Light: Reflection and Refraction Notes with PYQs
Light notes for exams: laws of reflection, mirrors and lenses with their formulae, refraction and Snell's law, total internal reflection and dispersion.
By GK24 Editorial Team· Published · 4 min read
Light is a form of energy that travels as an electromagnetic wave and needs no medium, which is why sunlight crosses the vacuum of space. Its speed in vacuum is about three hundred thousand kilometres a second, written as 3 into 10 raised to 8 metres per second, and it is the highest speed known in nature. In every other medium light travels more slowly, and almost everything a student must learn in this chapter follows from that single fact. Reflection, refraction, total internal reflection and dispersion together explain a mirror, a lens, a mirage, a rainbow and a pair of spectacles.
Reflection and mirrors
Reflection is the turning back of light from a surface. There are two laws. The angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. A plane mirror always forms a virtual, erect image of the same size as the object, as far behind the mirror as the object is in front, and the image is laterally inverted, which is why the word AMBULANCE is painted in reverse on the front of the vehicle.
A spherical mirror is a part of a hollow sphere. A concave mirror curves inward and converges light, so it can form a real image; a convex mirror curves outward and diverges light, so its image is always virtual, erect and smaller than the object. The focal length of a spherical mirror is half its radius of curvature. The mirror formula is 1/v + 1/u = 1/f and the magnification is m = -v/u. Concave mirrors are used as shaving mirrors, in dentists' mirrors, in torches and headlights and in solar cookers; convex mirrors are used as rear-view mirrors in vehicles and on blind turnings because they give a wide field of view.
Refraction and lenses
Refraction is the bending of light when it passes from one transparent medium into another, and it happens because the speed of light changes. A ray going from a rarer medium into a denser one, such as from air into glass, slows down and bends towards the normal; a ray going from a denser medium into a rarer one speeds up and bends away from the normal. A ray falling along the normal is not bent at all. Snell's law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a pair of media, and that constant is the refractive index. The absolute refractive index of a medium is the speed of light in vacuum divided by its speed in that medium, so a higher index means a slower and optically denser medium.
| Medium | Refractive index | Critical angle with air |
|---|---|---|
| Water | About 1.33 | About 48.6 degrees |
| Ordinary glass | About 1.5 | About 42 degrees |
| Diamond | About 2.42 | About 24.4 degrees |
A convex lens is thicker in the middle and converges light; a concave lens is thinner in the middle and diverges it. The lens formula is 1/v - 1/u = 1/f and the magnification is m = v/u. The power of a lens is the reciprocal of its focal length in metres, P = 1/f, and its unit is the dioptre. A converging lens has a positive power and a diverging lens a negative power, so a prescription of minus 2.5 dioptres means a concave lens of focal length forty centimetres.
Total internal reflection and dispersion
When light travels from a denser medium to a rarer one and the angle of incidence is larger than the critical angle, none of it passes out and all of it is reflected back into the denser medium. This is total internal reflection. It explains the brilliance of a cut diamond, whose small critical angle traps light inside, the working of optical fibres that carry telephone and internet signals, and the mirage seen on a hot road, where layers of air of different density bend light from the sky. Dispersion is the splitting of white light into its seven colours by a prism, first demonstrated by Isaac Newton. Red light is deviated the least and violet the most, because the refractive index of the glass is different for each colour. A rainbow is formed when raindrops refract, internally reflect and again refract sunlight.
The eye and its defects
- Myopia or short sight: the image forms in front of the retina and distant objects look blurred; corrected with a concave lens.
- Hypermetropia or long sight: the image forms behind the retina and near objects look blurred; corrected with a convex lens.
- Presbyopia: the eye muscles weaken with age and the near point recedes; corrected with bifocal lenses.
- Astigmatism: the cornea is not perfectly spherical; corrected with a cylindrical lens.
- The least distance of distinct vision for a normal adult eye is about twenty-five centimetres.
Exam Point of View
This chapter is asked in three ways. Direct facts: which mirror is used where, what image a plane mirror forms, the unit of the power of a lens, the refractive index of a medium. Small numerical questions on the mirror and lens formulae and on power, where the sign convention decides the answer and a concave lens must be taken with a negative focal length. And the explanation of everyday phenomena: why a pencil looks bent in water, why stars twinkle, why the sun is seen before it rises, why a diamond sparkles and why a mirage appears on a hot road. The usual traps are forgetting that a convex mirror can never give a real image, dropping the minus sign in the lens formula, and giving reflection instead of refraction as the reason for a mirage.
Important Facts
| Speed of light in vacuum | About 3 into 10 raised to 8 metres per second |
|---|---|
| Laws of reflection | Angle of incidence equals angle of reflection; both rays and the normal are coplanar |
| Plane mirror image | Virtual, erect, same size, laterally inverted |
| Focal length of a spherical mirror | Half the radius of curvature |
| Mirror formula | 1/v + 1/u = 1/f, with magnification m = -v/u |
| Lens formula | 1/v - 1/u = 1/f, with magnification m = v/u |
| Power of a lens | P = 1/f in metres; unit dioptre; convex positive, concave negative |
| Rear-view mirror | Convex mirror, for a wide field of view and an erect image |
| Refractive index | Water about 1.33, glass about 1.5, diamond about 2.42 |
| Total internal reflection | Denser to rarer medium, beyond the critical angle |
| Optical fibre | Works on total internal reflection |
| Dispersion | White light splits into seven colours; violet deviates most, red least |
| Myopia and hypermetropia | Corrected by concave and convex lenses respectively |
| Least distance of distinct vision | About 25 centimetres for a normal adult eye |
Practice MCQs on this topic
A ray of light travelling from a rarer medium to a denser medium:
- A.slows down and bends away from the normal
- B.slows down and bends towards the normal
- C.speeds up and bends away from the normal
- D.speeds up and bends towards the normal
Show answer
A denser medium has a higher refractive index, and the refractive index is the speed of light in vacuum divided by its speed in the medium, so light must travel more slowly inside it.
