Core Physics Topic 10 - Reflection

Light Rays

Light is an electromagnetic wave that travels in straight lines until it is either reflected or refracted.  We will look at the behaviour of light rays when they are reflected in a mirror.

We will explore different examples of reflection.  The same rules apply.

Reflection in a Flat Mirror

When light strikes a plane (flat) mirror, it is reflected as shown in the diagram:

We should note the following:

• There is an incident and reflected ray;

• The angle of incidence and the angle of refraction are measured from the normal line;

• The normal line is a construction line that is at 90 degrees to the surface of the mirror.

• The angle of incidence = angle of reflection.

A common bear trap is to measure the angle of incidence or reflection from the surface of the mirror.  You must remember to measure from the normal.

 Question 1 A light ray strikes a mirror at an angle to the surface of 30 degrees.  What is the angle of incidence?  What is the angle of reflection?

Sound waves reflect as well.  We hear the reflections of sound waves as echoes.

The Image in a Mirror

The picture below shows how the image in a mirror is formed.

We draw accurately two rays coming from the object and hitting the mirror at an angle.  Since angle of reflection = angle of incidence, the two rays will be reflected as shown.  We can then extend the rays back.  Where the two rays meet, that is where the corresponding part of the image is found.

You can see where this is done at the top and bottom of the image.

Try it for yourself.  Use graph paper if you want to.  And be a good chap and use a ruler and a sharp pencil.

 Question 2 What is the size of the object compared with the image?

There are two points to note about the image in a mirror:

• It laterally inverted.  This means that, although the image is the right way up compared with the object, left is swapped with right.

• It is virtual.  This means that if you look behind the mirror, you won't find the image there.

The picture below shows an example of lateral inversion:

Curved Mirrors

Concave Mirror

A concave mirror brings parallel rays of light together.

Each ray obeys the Law of Reflection. You can see that the rays come together, or converge.

Note how the shape of the mirror brings all the rays to a single point called the principal focus. The distance between the principal focus and the surface of the mirror is called the focal length.

If the object is close up to the mirror, it appears the right way up (upright) and is magnified (made bigger). If it's further away the image is upside down (inverted) and diminished (made smaller).

Other waves can be reflected by a concave mirror. A satellite dish is a concave mirror to reflect microwave waves onto an antenna. There was a device produced after the First World War to focus sound waves of incoming aircraft to give early warning of their presence.  And it worked.

 Question 3 What is the Law of Reflection?

This picture shows the radio telescope at Jodrell Bank.  The dish is a concave mirror that is 75 m across.  The concave mirror reflects and focuses radio waves on to the antenna (aerial) in the middle at the principal focus.

Photo by Mike Peel, University of Manchester

Convex Mirror

A convex mirror reflects light rays outwards as shown in the diagram.

If we extend the rays behind the mirror, we see that they meet at a principal focus. The image is virtual, upright, and diminished.

A convex mirror is used as a security mirror in a shop, or a wide angle mirror on a bus.

 Question 4 Do the matching questions that checks if you are familiar with some of the terms used in reflection.

We can work out the magnification of a lens with the simple formula:

magnification = height of image

height of object

• If the magnification is less than 1, the image is diminished;

• If the magnification is more than 1, the image is magnified.