Integrated Science — Force and Energy

Subtopic: Curved mirrors (Age 14, Kenya)

Curved mirrors are mirrors whose reflecting surface is part of a sphere. They change the direction of light and form images that can be magnified, reduced or virtual. Curved mirrors are useful in many Kenyan places: barber shops, car rear-view mirrors, security mirrors in shops and for concentrating sunlight in solar cookers.

Key terms

• Concave mirror — reflecting surface curves inward (like the inside of a spoon).
• Convex mirror — reflecting surface curves outward (like the back of a spoon).
• Principal axis — the straight line through the centre of the mirror.
• Centre of curvature (C) — centre of the sphere from which the mirror is a part.
• Focal point (F) — point where parallel rays meet (concave) or appear to come from (convex).
• Vertex (V) — centre point on the mirror surface.

How images are formed (simple ideas)

- Concave: Parallel rays of light reflect and meet at the focal point. Depending on the object's distance from the mirror, the image may be real (can be projected) or virtual (seen behind the mirror), and may be larger or smaller.
- Convex: Parallel rays reflect outwards and appear to come from a point behind the mirror. Images are always virtual, upright and smaller (useful for wide view).

Simple ray diagrams (look and copy)

Mirror formula (short)

For curved mirrors we use a simple relation (used later in higher classes):

1/f = 1/do + 1/di

where f = focal length, do = object distance, di = image distance. For age 14 you only need to know that: - Small f (strong curve) gives more bending of rays.
- Concave mirrors can produce real images (projectable) or virtual images. Convex mirrors always produce virtual, smaller images.

Energy and uses

• Mirrors change the direction of light energy but do not create energy.
• Concave mirrors concentrate light at the focal point — used in solar cookers and torch reflectors to increase brightness.
• Convex mirrors give a wide view — used as rear-view mirrors and security mirrors in shops and on roads.
• Concave mirrors are used by dentists and in shaving mirrors to produce magnified images.

Classroom experiment (easy, safe)

1. Take a metal spoon. Look at the inside curve — this is like a concave mirror. Move the spoon close to your face: the image may be magnified and upright or inverted if far.
2. Look at the back of the spoon — it behaves like a convex mirror and shows a smaller, upright image.
3. With a torch and a concave mirror (or shiny bowl), shine parallel light and try to find the bright spot (focal point) where light concentrates.

Questions for practice

1. Give two differences between concave and convex mirrors.
2. Why are convex mirrors used as rear-view mirrors in vehicles?
3. Describe one use of a concave mirror that takes advantage of concentrating light energy.
4. Draw a simple ray diagram for an object placed beyond the centre of curvature of a concave mirror and state whether the image is real or virtual, upright or inverted, larger or smaller.

Quick answers

• Differences: Concave curves inward and can form real images; convex curves outward and forms only virtual images.
• Convex mirrors give a wide field of view and produce smaller, upright images so the driver can see more area behind the vehicle.
• Concave mirrors concentrate sunlight to the focal point — used in solar cookers to heat food.
• When object beyond C: image is between C and F, real, inverted and smaller (use ray diagram to show this).

Tip: Try making small diagrams and practising with a spoon — it helps you remember how rays behave.