Refraction and Lenses — REET Science Notes

What is refraction

When light passes at an angle from one transparent medium into another, a change in the speed of light as the medium changes bends its path — this is called the refraction of light. There are two laws of refraction: (1) the incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane. (2) For a given pair of media and a given colour of light, the ratio of the sine of the angle of incidence (sin i) to the sine of the angle of refraction (sin r) stays constant — this is called Snell's law: sin i / sin r = constant = refractive index (n).

The refractive index tells us how much a medium bends light, and it equals the ratio of the speed of light in vacuum/air to the speed of light in that medium (n = c/v). In a denser medium (like glass or water), the speed of light decreases, so its refractive index is higher than in a rarer medium (like air).

Convex lens vs concave lens

Lens formula and power

Lens formula

1/v − 1/u = 1/f (using the new Cartesian sign convention)

Magnification (m)

m = h'/h = v/u

Power of a lens (P)

P = 1/f (f in metres); unit: dioptre (D)

Power of a convex lens

Positive (+D)

Power of a concave lens

Negative (−D)

Image formation by a convex lens

The image formed by a convex lens depends on where the object is placed. When the object is placed outside the focal length, a real, inverted image forms, whose size can be smaller than, equal to, or larger than the object depending on its distance. But when the object is placed within the focal length (between the lens and the focus), a virtual, erect, and magnified image forms — this is exactly the principle used in a magnifying glass.

A concave lens, on the other hand, always forms a virtual, erect image that's smaller than the object, no matter where the object is placed — this image forms between the lens and the focus. This is exactly why a concave lens is used to correct myopia (short-sightedness). Drawing ray diagrams to show the nature of an image (real/virtual, erect/inverted, magnified/diminished) is a useful practice for REET exam preparation, since it makes every situation clear.

Everyday examples of refraction

A pencil or a straight stick dipped in water looks bent — this happens because of the refraction of light, as light rays bend going from water (a denser medium) into air (a rarer medium). Similarly, a pond looks shallower than its actual depth, and the twinkling of stars also happens because of atmospheric refraction, as the density of the atmosphere's different layers keeps changing.

Another interesting example — even when the sun is below the horizon, we can still see it for a few minutes before sunrise and after sunset, because atmospheric refraction bends the sun's rays so they reach us. This makes it clear that refraction isn't just a laboratory phenomenon, but something happening continuously in nature.

Everyday uses of lenses

Spectacles

Convex and concave lenses are used to correct far-sightedness and near-sightedness respectively

Magnifying glass

A convex lens used to enlarge small objects (like small print in a newspaper) for viewing

Camera

A convex lens forms a real, inverted image on the film/sensor

Microscope and telescope

Combinations of convex lenses are used to view tiny or distant objects enlarged

How to teach this to students

Practice questions

Frequently asked questions

Read next