Reflection of Light
ICSE Class 9 · Physics · Chapter 7
What this chapter covers
- Reflection and the laws of reflection
- Images formed by a plane mirror
- Multiple images in two parallel plane mirrors
- Spherical mirrors
- Rules for drawing ray diagrams
- Image formation by spherical mirrors
- The mirror formula and sign convention
- Uses of spherical mirrors
Key points
- Laws of reflection: the incident ray, reflected ray and normal lie in one plane; ∠ i = ∠ r, both measured from the normal.
- A real image can be taken on a screen; a virtual image cannot.
- A plane mirror forms a virtual, erect, same-sized, laterally inverted image as far behind the mirror as the object is in front.
- Two parallel plane mirrors form a very large number of images that become fainter with distance.
- Concave mirrors converge parallel rays; convex mirrors diverge them.
- Pole P, centre of curvature C, radius of curvature R, principal axis, principal focus F and focal length f; f = R/2.
- Key rays: parallel to axis → through F; through F → parallel to axis; through C → back along itself; at P → equal angles with the axis.
- Concave mirror images are real and inverted except when the object is between F and P, when the image is virtual, erect and magnified.
- A convex mirror always gives a virtual, erect, diminished image between P and F.
- Cartesian sign convention: distances from the pole; along incident light positive; u always negative; f negative for concave, positive for convex.
- Mirror formula 1/v + 1/u = 1/f; magnification m = h2/h1 = -v/u.
- Concave: shaving mirrors, dentists' mirrors, headlights, solar cookers. Convex: rear-view mirrors, shop security mirrors, road-bend mirrors, street lamps.
Questions answered
State the two laws of reflection.
The incident ray, reflected ray and normal at the point of incidence lie in the same plane; the angle of reflection equals the angle of incidence.
A ray falls normally on a plane mirror. What are the angles of incidence and reflection?
Both are 0°, and the ray returns along its own path.
Distinguish between a real image and a virtual image.
A real image is formed where reflected rays actually meet and can be taken on a screen; a virtual image is formed where rays only appear to meet and cannot be taken on a screen.
List four characteristics of the image formed by a plane mirror.
Virtual, erect, same size as the object, as far behind the mirror as the object is in front, and laterally inverted.
Why is AMBULANCE written in mirror writing on the front of the vehicle?
A driver ahead sees it in the rear-view mirror, where lateral inversion makes it read correctly.
What do you see when a candle is placed between two parallel plane mirrors? Why do the images get fainter?
A long row of images in each mirror, in principle infinitely many. Some light is absorbed at each reflection, so successive images are fainter.
Define the principal focus of a concave mirror and its focal length.
The principal focus is the point on the principal axis where rays parallel to the axis meet after reflection. The focal length is the distance from the pole to the principal focus.
A convex mirror has a radius of curvature of 40 cm. What is its focal length?
f = R/2 = 20 cm (positive, by the sign convention).
Past board exam questions
ICSE 2019 sample paper · 2 marks
Why do we use a convex mirror as a rear view mirror?
Show answer
A convex mirror always forms an erect, virtual and diminished image of objects behind the vehicle, and it has a wider field of view than a plane mirror. So the driver can see a much larger area of the traffic behind, the right way up.
ICSE 2019 sample paper · 3 marks
An object of height 20 cm is kept at a distance of 48 cm in front of a mirror of focal length 12 cm. If the mirror forms a virtual, diminished image of the object then calculate (i) the distance of the image from the mirror. (ii) its magnification.
Show answer
A virtual, diminished image is formed only by a convex mirror, so f = +12 cm, u = −48 cm.
(i) (1)/(v) + (1)/(u) = (1)/(f) ⇒ (1)/(v) = (1)/(12) + (1)/(48) = (5)/(48) ⇒ v = +9.6 cm.
The image is 9.6 cm behind the mirror.
(ii) m = -(v)/(u) = -(9.6)/(-48) = +0.2 (image erect, 1/5 of the object, i.e. 4 cm high).