Grade 9 Integrated Science Notes: Curved Mirrors (Concave, Convex, Ray Diagrams and Uses)

Grade 9 Integrated Science · Strand 3.0 Force and Energy · Sub-strand 3.1 Curved Mirrors

These are free study notes for Grade 9 learners, written to the KICD curriculum design and taken from our full Grade 9 Integrated Science notes. Ray diagrams are easy marks once you know the three rules for rays. Practise drawing them with a ruler and a sharp pencil.

A car's side mirror, a torch reflector and a dentist's mirror are all curved mirrors. In this topic we study concave and convex mirrors, the terms used with them, how to draw ray diagrams to find their images, the characteristics of those images and how curved mirrors are used for safety, security and energy.

Types of curved mirrors

In the inside of a spoon held at arm's length your image is small and upside down; held very close it becomes upright and larger. In the back of a spoon your image is always upright and smaller.

Terms used with curved mirrors

Ray diagrams for concave mirrors

• Ray 1: parallel to the principal axis, reflects through F.
• Ray 2: through F, reflects parallel to the axis.
• Ray 3: through C, reflects back along itself.
• Any two rays are enough; where they meet is the top of the image.

Convex mirrors and image characteristics

For a convex mirror, F and C are behind the mirror. A ray parallel to the axis reflects as if it came from F; a ray heading for C reflects back along itself. The image is always behind the mirror, virtual, upright and diminished, with a wide field of view.

Describe an image by its position, nature (real or virtual), orientation (upright or inverted) and size. Magnification = image height ÷ object height.

Uses of curved mirrors

Test yourself: multiple choice

1. A concave mirror has a radius of curvature of 30 cm. What is its focal length?

   1. 60 cm
   2. 15 cm
   3. 30 cm
   4. 10 cm
   Show the answer

   B: f = 30 ÷ 2 = 15 cm.

2. Which mirror is used as a car side mirror?

   1. Convex
   2. Concave
   3. Plane only
   4. Parabolic
   Show the answer

   A: Convex mirrors give a wide view.

3. An object is placed at C of a concave mirror. The image is

   1. virtual and magnified.
   2. at F and small.
   3. at C, real, inverted, same size.
   4. behind the mirror.
   Show the answer

   C: Object at C gives an image at C of the same size.

4. Where is the bulb placed in a torch reflector?

   1. At C
   2. Beyond C
   3. Between C and F
   4. At F
   Show the answer

   D: A bulb at F gives a parallel beam.

5. An image is 2 cm tall and the object is 8 cm tall. What is the magnification?

   1. 4
   2. 0.25
   3. 16
   4. 6
   Show the answer

   B: 2 ÷ 8 = 0.25 (diminished).

Practice questions

1. Define (a) principal focus (b) focal length. (2 marks)

   Show the answer

   (a) The point where rays parallel to the axis meet (or seem to come from) after reflection. (b) The distance from the pole to the principal focus.

2. State the position and characteristics of the image when an object is between F and P of a concave mirror, and give one use. (4 marks)

   Show the answer

   Behind the mirror; virtual, upright, magnified. Use: shaving or dentists' mirror.

3. Give three differences between images formed by concave and convex mirrors. (3 marks)

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   Concave can be real or virtual; convex always virtual. Concave can be magnified; convex always diminished. Concave can be inverted; convex always upright.

4. Explain why convex mirrors are placed at sharp bends on roads. (2 marks)

   Show the answer

   They give a wide field of view, so drivers can see traffic coming round the bend and avoid accidents.