Chapter 9: Light: Reflection and Refraction
Reflection of light
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. The image in a plane mirror is virtual, erect, laterally inverted and of the same size as the object.
Spherical mirrors
Concave and convex mirrors. Key terms: pole P, centre of curvature C, radius of curvature R, principal focus F, focal length f = R/2.
Concave mirrors are used in dentists' mirrors, torches and vehicle headlights. Convex mirrors are used as rear-view mirrors in vehicles.
Refraction of light
The change in the direction of light when it passes from one medium to another is refraction. Snell's law: sin i / sin r = constant. Refractive index: n = c/v (water 1.33, glass 1.5, diamond 2.42). Light bends towards the normal when it enters a denser medium.
Lenses
A convex lens is converging and a concave lens is diverging.
A convex lens has positive power and a concave lens has negative power.
Important questions
Q1. State the laws of reflection.
(i) The angle of incidence equals the angle of reflection. (ii) The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
Q2. State the characteristics of the image formed by a plane mirror.
Virtual, erect, laterally inverted, the same size as the object, and as far behind the mirror as the object is in front.
Q3. What is the relation between focal length and radius of curvature?
f = R/2, that is, the focal length is half the radius of curvature.
Q4. Why is a convex mirror used as a rear-view mirror in vehicles?
It always forms an erect, diminished image and has a much wider field of view.
Q5. Why do dentists use a concave mirror?
When the object is between the pole and the focus, a concave mirror forms an enlarged, erect and virtual image, so the teeth look bigger.
Q6. Where should an object be placed in front of a concave mirror to get a real, inverted image of the same size?
At the centre of curvature C. The image is also formed at C.
Q7. Where and how is the image formed by a concave mirror when the object is at infinity?
At the focus F, real, inverted and highly diminished (point-sized).
Q8. Write the mirror formula.
1/v + 1/u = 1/f, where u is the object distance, v is the image distance and f is the focal length.
Q9. What is the magnification of a mirror and what does its sign tell?
m = h'/h = -v/u. A negative m means a real and inverted image; a positive m means a virtual and erect image.
Q10. An object is placed 10 cm in front of a concave mirror of focal length 15 cm. Find the position and nature of the image.
u = -10, f = -15. 1/v = 1/f - 1/u = -1/15 + 1/10 = 1/30, so v = +30 cm. m = -v/u = +3. The image is behind the mirror, virtual, erect and 3 times larger.
Q11. What is refraction and why does it occur?
The bending of light when it passes from one transparent medium to another. It happens because the speed of light is different in different media.
Q12. State Snell's law.
For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant: sin i / sin r = constant (the refractive index).
Q13. What is the refractive index?
The ratio of the speed of light in vacuum to its speed in the medium: n = c/v. It has no unit.
Q14. When light goes from air into water, does it bend towards or away from the normal?
Towards the normal, because water is a denser medium than air.
Q15. The refractive index of glass is 1.5. What is the speed of light in glass? (c = 3 × 10⁸ m/s)
v = c/n = (3 × 10⁸) / 1.5 = 2 × 10⁸ m/s.
Q16. What is the nature of convex and concave lenses?
A convex lens is converging (it brings rays together) and a concave lens is diverging (it spreads rays apart).
Q17. Write the lens formula.
1/v - 1/u = 1/f. The magnification is m = h'/h = v/u.
Q18. What is the power of a lens? What is its unit?
P = 1/f (f in metres). Its unit is the dioptre (D). 1 dioptre is the power of a lens whose focal length is 1 metre.
Q19. Find the power of a lens of focal length +50 cm.
f = 0.5 m, so P = 1/0.5 = +2 D.
Q20. Find the power of a concave lens of focal length 20 cm.
The focal length of a concave lens is negative: f = -0.2 m, so P = 1/(-0.2) = -5 D.
Quiz
1. If the angle of incidence is 30°, what is the angle of reflection?
By the law of reflection both angles are equal.
2. Which mirror is used as a rear-view mirror in vehicles?
A convex mirror has a wider field of view.
3. What is the formula for refractive index?
n = speed in vacuum / speed in the medium.
4. What is the unit of power of a lens?
The power of a lens is measured in dioptres.
5. Which lens has a positive power?
A convex lens has a positive focal length and positive power.