What do Mirrors do to Light?
Plane mirrors
When a ray of light reflects off a shiny surface it comes back at the same angle to the normal as it came in. The normal is an imaginary line, usually drawn hashed, at right angles to the mirror.
When we look in a mirror we see an image of ourselves. We can explain this by considering what happens to rays of light from the object that reflect off the mirror. Consider two rays from the tip of the candle above which reflect as shown. Our brain tells us that light travels in a straight line so the rays appear to be coming from a point on the other side of the mirror.
What we see is a virtual image. (Not a real image) It is not the kind of image you could project onto a screen.
Concave mirrors
The kind of image we get with a concave mirror depends on the position of the object. A concave mirror has a principal focus. This is where rays of light from a distant object (so they are parallel) would converge. You should learn the information below.
The magnification of the mirror (or lens) is how many times bigger the image is compared to the object. If the image is smaller than the object then this will be less than 1.
|
Position of object from mirror |
Position of image |
Nature of image |
|
Less than the focal length F |
Virtual, on other side of the mirror |
Upright and magnified |
|
Between F and 2F |
Greater than 2F |
Real and magnified |
|
Greater than 2F |
Between F and 2F |
Real, smaller and upside down |
You do not need to learn this table if you know haw to draw ray diagrams. Following simple rules as to what certain rays do you can figure out what happens to rays from the object and so what kind of image you will get.
Convex mirrors
These are easier to remember as the image is always virtual, upright and smaller. Convex mirrors have a very wide field of view which makes them useful for safety mirrors. Wing mirrors on a car are often convex.
_______________________________________________________________________________________
Revision Questions
1. Draw a diagram to show how an image is formed in a plane mirror.
2. What is the difference between a real image and a virtual image?
3. Look at the concave face of a large shiny spoon. Move it slowly towards your face. Describe and explain what you see.
4. Prove, by drawing a ray diagram, that if an object is greater than 2F from a concave mirror the image is as stated in the table above.
5. A candle is 5cm tall. Its image in a concave mirror is 12cm. Calculate the magnification of the mirror.
6. Move the convex face of a big shiny spoon towards yourself slowly. Describe and explain what you see.
7. Describe another use for a convex mirror.