Read the full explanation →A concave lens has a focal length of 15 cm. If the object is placed at 30 cm from the lens, what is the image distance?
- A.-20 cm
- B.-10 cm
- C.-15 cm
- D.-18 cm
Show answer
By the sign convention the focal length of a concave lens is negative, so f is minus 15 centimetres, and the object distance measured against the direction of the incident light is minus 30 centimetres.
Read the full explanation →Name the scientist who first used a glass prism to obtain the spectrum of sunlight.
- A.C. V. Raman
- B.Lord Rayleigh
- C.Isaac Newton
- D.S. Chandrasekhar
Show answer
Newton passed a narrow beam of sunlight through a glass prism in a darkened room and obtained a band of seven colours on the wall, which he named the spectrum.
Read the full explanation →The image formed by a plane mirror is always:
- A.Real, inverted and of the same size
- B.Virtual, erect and of the same size
- C.Virtual, erect and diminished
- D.Real, erect and magnified
Show answer
Rays reflected from a plane mirror diverge and only appear to come from a point behind the mirror, so the image cannot be caught on a screen and is virtual.
Read the full explanation →Which type of mirror is used as a rear-view mirror in vehicles?
- A.Plane mirror
- B.Concave mirror
- C.Convex mirror
- D.Cylindrical mirror
Show answer
A convex mirror curves outward and spreads the light that falls on it, so it always forms a virtual, erect and diminished image, whatever the distance of the object.
Read the full explanation →The power of a convex lens of focal length 50 cm is:
- A.+0.5 dioptre
- B.+2 dioptre
- C.-2 dioptre
- D.+50 dioptre
Show answer
The power of a lens is the reciprocal of its focal length expressed in metres, that is P equals 1 divided by f. Fifty centimetres is 0.5 metre, and the reciprocal of 0.5 is 2, so the power is two dioptres.
Read the full explanation →The formation of a mirage on a hot road is caused by:
- A.Reflection from the road surface
- B.Dispersion of sunlight
- C.Total internal reflection after refraction in air layers
- D.Scattering by dust particles
Show answer
On a hot day the air touching the road is heated and becomes rarer than the cooler air above it.
Read the full explanation →Optical fibres used in communication work on the principle of:
- A.Dispersion
- B.Total internal reflection
- C.Diffraction
- D.Polarisation
Show answer
An optical fibre is a thin thread of glass or plastic with a core of higher refractive index surrounded by a cladding of lower refractive index.
Read the full explanation →Myopia, or short-sightedness, is corrected by using which type of lens?
- A.Convex lens
- B.Concave lens
- C.Cylindrical lens
- D.Bifocal lens
Show answer
In myopia the eyeball is too long or the lens of the eye too strongly converging, so rays from a distant object meet in front of the retina and the person cannot see far objects clearly.
Read the full explanation →When white light passes through a prism, which colour is deviated the most?
- A.Red
- B.Green
- C.Yellow
- D.Violet
Show answer
White light is a mixture of colours, and the refractive index of the glass of a prism is slightly different for each of them, being greatest for violet and least for red.
Read the full explanation →Frequently Asked Questions
Why is a convex mirror used as the rear-view mirror in vehicles?
A convex mirror always forms a virtual, erect and diminished image whatever the position of the object, and because the image is smaller it covers a much wider field of view than a plane mirror of the same size. A driver can therefore see a long stretch of the road behind in a small mirror. The cost is that objects look farther away than they are, which is why such mirrors carry a warning.
Why does a pencil look bent when it is partly dipped in water?
Light coming from the part of the pencil inside the water passes from water, a denser medium, into air, a rarer one, and bends away from the normal at the surface. The eye traces these bent rays back in straight lines, so the submerged part appears raised and displaced from the part in air, and the pencil looks broken at the surface. The same refraction makes a pool look shallower than it is.
Why does a diamond sparkle more than glass?
The refractive index of diamond is about 2.42, much higher than that of glass, so its critical angle is only about 24.4 degrees. A ray entering a diamond strikes the inner faces at angles larger than this again and again, and is totally internally reflected each time before it finally leaves. The cutting of the stone is designed to make this happen many times, so the light that emerges is concentrated and the stone appears brilliant.
Why do stars twinkle while planets do not?
Stars are so far away that they act as point sources. Their light passes through layers of the atmosphere of changing density and is continuously refracted by varying amounts, so the apparent position and the brightness keep changing and the star seems to twinkle. A planet is much nearer and appears as a collection of many point sources; the variations from different points average out, so its light looks steady.
What is the difference between a real image and a virtual image?
A real image is formed where rays of light actually meet after reflection or refraction, it can be caught on a screen and it is inverted, as in a cinema. A virtual image is formed where the rays only appear to meet when produced backwards, it cannot be caught on a screen and it is erect, as in a plane mirror or a magnifying glass held close to the print.
Sources
- Science (Class X), chapter on light, reflection and refraction — NCERT
- Science (Class X), chapter on the human eye and the colourful world — NCERT
