Ray diagrams for convex lenses

Remember that they keep giving you mini-lenses on the diagram. Don't get fooled into drawing the rays through the lens. On their way of doing it, the vertical line is also part of the lens.

Reflection in a plane mirror

You've probably forgotten that we ever did this diagram for a plane mirror (flat mirror) but it would be a good idea to learn it.

Concave mirrors

Let's make a concave mirror. We start off with a complete circle. It seems quite obvious at the moment that the middle of the circle is the centre of curvature.



















Now let's mark out a section of the circle and then cut it out. You can see that it still has a centre of curvature even when it's just a cut section.



















Now let's fire rays of light at it. Notice that they don't focus at the centre of curvature. That means that we also need to know about a second point - the focal point (or principal focus).

















Now take a look at a proper ray diagram:

• It has C for centre of curvature and F for principal focus
• The curved lens has been drawn flat for the purposes of the diagram

There are 3 possible rays you could draw, although only 2 appear on any diagram:

Ray 1: Horizontally from the tip of the object arrow to hit the mirror and bounce back through F.
Ray 2: Slants down straight through F to hit the mirror then bounces back horizontally.

Ray 3: Is a perfectly straight line going through the top of the image arrow and through point C.

Real and virtual images

For the purposes of exam answers, this is what you have to say:

A real image can be projected onto a piece of paper.

• We did this is class to see an upside down picture of the room and to get pictures of the upside down triangle.
• You can spot it on a diagram because it is where solid rays cross.

A virtual image cannot be projected onto a piece of paper.

• You can spot it on a diagram because it is where dotted rays cross.

The National Grid

Click on the picture to get it to a bigger size.

On this diagram, A is a step up transformer at the power station.

• A step up transformer increases the voltage (potential difference).
• At the same time the current is reduced.
• Current is responsible for heating up wires and wasting energy.
• So reducing the current reduces the energy wasted by heat.

Near your house there is step down transformer C that decreases the voltage ( and increases the current). The reason is that the massive voltage could give a fatal electric shock.

The National Grid is the system of power stations and power lines. It means that each town no longer has to have its own power station. Clearly there are economies of scale.

PS B is an intermediate step down transformer. There's one for every town that reduces the voltage from 400,000V to 33,000V before it is sent to the local substations to be reduced to 230V for your house.

Elliptical orbits

The orbit of the Earth around the Sun is not a perfect circle. It is slightly oval. We say that it is an ellipse. Therefore the orbit is elliptical.

The Sun is not in the centre. It is slightly to one side at a point called the FOCUS.

All of the planets follow elliptical orbits.

On the mock exam there was a question about an object going around the Sun. Should scientists vote on whether or not it should be a planet? The answer was to wait for more evidence to decide whether it might be something else like a COMET. Note: moons don't directly orbit the Sun: they orbit planets. The orbits of comets are particularly elliptical.

Electromagnetic induction

The syllabus has certain crucial points they keep going over:

• Electromagnetic induction occurs when eg a coil cuts the magnetic field of a spinning magnet.
• You have to say that a potential difference (voltage) is induced.
• THEN the potential difference causes a current to flow.

Supernova

When a large star reaches the end of its life, there is a massive explosion called a supernova.

Stars normally make new elements by the process called NUCLEAR FUSION.

This involves heat and pressure making positive nuclei stick together to make larger elements. Positive nuclei normally repel each other.

In stars like the Sun, the temperatures are only enough to turn hydrogen into helium.

However, the heat and pressure in a supernova are so huge that nuclear fusion can make ALL of the elements that exist.

Hence all the atoms in your body were made in a supernova billions of years ago. FACT!

Transformers

The whole thing together is called a COIL. The individual parts are called TURNS.

You will only get a mark if you write like this:

"It is a step down transformer because there are more TURNS on the primary than on the secondary."