects look blurred; the image forms in front of the retina. Corrected by a concave (diverging) lens.
• Hypermetropia (long-sight): near objects look blurred; the image would form behind the retina. Corrected by a convex (converging) lens.
• Presbyopia: the age-related loss of accommodation; corrected by bifocal lenses.
• Cataract: clouding of the lens, treated by surgery, not spectacles.
• Astigmatism: uneven curvature of the cornea, corrected by cylindrical lenses.

Dispersion and atmospheric optics

• A glass prism splits white light into its component colours (the spectrum), in the order red, orange, yellow, green, blue, indigo, violet (VIBGYOR in reverse). Red bends the least, violet the most. This is dispersion.
• The rainbow is dispersion and internal reflection of sunlight by raindrops. The blue colour of the sky and the red of the rising and setting Sun are due to the scattering of light by air molecules (shorter blue wavelengths scatter most).

Key terms to fix

• Speed of light in vacuum: about 3 times 10 to the power 8 metre per second.
• Concave versus convex mirror: converging versus diverging; convex used as vehicle rear-view.
• Convex versus concave lens: converging versus diverging.
• Myopia (concave-lens correction) versus hypermetropia (convex-lens correction).
• Dispersion: splitting of white light into VIBGYOR by a prism.

CAPF angle

Optics underlies a wide range of defence and surveillance equipment: binoculars, telescopic and night-vision sights, periscopes, range-finders and laser-guided systems. Lenses and mirrors are the basis of reconnaissance optics and satellite cameras. The eye and its defects are a routine concern in the CAPF medical standards, where uncorrected and corrected vision limits are specified, so this topic links directly to the medical standards.

Authored practice

1. The rear-view mirror of a vehicle is convex rather than plane because a convex mirror: (a) magnifies the image (b) gives a wider field of view with an erect, diminished image (c) inverts the image (d) shows a real image. (Answer: b.) Authored practice, not a verbatim PYQ.
2. Myopia (short-sightedness) is corrected using which type of lens? (Answer: a concave, diverging lens.) Authored practice, not a verbatim PYQ.